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

  • WATER FLOW RATE CALCULATE or MEASURE - CONTENTS: measurements &calculations of water flow rate in gpm or lpm at faucets & fixtures tells us something about the condition of building pipes, fixtures, & water pressure or flow. Table of plumbing fixture water flow rates.
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    Measure water flow rate: how to measure water quantity delivered per minute at building plumbing systems.

    How to measure the water flow rate in gallons or liters per minute at building faucets & fixtures. Plumbing fixture flow rate data. Water flow rate vs pipe diameter & pressure. This article describes procedures for measuring the flow rate in gallons per minute or liters per minute at a building faucet or plumbing fixture. We explain what fixture flow rate means and we warn that measuring water flow in or at a building may give quite misleading data about the condition of the building water supply whether it's from a private well or from a municipal water main.

    Our page top photo shows typical flow rate at a kitchen faucet - around 3 gpm.

    Green links show where you are. © Copyright 2017, All Rights Reserved.

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

    Water pressure and flow measurement (C) Daniel FriedmanArticle Contents

    Empirical Flow Rate Measurement Using a Bucket & Stopwatch

    Our photo (left) shows water running into a five-gallon plastic bucket. If this is the only fixture running water in the building we can time the number of seconds or minutes needed to fill the bucket.

    For example, if the time required to fill the five gallon bucket is one minute, then the water flow rate at this plumbing fixture is 5-gallons per minute or 5 gpm.

    [Click to enlarge any image]

    One can purchase "flow meters" that connect to various plumbing fixtures to pretend to make this measurement, but remember that we are measuring the flow rate at the particular fixture - to obtain a number that does not necessarily describe the water flow rate capability of the water supply system.

    Watch out: measuring "flow rate" at any faucet or fixture served by a well pump system will be inaccurate and will reflect pump capacity, piping restrictions, fixture restrictions, and even actual well flow rate variations where pump protection tailpieces or similar devices are installed. Measuring flow rate at a fixture does not measure the well's true flue rate.

    The measurement of water flow rate at a particular plumbing fixture does not accurately measure the true water flow rate of the plumbing system because:

    On a pump and well system when we turned on water at just the kitchen sink (DYNAMIC WATER PRESSURE) the flow rate dropped slowly until the pump turned on. Then the water pressure rose slowly until the pump turned off. Water pressure varied between 38 psi (pump off) and 25 psi (pump on).

    When we turned on water at a bath tub faucet (photo just above) water pressure dropped to about 28 psi and stayed there as the well pump ran continuously, delivering water to the building at that rate. Here is a photo of our PRESSURE GAUGE reading 28 psi [image]

    Also see WATER PRESSURE REDUCER / REGULATOR for a discussion of how we reduce building water pressure to a safe level and how we assure uniform building water pressure and flow using a pressure reducing valve or pressure regulator.

    Readers whose building is served by a private pump and well system should

    Typical Building Water Flow Rates at Various Fixtures or Test Points

    If we actually measure the flow rate at various building fixtures and faucets we will see water flow, measured in gallons per minute or liters per minute in these ranges:

    Typical Water Flow Rates in Residential Properties

    Plumbing fixture or measurement location Water Flow Rate in GPM / LPM Comments / notes
    Bath tub faucet with no flow restrictor installed 3-5 gpm [1]
    Bathroom sink faucet 1-3 gpm [1]
    Kitchen sink faucet 2-4- gpm [1]
    Outdoor hose bib 3-6 gpm [1]
    Shower head, no flow restrictor 2-6 gpm [1]
    Shower head with flow restriction device installed 1.25 - 3.5 gpm [1], average 2 gpm.
    Shower heads prior to 1980 5 gpm (19 lpm)  
    Shower heads, flow restrictors ca 1985 3.5 gpm  
    Shower heads, flow restrictors, ca 1989 3.0 gpm  
    Shower heads, flow restrictors, 1992 2.5 gpm (9.5 lpm) US National Energy Policy Act 1992, requirement waived in 2010
    Shower head with high flow restriction device 1.8 gpm  
    Water pressure tank drain 3-6 gpm [1]

    [1] Typical field measurements by home inspectors

    Typical incoming water pressure at residential properties ranges between 20 psi and 70 psi. At properties served by a private pump and well system the actual flow rate will vary continuously between the pump's cut-in rate and cut-out rate. All flow rates are also affected by pipe diameter, length, restrictions, and other factors including water turbulence and building height.

    Aerating shower heads restrict water flow rate by adding air mixed in with flowing water to increase perceived water volume.

    Atomizing shower heads restrict water flow rate by water turbulence to create very fine high velocity water droplets.

    Shower head flow restrictors in simplest form consist of a disc insert with a small center hole to meter water flow.

    Some non-restrictive and non-compliant shower heads may permit water flow rates as much as 10 gpm (38 lpm).

    Calculate the Flow Rate for a Given Pipe Diameter, Pressure, & Length

    Reader Question: what is the flow rate of water out of a 6-inch diameter line at 90 psi static pressure?

    Chart relating prssure and flow inside of water pipes - InspectApedia & U Florida(Mar 15, 2013) Jim Mault said:

    Hi, we have a 6" water line with a static pressure of 90 psi. We want to put in a 6" stand pipe for truck filling at 1500gpm & 20psi. will that work?

    Reply: water & water pipe hydraulics give flow rate in feet per second, gallons per minute, etc.

    Whew!. OK so it depends ... that "static" pressure of 90 psi is not going to give us a true answer to the flow rate in gpm you'll be seeing, since we don't know the effects of line length, friction losses, and number of bends, valves, or obstructions in your water line.

    [Click to enlarge any image]

    Really this is a hydraulics question. The true figures for water movement through piping are interesting and complex. For example, the velocity of water moving through piping (measured in feet per second or fps) is not uniform across the diameter of the pipe.

    Water moves fastest in the middle and slows down at the edges of the cylinder or pipe walls - which makes sense if we think the walls impart friction losses on the water. OK so we'll skip that. Here are some basics:

    The cool chart at left relating water flow rate in GPM to pressure in psi makes some assumptions stated in the table's note.

    This data is from engineering work prepared by the University of Florida, Indian River Research & Eductation Facility. - Dr. Brian Boman, "Chapter 21, Hydraulics", retrieved 3/3/2014, original source: Water%20and%20FL%20Citrus/21%20Chap21.pdf

    The second chart shown at left relates pressure loss to length for 100 feet of hose of various diameters up to greater than the 6-inch you asked about. This data is from Dultmeier Sales also cited at REFERENCES. Click to enlarge for a more readable copy and you'll see that a six-inch diameter "pipe" (we don't know the material of your pipe) at a flow rate of 1500 gpm has a pressure loss of 4 psi per 100 feet.

    From there and considering I've got no other data about your system, you are on your own to calculate the approximate flow rate loss for your system and thus the net flow rate.

    Table of pressure loss through hose or pipe of varying diameters - Dultmeier - InspectApediaYou could ask Dultmeier or Dr. Boman for an accurate detail but I'm reluctant to bother an educator with these individual questions. Extrapolate from the table or try some basic ballpark calculations, or just give your local hydraulics engineer a call. Boman lays out the data and basic equation as follows:

    Volumetric Flow Rate Calculation

    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 


    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.

    Add Fluid Density to Flow Rate Calculations to Get Kilograms per Second Flow Rates

    If we need to add consideration of fluid density on flow rates, Omega (cited below) explains:

    W = rho x Q


    The flow rate will be 1 kilogram per second when 1 cubic meter per second of a fluid with a density of 1 kilogram per cubic meter is flowing.

    As we've reported elsewhere under improving water flow in buildings or "perceived water pressure", doubling the pipe diameter increases the liquid carrying capacity of the pipe by a factor of 4.

    FYI, friction losses for a 100 ft. length of 6" diameter PVC pipe are figured at 0.30 psi / 100 ft. of pipe. The friction loss data is different for different pipe materials, and is IMO always wrong for in-use systems where contaminants or usage have changed the surface properties of the piping to increase (roughing or wear or mineral deposits) or decrease (algae) friction losses.

    Pressure Versus Flow is described eloquently by Boman. As I've struggled for years to explain this and to help people understand that what they call "water pressure" experienced at the kitchen sink or bath shower is better understood as flow rate, I'm quoting him here:

    As water moves through any pipe, pressure is lost because of turbulence created by the moving water. The amount of pressure lost in a horizontal pipe is related to the velocity of the water, the inside diameter of the pipe, and the length of pipe through which the water flows. When velocity increases, the pressure loss increases.

    For example, in a 1-inch Sch 40 PVC pipe with an 8-gpm flow rate, the velocity will be 2.97 fps with a pressure loss of 1.59 psi per 100 ft.

    When the flow rate is increased to 18 gpm, the velocity will be 6.67 fps, and the pressure loss will increase to 7.12 psi per 100 ft of pipe.

    Increasing the pressure in the system increases the flow rate. In Fig. 21-3, the flow rate in a 2-inch pipe increases by 100 gpm when the pressure is in- creased from 20 psi to 50 psi. Using a smaller pipe size does not increase the flow.

    Note that the smaller pipe sizes have considerably less flow at any given pressure. Since decreasing the pipe size does not increase the pressure at the source, the result of decreased size is reduced flow.

    Using a smaller pipe size does not increase pressure. In contrast, it will result in lower pressure because there will be greater pressure loss in the lines. In Fig. 21-3, a flow of 20 gpm would require about 9 psi pressure in a 1-inch pipe. In order to maintain a 20-gpm flow in a 1/2-inch pipe, over 50 psi would be required at the source. Smaller pipes result in greater pressure loss, not higher pressure. - Op. Cit.

    Sources of Variation in Building or Fixture Water Flow Rate Measurements

    Measurements of actual flow rate at a fixture (explained at MEASURE ACTUAL FLOW RATE) will not, without additional measurements, reflect variations due to changes in the source pressure such as the effects of a well pump and pressure tank or temporal variations in the water pressure delivered from a municipal source or variations in the behaviour of a pressure regulator.

    If it takes 5 minutes to fill a 5 gallon joint compound bucket (that's what we used to use) at the bath tub, then we're seeing a water flow rate of one gallon per minute.

    As we calculated at SUMP PUMPS, using an 18-inch diameter joint compound bucket (or sump pit of that size), one inch in the bucket equals 1.1 gallons of water. Therefore,

    If our 5-gallon 18" diameter bucket is filled up 10 inches in one minute of running water, that gave us about 10 gallons (actually 11 gallons) of water "per minute" at that plumbing fixture.

    What's wrong with measuring well flow rate in the building at the bath tub using a bucket and a stopwatch?

    The water flow rate you observe at an individual plumbing fixture is affected by:

    Watch out: This bucket in the house well flow test assumes that the water flow rate at the plumbing fixture somehow reflects the water in-flow into the well too. This is a big mistake. Those figures could be totally different.

    For example we could have a well with a horrible in-flow rate of .5 gpm but a huge water reservoir tank or a huge static head in the well itself.

    The well or water flow rate we can measure at or inside of a building reflects the conditions of the pump, pressure tank, building piping, valves, and fixture, not the actual well flow rate of water from the ground into the well.

    Nevertheless, an at-building water "flow test" is a practical or functional test that might tell you something important. At countless building inspections we ran water in the building, pulling perhaps 200 gallons or so out during a septic loading and dye test.

    And at countless inspections we ran out of water, discovering that the well and water supply system had a poor well flow rate combined with a small static head in the wall and a small in-building water tank. But when you see a poor water flow in the building, you have found a problem but you have not diagnosed the problem.

    Omega, a provider of accurate flow metering equipment adds these technical details about measuring water flow:

    The fluid and its given and its pressure, temperature, allowable pressure drop, density (or specific gravity), conductivity, viscosity (Newtonian or not?) and vapor pressure at maximum operating temperature are listed, together with an indication of how these properties might vary or interact. ... Expected minimum and maximum pressure and temperature values should be given in addition to the normal operating values when selecting flowmeters.... Concerning the piping and the area where the flowmeters are to be located, consider:

    For the piping, its direction (avoid downward flow in liquid applications), size, material, schedule, flange-pressure rating, accessibility, up or downstream turns, valves, regulators, and available straight-pipe run lengths. The specifying engineer must know if vibration or magnetic fields are present or possible in the area, if electric or pneumatic power is available, if the area is classified for explosion hazards, or if there are other special requirements such as compliance with sanitary or clean-in-place (CIP) regulations.
    - Omega, retrieved 11 Aug 2015, origonal source:

    How to Control Water Flow Rates & Usage

    Reader Question: How to Both Measure & Restrict Water Usage

    7/25/14 said:

    We are looking some sort of 'Auto valve/Flow meter' which will allow us to restrict the supply of water to each residential flat to a specified limit e.g. 200-300 liters per day. After supply of specified limit of water, valve should automatically close & stop the further flow of water.

    Reply: control water usage by metering, timed valves, or sub-meters charging for actual water use


    It is easy to measure the volume of water delivered to a specific residential flat provided that each flat receives all of its water from a single delivery pipe: install a water meter at each of those points. If however building hot water is provided from a central source though a separate piping network you may need two water meters to measure both hot and cold water use.

    It is also easy to restrict water flow rate through piping using controls, valves, and low-flow plumbing fixtures.

    But to restrict water usage to a specific quantity in cubic feet, gallons or liters is another matter. You'd need to combine a water metering device with an automatic shutoff valve. Or for a less costly approach you can install timers to control water supply valves, opening and closing the valves only during certain intervals - something that few of your residents will appreciate.

    In our opinion best, is to control water usage by metering and charging individual water users by installing sub-meters. This approach encourages voluntary water conservation.

    I suggest that before going to that considerable expense you invest in

    Sources of Water Flow Meters

    Take a look at the Ista line of process and flow controllers as well as those made by other manufacturers listed below. Some of these flow meter producers such as Omega include flow control devices. See


    The list of brands of flow metering equipment is long as you can see. Here are more flow metering manufacturers you can consult:

    Aalborg, Alicat Scientific, AquaMetrix, AW Gear Meters, Badger Meter, Blancett, Dwyer Instruments, Dynasonics, Flocat, Flo-tech, Flowline, Fox Thermal Instruments, Fuji Electric, GE Panametrics, GE Rheonik, Gems Sensors & Controls, General Tools, Georg Fischer / GF Signet, GPI, Greyline Instruments, Hedland, KEP, King Instrument, Kobold, Krohne, Lake Monitors, Mace, Macnaught, Rosemount, Seametrics, Sondar, Teksco, Universal Flow Monitors, W.E. Anderson, Yokogawa

    Watch out: Measurements like the

    are all useful, but taken by themselves some of these numbers can give a false reading about the basic question of how much water is in the well?

    Even calculations of theoretical flow rate through a pipe of given diameter, length, pressure, and bends (explained at CALCULATE WATER FLOW RATES for PIPE DIAMETER, LENGTH, PRESSURE) will not give the actual flow rate that will be experienced in most real-world situations where other details that affect flow rate have not been considered such as the effects of height, flow restricting fixtures, variations in the actual water source pressure, unidentified restrictions in piping such as mineral deposits or rust, etc.

    Before assuming that a water pressure, flow, or quantity problem is due to the well itself, see WATER PUMP REPAIR GUIDE for an example of diagnosis of the cause of loss of water pressure, loss of water supply, and analyzes the actual repair cost.


    Continue reading at WATER PRESSURE VARIATION CAUSES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

    Or see WATER FLOW RATE FAQs - questions & answers on how to measure or calculate the flow rate of water from a well or through a pipe

    Or see WATER CONSERVATION MEASURES - how to conserve or reduce water usage and water wastage

    Or see WATER PRESSURE MEASUREMENT - what is the dynamic or static water pressure in a water supply system of faucets, fixtures, pipes, tanks, pumps, meters, pressure regulators?

    Or see WELL FLOW RATE - how much water can we actually get out of a well, at what rate, for how long?

    Suggested citation for this web page

    WATER FLOW RATE CALCULATE or MEASURE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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