Old wells, lots of things to fix (C) Daniel FriedmanWater Well Casing Size
Diameter choices for well bores

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Water well casing diameters:

This article explains the reasons that a well driller might recommend installing a larger diameter well bore or well casing.

We include simple calculations showing the change in the volume of water in the well's static head with larger diameters.

A photo of the above-ground portion of a typical residential 6-inch diameter steel well casing is shown in the photo at the top of this page.

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Well Casing Size or Well Casing Diameter Choices

Pitless adapter sketch (C) InspectApediaA well casing is a steel pipe that is used to seal and support the sides of a drilled water well. The diameter of a residential water well casing is 4", 5", 6" or less-often, 8" in circumference. Much larger steel casings are used in high capacity community water wells, commercial or agricultural wells, and in some other water wells.

While soil and well designs vary as ground, soil, rock and water availability also vary in different areas, generally, after the well has been drilled the casing is driven into the drilled opening in the earth to a dept that (usually) inserts the bottom of the casing in bedrock. The drilled well depth continues downwards into bedrock, through which water flows into the well.

[Click to enlarge any image]

The well casing is shown in cross section, represented by the two vertical red lines in our drawing at left. But of course actually the well casing is a round steel pipe. The water piping that picks up water from the well near its bottom is routed up the center of the well casing until it reaches that exit opening in the casing side.

There the piping (and the water it carries) exits the well casing through a pitless adapter (a fitting that seals the hole in the casing), continuing on to bring water to the building the well is serving.

Question: Would an eight inch well as opposed to a six inch well deliver more water pressure?

Di said:

Would an eight inch well as opposed to a six inch well deliver more water pressure? Are plastic or metal pipes better for a residential well?

This question was posted originally at WELL LIFE EXPECTANCY

Reply: no. but you might want a larger diameter well casing for other reasons we'll give here


THANK YOU for asking a great question: why would we want to drill a larger-diameter well bore or put in an 8" casing rather than a 6" well casing?

Summary: Larger casing diameter = more static head reserve

In short, a larger diameter well casing will not quite give you more water pressure, but it might give you more water while saving your pump.

Summary: Using plastic well pipe casings, benefits, risks

Some sources, no surprise the manufacturers of plastic or PVC well casings describe the advantages of PVC rather than the traditional steel well casing materials. In my amateur OPINION, the title of one of the best documents I found on this also hints at where there could be an achilles heel in plastic well casings: the risk of breakage or hydraulic collapse from the pressure of surrounding earth, a problem that will be more-likely to occur if the installer chooses the wrong material.

Improper installation of PVC well casing can also lead to a later collapse. Certainteed (2013) points out that the heat generated by curing portland cement used as well bore grout around the plastic pipe can generate heat that causes the plastic to become more prone to breakage. The company recommends using bentonite clay instead of portland cement for well bore grout around PVC casings.

Resources on Using Plastic or PVC Well Casings

At PLASTIC PIPE LEAK CAUSES we discuss common causes of leaks in plastic piping.

Comparison of 6-Inch vs 8-Inch Water Well Casings

Now I'll carry on with the reasons to go to a larger diameter well casing. Or not.

Definitions of Water Quantity, Water Pressure, Water Flow

I admit that normal people refer to the force of water exiting their shower head or sink faucet as "water pressure" and it's true that at higher system pressure more water will be delivered through a pipe. But a lager diameter water pipe system could also give more gallons of water flow at the same fixture at the same actual water pressure!

Provided we are not changing the water piping size in the building itself, the well or water pump type, location, horsepower and other features determine both the water pressure and water flow rate - as it is taking water out of the well and sending it to the building.

If you want more details about how we measure the actual water flow rate in a building or through a pipe, see WATER FLOW RATE CALCULATE or MEASURE

Limitations on Building Water Supply Flow or "Pressure" from a Pump

The pump's flow rate in gallons per minute (GPM) to the building is limited by

  1. By the capacity of the pump itself, its design and horsepower. We describe the range of water pump capacities at WATER PUMP CAPACITIES TYPES RATES GPM
  2. By the lift height - how high the pump has to lift the water to get it to you since lifting higher reduces the flow's output GPM. Lift height is also discussed in the pump capacities article above.
  3. By restrictions in the water piping system: pipe diameter, lengths, number of bends or elbows, even to a degree by the actual material of the pipe and its internal friction
  4. By the well's in-flow rate: this is the upper limit of the water system once any reserve of water in pipes or casing or tank has been exhausted: the rate at which water flows INTO the well while you're taking water out.

    Details are at WELL FLOW RATE
  5. By the presence of flow-limiting devices like a water regulator or a tailpiece in the well or something else designed to protect the pump from running dry

    See WELL PIPING TAIL PIECE or take a look at WATER PUMP PROTECTION SWITCH - for examples of devices to protect the pump if a well runs dry

What If the Pump Exceeds the Well Flow Rate?

A pump that has a GPM rate (GPM-PF) that's taking water out of the well that is above than the rate at which water from the earth flows into the well (GPM-WF) can deliver water to the building at GPM-PF only long as it is
drawing water out of the well bore itself - the static head or volume of water that is in the well bore when the well is at rest and fully recovered.

But if GPM-PF is higher than GPM-WF, once the pump has drawn water down to where the pump's intake opening is in the well, one of two things happens:

1. The pump starts to suck air, or runs dry, or burns up -


2. A pump limiting device built into the water system right at the pump or at other controls that may be elsewhere will actually begin to operate to slow the pump's output to protect it from running dry and being ruined.

Benefits of a Larger Diameter Well Casing or Well Bore

If for the same depth we drilled for and installed a larger diameter well bore and well casing, what do we get?

The larger diameter increases the volume of water in the well bore or the well's static head.

Static head is explained in detail at STATIC HEAD, WELL DEFINITION

There I note that when the well is fully-recovered, that is, water has risen in the well as high as ground water will push it when no water is being taken out of the bore, then the volume of water in the well is called it's static head.

I found my numbers on the volume of water you'll get in a foot or meter of well casing for different casing diameters like a 6-inch or 8-inch casing by using our on-page search box of to look for “volume of water in a well casing

Among other well casing diameters given in that article in this TABLE OF WELL WATER VOLUME / QUANTITY you'll see that

If you want to see how to calculate the volume for a well of any diameter and depth, that same article includes the formula at STATIC HEAD VOLUME CALCULATION

How Much More Water is in a Two-Inch Larger Diameter Well Casing?

That means that your 8-inch well casing gives you almost double the volume of water in the static head (actually 1.86 times as much as a 6 inch casing if my math is right).

So you will NOT get more PRESSURE from your fatter water well casing but your well will have a larger static head.

When Do We Want a Larger Diameter WelL Casing?

Whether or not that makes the slightest difference depends entirely on the total well depth and even more-so on the actual height of water in the well at rest - the true static head.

If your particular well were just twenty feet deep (chosen to make the math easy) and if the static head were ten feet, then you'd have a static head of (2.8 x 10) = 28 gallons instead of (1.5 x 10) 15 gallons.

Now suppose somebody actually installed that not-very-deep well.

Suppose the installer also put in a pump that has a capacity of GPM-PF just 5 gallons per minute GPM of output. And let’s pretend that the lift height nd piping and other restrictions were zero.

Then the pump in the 6-inch well could run for just 3 minutes (15 gallons / 5 GPM-WP) before it would depend entirely on the flow rate INTO the well - not so nice, right,

But suppose the well has a well inflow rate GPM-WF that is 10 gallons a minute.

A pump with a GPM-PF of 5 is sucking water out at 5 gallons a minute but the well admits water in to its static head at 10 GPM-WF - the well is always ahead and the pump can keep sending water to you.

Hot dog!

For that case, an 8-inch well casing and the 28 gallons didn’t buy you anything because the well flow rate was already greater than the pumping rate.


What if the well flow rate is terrible, say 1 gallon per minute.

In our well with a 10 foot static head holding 15 gallons, during the 3 minutes we are sucking water out at 5 gpm and getting water in at 1 gpm so over 3 minutes we took in 3 gallons and sent out 5.

We have 2 gallons spare - that gets sent out in the next minute and the pump quickly falls behind the well's flow-rate.

The pump either runs dry and burns up or its output must be limited by a controller to that of the well: 1 gallon a minute - and that’s all you’ll ever see in the house after the first 3 minutes (approximately)

Big Well Casing to the Rescue

So what does the well driller do about this mess?

She drills a deeper well AND / OR she uses a larger well casing (perhaps both) so that she gives you a well with a huge static head, maybe a 100 foot static head holding 150 gallons of water, figuring that you’ll usually turn off the shower well before you’ve sucked out all of that 150.

THAT’s why a driller might want a bigger diameter well casing - you might get a bigger static head without having to drill deeper in nasty ground.

Well flow rate is explained in detail at a live link we give below WELL FLOW RATE

The total water quantity that can be gotten out of a well is explained at a live link given below for WELL QUANTITY TOTAL


Continue reading at STATIC HEAD, WELL DEFINITION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see WELL CAPS & COVERS for well casing cap sealing and venting requirements

Or see WATER PUMPS, TANKS, DIAGNOSTICS for explanation of well pumps and water storage tanks or pressure tanks and their controls.

Or see these well water flow and quantity articles

Suggested citation for this web page

WELL CASING DIAMETER CHOICE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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