Hong Life CalculatorsUnderstanding the well volume calculator — concept and principle
This calculator finds how much water is standing in a drilled well from the casing diameter, the total depth and the depth to the water. Use it before shock-chlorinating a well, when estimating how much water is available at once, or when sizing a purge before sampling.
How it is calculated
Water column height h = total well depth − depth to water
Volume V = π × (d ÷ 2)² × h
- d is the inside diameter of the casing: cm in metric mode, inches in US mode.
- Depths are in metres or feet; the result is in litres and US gallons (1 US gal = 3.785411784 L).
- Litres per metre of water = π × (d ÷ 2)² × 1,000 with d in metres; US gallons per foot are shown too.
- Full casing volume uses the total depth instead of the water column.
The principle
A well casing is a vertical pipe, and the water inside it forms a cylinder. A cylinder's volume is its cross-section area times its height. The water does not fill the casing to the top: it stands at the static water level, so only the part below that level counts. Because area grows with the square of the diameter, a 15 cm casing holds four times as much per metre as a 7.5 cm one. A common rule of thumb in US units is 0.0408 × d² gallons per foot with d in inches.
Worked example
Default inputs: metric, casing 15 cm, well depth 60 m, water at 20 m.
- Water column = 60 − 20 = 40 m.
- Radius = 0.075 m; area = π × 0.075² = 0.017671 m², which is 17.67 litres per metre.
- Volume = 17.67 × 40 = 707 litres = 707 ÷ 3.785 = 187 US gallons.
- Per foot: 0.017671 × 0.3048 × 1,000 ÷ 3.785 = 1.423 US gallons.
- Full casing = 17.67 × 60 = 1,060 litres.
| Inside diameter of the casing | Water stored (litres) |
|---|---|
| 3.75 | 44 |
| 7.5 | 177 |
| 11.2 | 398 |
| 15 | 707 |
| 18.8 | 1,104 |
| 22.5 | 1,590 |
| 30 | 2,827 |
How to read the result
The stored volume is what is in the casing now, not how much the well can supply. Recovery depends on the aquifer: some wells refill in minutes, others take hours. For disinfection, chlorine dose instructions are usually given per volume of water in the well, so this number is the starting point. Measure the water level with the pump off and after it has settled.
Common mistakes
- Using the outside diameter of the casing.
- Measuring the water level while the pump is running.
- Treating stored volume as the well's yield.
- Mixing centimetres for diameter with feet for depth.
Frequently asked questions
How many gallons per foot is a 6-inch well?
About 1.47 US gallons per foot (0.0408 × 6²).
How do I measure the depth to water?
With a well sounder or water-level meter lowered from the top of the casing.
Does a deeper well always hold more water?
Only if the water column is taller; a deep well with a low water level may hold little.
Sources: NIST Special Publication 811, Appendix B (1 in, 1 ft, 1 US gallon) · Cylinder volume V = πr²h (standard geometry)