Quick answer: gallons in a well = gallons per foot × water column height. A 6 in casing holds 1.47 gallons per foot, and a 4 in casing holds 0.65.
How to calculate the water in a well
- Water column height = total well depth − static water level, both in feet.
- Casing inside diameter in inches; divide by 24 to get the radius in feet.
- Cubic feet = π × radius² × water column height.
- Gallons = cubic feet × 7.4805.
Gallons = 0.0408 × D (in)² × water column (ft)
Gallons per foot by casing size
| Inside diameter | Gallons per foot | Gallons per 100 ft |
|---|---|---|
| 2 in | 0.163 | 16.3 |
| 4 in | 0.653 | 65.3 |
| 5 in | 1.020 | 102.0 |
| 6 in | 1.469 | 146.9 |
| 8 in | 2.611 | 261.1 |
| 24 in (dug well) | 23.50 | 2,350 |
| 36 in (dug well) | 52.88 | 5,288 |
Sizes are inside diameters. Steel and PVC casings are often named by nominal size, so check the actual inside diameter if you need a precise figure.
Worked example
A 300 ft well with a 6 in casing and a static water level of 80 ft: water column 300 − 80 = 220 ft. 220 × 1.469 = 323 gallons standing in the casing.
- Storage and pressure tanks: the tank volume calculator covers vertical and horizontal tanks. For the line from the well to the house, use the pipe volume calculator.
Frequently asked questions
How do I calculate how much water is in my well?
Subtract the static water level (the depth from the top down to the water) from the total well depth to get the height of the water column. Then gallons = π × (casing diameter ÷ 2)² × height × 7.48, with the diameter in feet. A 6 in well with 150 ft of water holds about 220 gallons.
How many gallons per foot are in a 6 inch well?
About 1.47 gallons per foot of water column. A 4 in casing holds about 0.65 gallons per foot, and a 2 ft dug well about 23.5 gallons per foot.
What is the water column height?
The depth of the water standing in the well: total well depth minus the static water level. Both are usually on the well log or driller’s report. A water level meter or a weighted tape can measure the static level.
Is the stored water the same as the well’s yield?
No. This calculator gives the water standing in the casing right now. The yield (gallons per minute the well can supply) depends on how fast water flows back in from the aquifer, which a pump or yield test measures.
Why would I need the volume of water in my well?
For disinfecting the well (the chlorine dose depends on the volume), for knowing how much water is stored at the start of a pumping cycle, and for purging a well before taking a water sample.
How much chlorine do I need to shock a well?
The dose depends on the volume and on the chlorine product. Use the volume from this calculator with the instructions from your state or local health department or the product label, which give the exact amount and the safety steps.
Is this well volume calculator free to use online?
Yes. The CalcWall Well Volume Calculator is completely free and works online in any browser, on a phone, tablet or computer. There is nothing to download and no account to create, and the math runs right in your browser, so results appear as you type.