Groundwater sampling protocols usually require purging three to five well volumes so that the sample represents fresh aquifer water, not stagnant water in the casing. Well volume also tells you how much chlorine is needed for shock chlorination and how much water storage the well provides.
How to use the Well Volume Calculator
- Casing inside diameter
- Total well depth
- Depth to static water
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the inside diameter of the casing, the total well depth and the depth from the top of the casing to the water.
Tips
- Use the figures printed on your product or its manual when you have them.
- Round quantities up so you are never short.
- Measure twice before you buy anything cut to size.
- Everything runs in your browser – try different values to compare options.
Common mistakes to avoid
- Mixing units (inches and centimetres, mAh and Ah).
- Ignoring efficiency losses and safety margins.
- Treating rules of thumb as exact.
Inputs explained
| Field | What to enter |
|---|---|
| Casing inside diameter | Units: in, cm, mm |
| Total well depth | Units: ft, m, in, cm, yd |
| Depth to static water | Units: ft, m, in, cm, yd |
What each result means
- Water column = max([Total well depth] – [Depth to static water], 0)
- Well volume (water) = pi÷4×[Casing inside diameter]^2×[Water column]
- Three well volumes (purge) = 3×[Well volume (water)]
- Water per foot of casing = pi÷4×[Casing inside diameter]^2×0.3048
- Total casing volume = pi÷4×[Casing inside diameter]^2×[Total well depth]
Example values
How the main result changes when Casing inside diameter changes and everything else stays at the example values above:
| Casing inside diameter (in) | Well volume (water) (US gal) |
|---|---|
| 3 | 55.1 |
| 4.5 | 123.9 |
| 6 | 220.3 |
| 7.5 | 344.2 |
| 9 | 495.7 |
| 12 | 881.3 |
Formula
Water column = total depth − depth to static waterWell volume = π ÷ 4 × diameter² × water columnPurge volume = 3 × well volumeWorked example
Inputs — Casing inside diameter: 6 in; Total well depth: 200 ft; Depth to static water: 50 ft.
Results — Water column = 150 ft; Well volume (water) = 220.3 US gal; Three well volumes (purge) = 661 US gal; Water per foot of casing = 1.469 US gal; Total casing volume = 294 US gal.
Frequently asked questions
How much water is in a 6-inch well per foot?
About 1.47 gallons (5.6 L) per foot of water column.
Why purge three well volumes?
So the sample comes from fresh aquifer water rather than water that has stood in the casing.
How do I measure the static water level?
With an electric water-level meter or a weighted tape lowered until it touches water.
What formula does the Well Volume Calculator use?
Water column = total depth − depth to static water.