Construction Calculator
Well Volume Calculator
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Work out how much water is standing in a well — in gallons, litres and cubic feet — from the casing diameter and either the water column or the total depth and static water level.
Well Volume Calculator
Water standing in a well casing
Results
Water stored
205.6 gal
27.49 ft³
Per foot
1.469 gal
of water, 6 in casing
Common Casing Sizes — 140.0 ft of Water
| Casing | gal/ft | Gallons | Litres |
|---|---|---|---|
| 2 in | 0.1632 | 22.8 | 86 |
| 4 in | 0.6528 | 91.4 | 346 |
| 5 in | 1.0200 | 142.8 | 541 |
| 6 in ← | 1.4688 | 205.6 | 778 |
| 8 in | 2.6112 | 365.6 | 1,384 |
| 10 in | 4.0800 | 571.2 | 2,162 |
| 12 in | 5.8752 | 822.5 | 3,114 |
Storage scales with diameter squared: an 8 in casing holds four times as much per foot as a 4 in casing.
Each foot of water in a 6 in casing holds 1.4688 US gallons (18.24 L per metre), so the volume is simply that rate times the water column.
Volume grows with the square of the diameter: doubling the casing from 4 in to 8 in quadruples the water per foot. A diameter that is 2% off gives about 4% error, so use the measured inside diameter rather than the nominal casing size.
Constants are exact: a US gallon is defined as 231 cubic inches, so a cubic foot is 1,728 ÷ 231 = 7.480519 gallons, and a cubic foot is 28.316847 litres. The rounded 7.48 and 28.316 are low by less than 0.01%.
This is the water standing in the casing. How much a well can supply depends on how quickly it refills, which a pump or recovery test measures — not on the stored volume alone.
Step-by-Step Calculation
Step 1 — Water column
total depth 200 − static level 60 = 140 ft
Step 2 — Radius in feet
6 in ÷ 2 ÷ 12 = 0.25000 ft
Step 3 — Volume
π × 0.25000² × 140.000 = 27.4889 cu ft
Step 4 — Convert
27.4889 × 7.480519 = 205.63 US gal
27.4889 × 28.316847 = 778.4 L
Result — 205.63 gallons (27.489 cu ft) in the well
The Water Column in a Well
Only the water standing in the casing counts. The water column is the total well depth minus the static water level (the depth to the water surface when the pump is off), and the volume is that column times the casing's cross-section.
A Cylinder of Water
The water in a well is a cylinder: as wide as the inside of the casing, as tall as the water column. So the volume is π × radius² × water column, and a 6 in well with 50 ft of water holds 9.8175 cubic feet — 73.44 US gallons.
The one thing to get right is the water column. Well logs record the total depth and the static water level — how far down the water stands when the pump is off — rather than the column itself. The column is the difference, so a 200 ft well with water at 60 ft holds 140 ft of water: 205.63 gallons in a 6 in casing.
Well Volume Formulas
1. Water Column
2. Volume
3. Gallons and Litres
Gallons per Foot by Casing Size
Water held by each foot (or metre) of column, by inside diameter:
| Casing | Gal per ft | L per m | 100 ft of water |
|---|---|---|---|
| 2 in | 0.1632 | 2.03 | 16.3 gal |
| 4 in | 0.6528 | 8.11 | 65.3 gal |
| 5 in | 1.0200 | 12.67 | 102.0 gal |
| 6 in | 1.4688 | 18.24 | 146.9 gal |
| 8 in | 2.6112 | 32.43 | 261.1 gal |
| 10 in | 4.0800 | 50.67 | 408.0 gal |
| 12 in | 5.8752 | 72.97 | 587.5 gal |
Storage grows with the square of the diameter: an 8 in casing holds exactly four times as much per foot as a 4 in one. It is also why diameter needs care — a 2% error in diameter becomes about a 4% error in volume — so use the measured inside diameter rather than the nominal casing size.
Exact Conversion Constants
The familiar 7.48 gallons per cubic foot is a rounding. A US gallon is defined as exactly 231 cubic inches, so a cubic foot holds 1,728 ÷ 231 = 7.480519 gallons. Likewise a foot is exactly 0.3048 m, so a cubic foot is 28.316847 litres.
The rounded values are low by 0.007% and 0.003% — far below any measurement error in a well — but there is no reason not to use the exact ones. The bigger rounding risk is intermediate: rounding 9.8175 cu ft to 9.82 before converting turns 73.44 gallons into 73.46.
Stored Volume Is Not Supply
The figure here is the water standing in the casing at one moment. It is useful for knowing what a pump can draw before the level falls, and for any task that depends on the amount of water in the well itself. It is not how much water the well can supply.
Supply depends on how quickly the well refills from the aquifer — its recovery rate or yield — which a pump test measures. A deep, wide well holding hundreds of gallons can still be drawn down under heavy use if it recharges slowly, and a modest well can keep up with demand if it recharges quickly.
Example Calculations
Three common casing sizes, showing how much diameter matters:
Example 1 — 6 in Casing, 50 ft of Water
The most common domestic well size.
Radius = 6 ÷ 2 ÷ 12 = 0.25 ft
Volume = π × 0.25² × 50 = 9.8175 cu ft
Gallons = 9.8175 × 7.480519 = 73.44 gal
Litres = 9.8175 × 28.316847 = 278.0 L
That is 1.4688 gal for every foot of water
Example 2 — 8 in Casing, 80 ft of Water
A wider casing holds far more per foot.
Radius = 8 ÷ 24 = 0.3333 ft
Volume = π × 0.3333² × 80 = 27.925 cu ft
Gallons = 208.90 gal (790.8 L)
2.6112 gal per foot — 1.78× the 6 in casing
Example 3 — 4 in Casing, 120 ft of Water
A deep narrow well can still hold little.
Radius = 4 ÷ 24 = 0.1667 ft
Volume = π × 0.1667² × 120 = 10.472 cu ft
Gallons = 78.34 gal (296.5 L)
Over twice the water column of Example 1,
yet only 5 gallons more — 0.6528 gal per foot
Getting Good Figures
Take the total depth from the well log, and measure the static water level with the pump off long enough for the water to recover — a level taken just after pumping will read low and understate the volume. Measure both from the same reference point, usually the top of the casing. Use the inside diameter of the casing, since volume depends on its square and nominal sizes are names rather than measurements. Expect the result to change through the year as the water table rises and falls. And keep in mind that this is the water standing in the well, not what it can supply: that depends on how fast the well recharges, which only a pump test can tell you.
Frequently Asked Questions
- How do you calculate the volume of water in a well?
- Treat the water as a cylinder: π × radius² × water column. Convert the casing diameter to feet first. A 6 in casing has a radius of 0.25 ft, so 50 ft of water is π × 0.25² × 50 = 9.8175 cu ft. Multiply by 7.480519 for US gallons — 73.44 gal — or by 28.316847 for litres.
- How many gallons per foot are in a well?
- It depends only on the casing diameter. Per foot of water: 2 in holds 0.1632 gal, 4 in 0.6528, 5 in 1.0200, 6 in 1.4688, 8 in 2.6112, 10 in 4.0800 and 12 in 5.8752. Multiply by the depth of water in feet for the total.
- How do I find the water depth in my well?
- Subtract the static water level from the total depth. The static level is how far down the water stands when the pump has been off; both figures are usually on the well log and should be measured from the same point, such as the top of the casing. A 200 ft well with water at 60 ft has a 140 ft column.
- Why does a wider well hold so much more water?
- Volume grows with the square of the diameter. Doubling a casing from 4 in to 8 in quadruples the water per foot, from 0.6528 to 2.6112 gallons. That is why a 4 in well with 120 ft of water holds only 78 gallons — barely more than a 6 in well with 50 ft.
- Is 7.48 gallons per cubic foot exact?
- Nearly. A US gallon is defined as exactly 231 cubic inches, so a cubic foot is 1,728 ÷ 231 = 7.480519 gallons. The rounded 7.48 is low by 0.007%, and 28.316 litres per cubic foot is low by 0.003% against the exact 28.316847. This calculator uses the exact values.
- Should I use the nominal or inside diameter of the casing?
- The inside diameter. Because volume depends on diameter squared, a diameter that is 2% off gives about 4% error in the volume. Nominal casing sizes are names rather than measurements, so measure the inside or take it from the casing specification.
- Does well volume tell me how much water my well can supply?
- No. It is the water standing in the casing at one moment. What a well can supply depends on how fast it refills from the aquifer — its recovery or yield — which a pump test measures. A deep well with a large standing volume can still run dry under heavy use if it recharges slowly.
- How many litres are in a well?
- Use the same cylinder, in metres: π × (diameter in m ÷ 2)² × water column in m × 1,000. In a 152.4 mm (6 in) casing, each metre of water holds 18.24 litres; 15.24 m of water is about 278 litres.
- Why does my well volume change through the year?
- Because the static water level rises and falls with the seasons and with demand on the aquifer. The casing does not change, so every foot the water drops removes a fixed amount — 1.47 gallons in a 6 in casing.
- What if my water level is below the total depth I entered?
- Then the figures are swapped or measured from different points. The static water level must be shallower than the total depth for there to be water in the casing. The calculator flags this rather than returning a negative volume.