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Water pressure and flow measurement (C) Daniel FriedmanBuilding Water Flow Rate Calculation & Measurement Procedures

Measure water flow rate FAQs:

Questions & answers on how to how to measure water quantity delivered per minute at building plumbing systems - what is the flow rate of water out of a well or through a pipe?

This article series explains how to measure the water flow rate in gallons or liters per minute at building faucets & fixtures. Our page top photo shows typical flow rate at a kitchen faucet - around 3 gpm.

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How to Calculate or Measure Actual & Theoretical Water Flow Rates in a Building

Water pressure and flow measurement (C) Daniel Friedman

Recent water flow rate measurement questions & answers

These questions and answers about measuring, estimating or understanding water flow rates at buildings or through pipes and fixtures were posted originally at WATER FLOW RATE CALCULATE or MEASURE - be sure to read that article.

On 2018-07-17 by (mod) - what's the theoretical maximum amount of water that could be delivered to a building?

Matt

You ask a perfectly good question, a great one, but one that is in my opinion very difficult if not impossible to answer by an equation, because of the number of variables and because no home will precisely fit any generic formula or answer.

The theoretical answer is given in WATER FLOW RATE CALCULATE or MEASURE first as a theoretical equation based on pipe diameter, length, and water pressure (see the tables in that article), and second from an equation from which I will excerpt:

The equation of continuity states that flow rate can be calculated from the multiple of the velocity times the cross-sectional area of flow.

Q = A x V or V = Q/A

where

Q = flow rate in ft3 / second
A = cross sectional area flow in sq. ft. of the pipe

(A = π x D2/4 and of course pi or π = 3.1416)

V = velocity in ft/sec - for systems where V varies use the average fluid velocity in the pipe

The flow of a fluid traveling at an average velocity of a 1 meter per second through a pipe with a 1 square meter cross-sectional area is 1 cubic meter per second - volumetric flow rate before considering fluid density. .

For building water supply systems and many other applications the above flow rate calculation is sufficient, but it is not accurate for all types of fluids of various densities.

But for an actual home there are practical considerations that argue that the theoretical answer is likely to be wrong.

Even starting at a water pressure regulator, that device will pass more water volume (at the same pressure) as more faucets are opened in your home.

If we had just a single open pipe pouring water into a home, say at a bath tub, we could calculate the flow rate using an engineering approach that considers pressure and pipe diameter, but that's not the case in a normal home: you have many pipes with elbows, lengths variations in diameter, and constrictions at fixtures. Even the constrictions are not fixed over time

In addition, the pressure of water delivery from the municipal supply - if it is a municipal supply in your case (you don't say) - in many communities varies from hour to hour and day to day.

If your water supply is from a private pump and well system, the delivery pressure is not fixed, by cycles from a low CUT IN pressure to a higher CUT OUT pressure, and worse, the cycle time will vary widely depending on how many fixtures are being run simultaneously, unless and until you are running so much water that the pump simply runs continuously.

In that case you've reached the delivery limit for the private pump and well.

So what's to be done to guesstimate how much water could possibly be delivered:

You could calculate the potential flow rate at your entering water main at your building by assuming a fixed pressure and diameter and by removing the pipes at the incoming main, say just past the shutoff valve.

Then you'd open the valve and measure the pipe diameter and the dimensions of the arc of the stream of water. There are engineering procedures to translate that arc and the other measurements into a fair estimate of flow rate.

Or you could read your water meter over some period of time to see your actual, not your theoretical maximum water usage rate.

Or you could run water at every fixture in your home at once, emptying the flow into a measured container - say a 5 gallon bucket - and time the minutes to fill each bucket. Those flow rates would be additive and would guess at what your house flow rate sum could be for the fixtures you test.

It'll be incomplete as you'll find doing that with a washing machine or toilet or dishwasher is just too much trouble.

On 2018-07-16 by Matt

How do you calculate the amount of water that could possibly be distributed to a home per day. Im not sure if I worded that correctly.

On 2018-06-16 by Ivan

Question about pumping water uphill. Here is what I have: 1500 gallon cistern that needs to be filled on a regular basis uphill from where the water truck would go. 120' of pipe with about 25-30' of rise. Naturally, the water delivery guy wants to save time and asked that I install 3" pipe as the fill line to increase water flow.

But his hose and also the connection that I must install on the end is 2". With a 2" input and a rise, is there really any benefit to switching to 3" pipe for the run?

My gut instinct tells me that this would lower the pressure and the flow but I could be wrong. I can't find any numbers for calculations anywhere, and cisterns as the water source for a house are a standard where I live but have neither code or inspection so I'm kind of on my own to work this stuff out

On 2018-05-11 by (mod) -

Danny

Please try the search box on this page to find our article titled

WELL PUMP SHORT CYCLING

To read how to diagnose and fix the problem that you described

On 2018-05-11 by danny

turn the shower on pump runs every 30 seconds

On 2018-03-05 by (mod) - Any increase in even part of the run of supply piping can make a significant improvement in the fixture flow rate, all else being unchanged.

JW

What question are we really trying to answer here? I'll be glad to research or clarify when you help me understand.

The WATER FLOW RATE CALCULATE or MEASURE article is about rough measurements of water delivery at various fixtures; the actual flow rate is affected by a number of factors:
pipe size, length, elbows valves, restrictions, starting pressure, for pump systems, pump capacity, valve positions, lengths of small diameter fixture riser pipes, and properties of faucets and control valves themselves, even clogged strainers.

So with all of that I would not look for precision in any measurement made at a fixture. I've tripled water flow just by removing a clogged faucet strainer.

Water temperature has no significant effect on flow rate, but I note that hot water flow rate is often a bit less than cold because of the restriction of the water heating equipment, including sometimes deliberate restriction such as using a flow control valve. When hot water flow is much less than cold I look for scale clogging, such as in a tankless coil.

Any increase in even part of the run of supply piping can make a significant improvement in the fixture flow rate, all else being unchanged.

Eg increase from 1/2" to 3/4" pipe diameter.

On 2018-03-05 by JWall

I think I get it. Let me recap to be sure.
The flows in all your equations and charts are for cold water..

Some of that cold water will be heated, but the fixtures handle the same amount of water whether hot, cold, or a mix.
The hot water flow required is beyond the scope of this exercise, and is complicated and dependent upon equipment selection and operating temperature.

So is there a hot water rule of thumb for typical residential housing using an electric hot water heater and commonly available fixtures?
Does an order of magnitude of 1/3 or 1/2 hot flow sound reasonable for sizing hot water distribution lines in residential housing?

On 2018-03-03 by Anonymous

This article below might help you.

I suspect that the foot valve in your well has been leaking all along but you never lost Prime because the pump would run to reprime itself before things got bad.

When you have lost power of course they pump can't run to keep it self primed.

You might be able to reprime your well I connected via garden hose to a neighbor if someone is close to enough

See REPEATED PUMP PRIME LOSS at

https://inspectapedia.com/water/Repeated_Pump_Prime_Loss.php

On 2018-03-02 by (mod) - determine the flow rate of a commercial flush valve for a toilet

Robert,

You could get close by taking the dimensions of the interior of the toilet tank, and then measuring the change in elevation or depth of water in the tank over the time that the fill valve is operating. Take care it's possible that they flow rate is not uniform throughout all of the valve positions.

On 2018-03-02 by Robert

Is there a way to determine the flow rate of a commercial flush valve for a toilet with out taking valve about and flushing in a calibrated bucket?

On 2018-03-02 by (mod) - So, how do you tally hot water flow?

J

What we are discussing at WATER FLOW RATE CALCULATE or MEASUR is a simple approach to measuring the flow rate at an individual Plumbing fixture.

You could measure that only looking at cold or only looking at hot or looking at both together. Keep in mind that we've tried to make clear that the flow rate at a fixture is not necessarily describing the capacity of the water source Or fixtures in other areas in the building.

It's also important to keep in mind that measuring flow rate of fixtures indoors tells us very little about the water delivery capacity of a private water well.

the tables in this article that give typical flow rates for various Plumbing fixtures assume that you are running only cold water. That's in part because depending on how hot water is being produced we will often see a slower flow rate going through the hot water system.

On 2018-03-01 by JWall

So, how do you tally hot water flow?
Are your charts mainly for sizing the main supply to the building (the cold water)?
If so, would that not include hot and cold since the cold water obviously supplies the water heater.

On 2018-02-28 by (mod) -

Thanks for an excellent question. My answer is no. Those are cold water flows.

On 2018-02-28 by JWall

Are the flows shown in your table "Typical Water Flow Rates in Residential Properties" considered to be a combination of hot and cold water?
I assume so.

If so, what proportion would typically be hot or cold for fixtures that require both? half and half? More hot than cold?

This must be known in order to size the two different main supply lines (hot and cold).

On 2017-12-14 by (mod) - what is 25 litres of water flow per minute in terms of bar measurement

Russell

You ask an understandable question but despite the Volumetric Flow Rate Calculation (Q = A x V or V = Q/A ) given in the article above, since that is a theoretical calculation, yours is not a question for which there is a single "correct" answer.

Flow is measured in litres per minute of something (liquid for example). You'll find plenty of online websites that will convert a liters per minute flow rate into other volumes per minute, for example,
25 LPM = 0.025 m³/min

But you won't find flow rates directly convertible to a system pressure.

Bar is a measure of pressure.

The pressure in a water piping system when NO water is being run is static pressure and might be measured in bar.

But as soon as we open a faucet to permit water to flow, our pressure gauge, presumably mounted somewhere on that system, will drop to a lower bar.

Let's compare two examples in which we calculate water velocity (we cannot calculate water pressure)

Flow rate = 1 lps, Pipe diameter = 2 cm. Water flow rate = 10.44 cubic feet per second

Flow rate = 1 lps, Pipe diameter = 20 cm. Water flow rate = 0.10 cubic feet per second

But both of these systems are delivering water at 1 LPS.

Just how low the bar (pressure) falls is not a constant. Instead the pressure in the piping system will be a function of

- the pressure and flow capacity in liters per minute (LPM) at the building water source - e.g. the municipal supply pressure, pump capacity, or the incoming water pressure regulator setting

- the diameter of the water piping
- the length of the water piping, number of elbows, bends, valves, and other restrictions
- the restriction caused by the internals of the faucet, shower head, or other exit valve through which water is running

There are, however, engineering models that will give us flow rates through a water system for cases where we specify values for those variables.

We CAN however easily convert water in a vertical pipe to psi.
IF we have a vertical pipe that is filled 2.31 feet of water (not moving, just sitting there), that water will exert 1 psi (or 0.069 Bar) of pressure. That's independent of the diameter of the pipe.

See WATER PRESSURE MEASUREMENT https://inspectapedia.com/water/Water_Pressure_Measure.php for details.

On 2017-12-14 1 by Russell

what is 25 litres of water flow per minute in terms of bar measurement

On 2017-12-12 by olivia

me and my friends have a lot of questions

On 2017-12-12 by olivia

what is the fastest speed

On 2017-05-05 by (mod) -

Sathee

That's a tremendous flow rate you're describing, 3000 liters per minute. That's off the curve of most water system pump capacities even before we consider the reduction in pumping capacity that occurs when the pump has to lift water up to high head levels, loss of flow from friction, bends, etc.


Fristan, an Australian pump producer, offers a nice pump sizing guide
http://www.fristam.com/DesktopModules/Bring2mind/DMX/Download.aspx?language=en-US&Command=Core_Download&EntryId=254&PortalId=0&TabId=301

On 2017-05-05 by satheeskumatr

we need to fill our water tanker 18000lits capacity within 6mins suggest which size pipe should fitted in stock tank

On 2017-04-02 by (mod) -

Raja

With the understanding that water pressure is not the same measurement as water flow, you will need to specify the type of fountain of interest: drinking water? Decorative? Size? Pump capacity?

Typically decorative water fountains operate by a recycling pump that is rated by flow rate in gallons or liters per minute. Typically an outdoor fountain pump will have capacity expressed as a maximum flow rate, such as 1058 GPH (4000L/H).

On 2017-04-02 by raja

i need water pressure calculation in water fountains. how to calculate that

On 2017-01-18 by (mod)

M. You can do this. You just need the volume of a cylinder (7m x .5m)

On 2017-01-18 by M aourangzaib - how much water is discharged in an hour?

If the velocity of water is .9m/s and length is 7 meter width is .5m how much is discharged in per hour

On 2016-12-05 by Need some water?

Thank you very much for your reply.
Yes, and it could have been much much worse.

It was 11 o'clock at night. We were told that the workers were about ready to call it a day.

If they had left before the water main burst, it might have been an hour for the water to have been turned off. We would then have suffered from severely bad flooding.

We live right on an old river system that has dried up due to the sewer system. We are very close to being the lowest point in the whole neighbourhood. Luckily the actual bottom of the river is a few feet lower than even our house is so we were spared from the full brunt of all that water.

We were also told by the Water Department that there were three workers in the hole when the main burst and that in the hole there was a 4 foot wide open grate into the storm sewer. It is very unclear how anyone managed to get out alive. It was extraordinarily dangerous.

Sorry I was not clear about that. The 12 inch water main at 120 psi opened up in a 30 foot hole at the end of our driveway. This released a river of water. The Water Department estimated that 100000 GPM would have flowed out of
the burst water main.

I have been trying around with different online calculators and my numbers are lower, though I have no idea what I am doing.

I used a primary pressure of 120 psi, secondary pressure of 0 psi and an orifice of 12 inches that gave 31000 GPM.

www.tlv.com/global/TI/calculator/water-flow-rate-through-orifice.html

For the next formula, I used D=12, P=120, C=1

This gave 47000 GPM.

www.hosemonster.com/filebin/images/pdfs/Calculating_Flow_Rates_and_Coefficients_v2012Catalog.pdf

I just wanted to get in the right ballpark. I will have to carefully think about the assumptions used in the urls that you provided. I found the idea of needing to know the length of the pipe very confusing.

The water main is probably miles lone. What I want to know is what happens when the full water main turns into nozzle and dumps all of its water.

I suggested to the water department that the whole problem with the flooding might have been avoided if they had simply dumped the 50000 GPM down the sewer grate that is right beside the excavation site.

(I suggested that as a precaution they could have drilled a diversion pipe right into the down sewer to protect the workers on site.)

I had been told that there is an eight foot diameter sewer line on our street, so I thought there should be no problem at all in draining an enormous amount into the sewer. It took them a while to make me understand that much of the 50000 GPM was going down this sewer. It was just that there was no possible way that a 3 foot sewer could handle that much water.

The 8 foot main sewer could, though not the 3 foot sewer grate. There is a stream of mud hundreds of feet downstream of our house which demonstrates where some of the water that did not go down our driveway or beside or house went.

The day after the incident they appear to have opened up an even larger temporary direct access to the sewer system to prevent another water release.

The other water problem that we are now coping with is somewhat more manageable. For some reason we have now lost water pressure in our house. It was working fine before they dug a 30 foot excavation hole on our driveway and a fair amount of mist drifted our way after the water main failed. Now no pressure.

The Water Department says our shut off valve is broken and that we must fix it. Does that make sense?

About the only idea that we have is that a rock or some debris has lodged in the valve and is preventing us from having full pressure. The most we were getting was a few minutes of pressure cycled with about an hour of no pressure. Does anyone have any idea what could have caused this?

Thank you for your help.

On 2016-12-04 by (mod) - A 50,000 gpm water flow into a 36" diameter pipe gives this enormous water flow rate: 15.76 feet per second.

What a snafu.

A 50,000 gpm water flow into a 36" diameter pipe gives this enormous water flow rate: 15.76 feet per second.

The article WATER FLOW RATE CALCULATE or MEASURE gives a procedure for guesstimating the flow rate of a pipe of any given diameter. The pressure of course in this case is a function of gravity - the heights involved, as well as bends, elbows, restrictions, and friction losses.

Storm drainage rate capacity calculations are based on

1. the area being drained

2. A coefficient of runoff (water runs off of grass slowly but paved surface quickly, where a concrete roof = 1 and grass - .35)

3. A rainfall rate in inches per hour

4. Slope of a pipe drain in inches per foot

But typically a storm drain calculator assumes no more than 3 inches of rain per hour - that's about 300 gpm for 1000 sq ft area, or about 0.69 cfm - about an 8-inch storm drain would handle that.

A Washington State U drainage calculator works to examine your question.
http://irrigation.wsu.edu/Content/Calculators/General/Pipe-Velocity.php

See this drainage calculation handbook for the state of Florida http://www.fdot.gov/roadway/drainage/files/exfiltrationhb.pdf

See http://www.lawrencevillega.org/userfiles/file/Government/Planning_Zoning/StormWaterDesignManual/Chapter%203%20%20section%20%203.7.pdf
for a detailed calculation.

At https://InspectAPedia.com/water/Well_Static_Head_Definition.php STATIC HEAD, WELL DEFINITION we include an explanation of how to calculate the volume of a cylinder - in this case a 3 foot diameter pipe - storm drain.

On 2016-12-04 by Need some water?

Great site!

A few nights ago we had a knock at our door and a fine gentleman wanted us to be aware that the water main in front of our house had burst and that 50000 GPM was flowing down our driveway towards our house.

Is that roughly what you would expect if a 12 inch pipe at 120 psi were to break? Apparently some of this water was being drained through the sewer system. How much water could a 3 foot wide storm sewer accommodate? The city water workers explained that the work to upgrade our sewers had been contracted out, and that corners were being cut. Made a mess of our garage.

On 2016-05-25 by Baffled Bill - How accurately would a flow test work from that spigot?

So my ego has deflated enough to post this question( Possibly more details later). There is a spigot at the Well head. How accurately would a flow test work from that spigot?

On 2015-09-17 by Derby

Thanks for the reply, danjoefriedman. Since the fullest the barrel can be is only a little over two feet, I guess I would only have about .866 psi. Guess I'll need to use an on-demand pump. I was hoping to avoid that expense and bother. Thanks again for the information.

On 2015-09-17 by (mod) - weight of or pressure exerted by a column of water.

Thanks for the question, Derby.

Over at a companion article on pressure at WATER PRESSURE MEASUREMENT

we do the math on the weight of or pressure exerted by a column of water. It's only the height of the water column that is critical not the diameter.

If your barrell were say five feet tall and completely full of water, you'd have a 5 ft. column of water;

In the article I cite you'll see that The pressure of a column of water is about 0.433 psi per foot of column height

If you measured pressure at the very bottom of the barrel you'd have 2.16 psi of water;

Most water pressure gauges designed to operate in the range of water pumps, tanks, and wells, will not register water pressure at such low levels.

On 2015-09-17 by Derby

I have a barrel that I want to gravity clean waste water to the toilet tanks. The water flow out of the spigot fast enough to fill a five gallon bucket in 4 minutes but when I put a psi gauge on the spigot it does not register anything at all.

The same gauge registers 33 gpm when attached to a 175 foot hose attached to the garden spigot. The barrel spigot is about 3 inches above the bottom of the barrel and I had 50 gallons of water in the barrel. Is this normal?

On 2015-09-16 by Abbey Major

My home is supplied by a well. Recently we started seeing water flow issues - when the sprinkler, clothes washer, or dishwasher was running, fixtures in various parts of the house would have little to no flow.

While investigating the problem to see if the fault is with the system pressure switch, pump, pressure tank, etc, my husband hooked the sprinkler to the hose bib right next to the well.

Here the sprinkler has much increased water flow/pressure (as expected) AND there are no issues with water flow at any of the other fixtures in the house. I do believe we have a LONG run of pipe between the well and house.

I don't understand why running the sprinkler from the house hose bib would give such different flow results at the house fixtures compared to running the sprinkler from the well hose bib. Any thoughts?

On 2015-08-11 by (mod) I need to find out the flow rate for a 2" PVC pipe 300 feet from meter at 45 psi.

Flow

In the article WATER FLOW RATE CALCULATE or MEASURE please see the detailed calculation procedure for flow rates in the section titled CALCULATE WATER FLOW RATES for PIPE DIAMETER, LENGTH, PRESSURE

and let me know if that leaves you with any questions.

On 2015-08-11 by Flow rate

I'm working for a food processing company I need to find out the flow rate for a 2" PVC pipe 300 feet from meter at 45 psi.

On 2015-03-03 by (mod) - What information (other than the specific heat capacity of water) is required to calculate the required water flow rate.

Julian you're missing a lot of pieces - what textbook are you using?

Surely the incoming temperature of the water, the heat transfer rate into the room, are critical.

On 2015-03-02 1 by julian peacegood-

hi. I am doing a college course and 1 of my questions is.

A room has a calculated heat loss of 5kw. What information (other than the specific heat capacity of water) is required to calculate the required water flow rate. I hope you can help as I am totally confused as to what I am need to know in oreder for this to be answered.


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