💧 Well Volume Calculator

How Many Gallons Are in Your Well?

Enter the casing diameter in inches, total well depth, and the static water level (how far down the water sits when the pump is idle). The calculator returns the standing gallons in the casing. A 6-inch casing, 200 feet deep, water at 100 feet: 100 feet of water column holding 146.9 gallons, about 1.47 gallons per foot.

The Formula

A well casing is a cylinder: cross-section area times the water column gives cubic feet, and each cubic foot holds 7.48052 gallons. Only the submerged part counts, which is why the static level matters more than the total depth. A 4-inch casing holds 0.65 gallons per foot; a 6-inch holds 1.47; an 8-inch holds 2.61.

Worked Example

Six-inch casing, 200-foot well, static level at 100 feet: radius 3 inches = 0.25 feet. Area = pi x 0.0625 = 0.196 square feet. Times 100 feet = 19.6 cubic feet. Times 7.48052 = 146.9 gallons standing in the casing. That is the reservoir your pump draws from before the aquifer must refill it.

Why This Number Matters

Chlorinating a well (shock chlorination) doses by total water volume: too little fails, too much wastes chemical and time. Pump sizing uses it too: the stored gallons are the buffer between pump cycles. And when the well runs dry, this number explains the recovery wait: the aquifer refills the casing at its own rate, often just gallons per minute.

Measuring Static Water Level

Drop a weighted tape or an electronic sounder down the vent until it hits water, with the pump off for several hours first. Measure from ground level. Levels move seasonally: measure in the dry season for the conservative figure, since that is when the water column is shortest and the math matters most.

Stored Gallons vs Well Yield

Do not confuse the two: stored gallons are the static reserve; yield (gallons per minute) is what the aquifer delivers continuously. A well with 150 stored gallons and 5 gpm yield performs far better than one with 400 stored gallons and 0.5 gpm yield. The calculator gives the reserve; a pump test gives the yield; you need both.

When to Call a Professional

Sudden drops in the static level, sediment in the water, or the pump cycling rapidly all warrant a well contractor, not a calculator. Wells are regulated in most states: chlorination and repairs must meet local code. Use this page for planning and understanding; use a licensed pro for the work.

Keeping Records

Log your static level readings with dates: a slow decline over years signals aquifer trends worth discussing with neighbors and your contractor, while a sudden drop means something changed now. Pair the readings with the stored-gallon figures from this calculator and you will have the complete picture of your well's health in one notebook.

A Second Worked Example: The Recovery Wait

A 6-inch casing, 200 feet deep, static level at 100 feet: 146.9 gallons standing, as the worked example shows. The well pump draws 10 gallons a minute, and the aquifer refills at 3 gallons a minute. Run the pump 20 minutes: 200 gallons demanded, but only 146.9 stored plus 60 refilled, so the well runs dry around minute 15. Recovery: the deficit refills at 3 gallons a minute, so roughly an hour of rest restores the buffer. This is why low-yield wells need storage tanks or timed pump cycles.

Stored gallons are the buffer, not the supply. The aquifer's yield decides the sustainable rate; the casing decides how long you can exceed it. Know both numbers before sizing a pump or planning irrigation.

Static Level vs. Pumping Level

The static level is where water sits when the pump is idle. Start pumping and the level drops to the pumping level; the difference is the drawdown. A 20-foot drawdown at your pumping rate is normal; a 100-foot drawdown means the aquifer struggles to keep up. Measuring both levels tells you the well's real capacity in a way the static level alone cannot.

Levels also move seasonally. Dry seasons and heavy nearby pumping lower the static level; wet seasons raise it. A well that performs fine in spring can run short in late summer, so measure in the tough season for honest numbers. Keep a log: declining levels over years signal aquifer depletion worth investigating.

Common Well-Volume Mistakes

First: using total depth instead of the water column. Only the submerged part holds water; a 200-foot well with water at 150 feet holds 50 feet of water, not 200. Second: diameter versus radius. The formula squares the radius; entering the diameter quadruples the answer. Halve first, always.

Third: confusing stored gallons with yield. The casing buffer empties in minutes at pump rates; the aquifer's gallons-per-minute is the sustainable supply. Fourth: the seasonal blind spot. Measuring the static level once in a wet spring overstates the dry-season reality; measure when it matters. Fifth: skipping the pro. Well work involves electricity, deep holes, and water chemistry; the calculator does the math, but drilling, pump service, and contamination testing belong to licensed professionals.

The Recovery-Rate Reality Check

Stored gallons are only half the story; the other half is how fast the well refills. A well yielding 5 gallons per minute recharges 300 gallons an hour, so a 400-gallon stored volume supports heavy use with recovery to spare. A 1-gallon-per-minute well recharges just 60 gallons an hour, which means a long shower can outrun it no matter how much is stored. If your pump ever sucks air, the fix is not a bigger pressure tank but managing draw against recovery: stagger laundry and showers, and consider a larger storage or cistern system. Volume is the bank account; recovery rate is the paycheck.

Why Well Depth Is Not Water Depth

A "300-foot well" does not hold 300 feet of water. Total depth is where the drilling stopped; the water column is total depth minus the static water level. A 300-foot well with water at 100 feet holds 200 feet of water, and that is the number the volume formula needs. Confusing the two overstates storage by 50 percent in this example, which is why the calculator asks for both numbers separately.