What this calculator does
A well casing is a cylinder, so the volume is the area times the water column depth. A 6 inch casing with 100 feet of water holds 147 US gallons, or about 556 litres.
The gallons-per-foot figure is the one worth remembering, because it makes any depth easy. A 6 inch casing holds 1.469 gallons per foot; a 4 inch holds 0.653 and an 8 inch 2.611. Diameter matters far more than it appears, since volume goes with the square of the radius.
The formula
The volume is pi times the radius squared times the water column depth, converted from cubic inches at 231 cubic inches per US gallon. The figure is the standing water in the casing only, not the yield of the well, which depends on how fast the aquifer recharges and is a completely different quantity.
| Term | Meaning |
|---|---|
| Casing | The pipe lining the well bore, whose internal diameter sets the volume per foot. |
| Water column | The depth of standing water, from the water level down to the well bottom. |
| Static water level | Where the water sits when the pump is off. |
| Yield or recovery rate | How fast the well refills, which is what actually limits supply. |
The inputs explained
| Field | What to enter |
|---|---|
| Casing diameter (in) | Casing internal diameter in inches. Domestic wells are commonly 4 to 8 inches. |
| Water column depth (ft) | Depth of the water column in feet, which is the well depth less the static water level. |
When to use it
Chlorinating a well
Shock chlorination is dosed by volume, so the standing water must be calculated first.
Estimating available water
The casing volume is what can be drawn immediately, before recovery becomes the limit.
Sizing a pump test
Knowing the standing volume helps interpret drawdown during a yield test.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much water in each casing size?
The same water column in a range of casing diameters.
| Casing diameter | Water volume in the casing | In litres | Gallons per foot of casing |
|---|---|---|---|
| 4 in | 65 US gal | 247 L | 0.653 gal/ft |
| 6 in | 147 US gal | 556 L | 1.469 gal/ft |
| 8 in | 261 US gal | 988 L | 2.611 gal/ft |
Questions
Is this the same as the well yield?
No, and the distinction matters. This is the standing water in the casing, which you could draw immediately. Yield is how fast the aquifer refills the well, measured in gallons per minute, and it is what actually limits sustained supply.
How do I find the water column depth?
Subtract the static water level from the total well depth. A 200 foot well with water standing at 100 feet has a 100 foot column. The static level is measured with a water level meter or a weighted tape.
How much chlorine does a well need?
Shock chlorination doses are given per hundred gallons, so the casing volume is the starting point. Follow the specific guidance for your situation, and have the water tested afterwards. Disinfection is one case where getting the dose right genuinely matters.
Does the pump and pipe volume count?
Not materially. The drop pipe and pump displace a little of the casing volume, typically under a percent for a domestic well, which is well inside the uncertainty in the static water level. For practical purposes the casing volume is the figure.
For a tank instead, see the water tank volume calculator. For pipe volumes, see the pipe volume calculator.