How this instrument works
A drilled well's standing water is, geometrically, just a cylinder: a circular cross-section, set by the casing's inside diameter, extended down through the depth of the water column above the well's bottom. This calculator finds that cylinder's volume using the standard formula — pi times the radius squared times the height — then converts the result from cubic feet into US gallons, the unit well owners actually need for tasks like sizing a pump or dosing a chemical treatment.
The conversion from cubic feet to gallons uses a fixed, exact factor: one cubic foot equals 7.48052 US gallons, which follows directly from the legal US definition of the gallon as 231 cubic inches. That factor is why a modest-looking well can hold more water than intuition suggests — a standard 6-inch-diameter well with just 100 feet of standing water column already holds close to 147 gallons, all from a column only half a foot across.
Well volume matters most in practice when a well needs shock chlorination to disinfect it, since the standard bleach dosage is specified per gallon of water in the well — using too little bleach under-treats the well, and too much leaves an unpleasant chlorine taste to flush out afterward. It's also useful for a rough sense of a well's water storage before deciding on pump sizing or troubleshooting a well that seems to run dry faster than expected.
- Enter Well diameter, feet — the casing's inside diameter; a standard 6-inch casing is 0.5 feet, a 12-inch casing is 1 foot.
- Enter Water column depth, feet — the standing water depth, found by subtracting the static water level from the total well depth, both measurable with a weighted tape.
- Read Water volume, US gallons — the cylinder volume converted to gallons, recalculated as either figure changes.
- Convert a diameter given in inches to feet first by dividing by 12, since the calculator expects feet for both fields.
- Use the result alongside a shock chlorination guide's bleach-per-gallon dosage table if you're disinfecting the well.
Worked example — a 6-inch well with a 100 ft water column
Enter 0.5 into Well diameter, feet and 100 into Water column depth, feet — a standard 6-inch-diameter (0.5 ft) drilled well with a 100-foot standing water column. The cylinder's cubic-foot volume is pi x (0.25)^2 x 100, which then gets multiplied by 7.48052 to convert to gallons.
Water volume, US gallons returns 146.879666731, or about 147 gallons — a substantial amount of standing water from a casing only 6 inches across, which is exactly why even a modest-diameter well needs a real gallon-based calculation rather than a guess before dosing it with chlorine or estimating its storage.
Questions
Why does a narrow 6-inch well hold so much water?
Because volume grows with depth in a straight line but the cross-sectional area, while small, still multiplies against every one of those feet of depth — a 100-foot column of water, even through a 6-inch casing, adds up to close to 147 gallons. Depth matters more than diameter for total volume in a typical drilled well, since diameter only affects the cross-sectional area while depth is a much larger number to begin with.
How do I measure my well's water column depth?
Subtract the static water level from the total well depth. Total well depth is usually on the well completion report or driller's log; static water level — how far down the water surface sits when the well isn't being pumped — is measured by lowering a weighted tape or an electronic water-level meter down the casing until it contacts water.
Why does this calculator need diameter and depth in feet specifically?
Because the formula computes volume in cubic feet before converting to gallons, and mixing units — entering a diameter in inches without converting — would multiply the error into a badly wrong gallon figure. Divide an inch measurement by 12 to get feet before entering it; a 6-inch casing becomes 0.5 feet, an 8-inch casing becomes about 0.667 feet.
How is well volume used when shock chlorinating a well?
Shock chlorination guides specify bleach dosage per gallon of standing water in the well, commonly a few pints of unscented household bleach per 100 gallons, plus extra for the household plumbing. Knowing the well's actual volume in gallons, rather than guessing, is what keeps the dose strong enough to disinfect the well without leaving it so over-chlorinated that the taste takes days to flush out.
Does this calculator account for the well casing's wall thickness?
No — it uses whatever diameter you enter as the water-holding diameter, so entering the casing's outside diameter instead of its inside diameter will slightly overstate the volume. Use the inside diameter of the casing, which is usually the figure quoted on a well completion report or printed on the casing material itself, for the most accurate result.