Well Volume Calculator Gallons: How to Find It

This calculator works out the volume of water in a cylindrical well in gallons, based on the well’s diameter and water depth. You enter the inside diameter of the well casing and the depth of the water (not the total well depth), and you get the volume in US gallons, plus the volume in cubic feet and liters for reference. The calculation uses the standard cylinder volume formula and converts the result to gallons.

Formula

The volume of a cylindrical well is the same as the volume of any cylinder:

V = π × r² × h

Where:

• V = volume in cubic feet (ft³)
• Ï€ ≈ 3.14159
• r = radius of the well in feet (half the diameter)
• h = height of the water column in feet (water depth, not well depth)

To convert cubic feet to US gallons, multiply by 7.48052 (1 ft³ = 7.48052 US gallons). To convert to liters, multiply by 28.3168 (1 ft³ = 28.3168 L).

Worked Example

Using the default values from the calculator: well diameter = 2 ft, water depth = 50 ft.

1. Radius = diameter ÷ 2 = 2 ÷ 2 = 1 ft
2. Radius squared = 1² = 1 ft²
3. Area of the circle = π × 1 = 3.14159 ft²
4. Volume in cubic feet = 3.14159 × 50 = 157.08 ft³
5. Volume in US gallons = 157.08 × 7.48052 = 1175.0 gallons

The calculator shows 1175.0 US gallons, matching the step-by-step result.

When to Use This Calculator

You might need the water volume in a well for several practical reasons:

• Well disinfection: Chlorination (e.g., shock chlorination) requires knowing the water volume to add the correct amount of bleach.
• Water treatment sizing: Choosing a water softener, iron filter, or other treatment system often requires knowing the water volume and flow rate.
• Well recovery testing: Estimating how much water is stored in the well casing helps interpret recovery rates.
• Filling or pumping: If you plan to pump out the well for repairs, you need to know how much water to dispose of.

This calculator only gives the static volume of water in the casing. It does not account for water that flows into the well from the aquifer during pumping.

How to Measure the Inputs

Getting accurate measurements is the key to a correct answer.

• Well diameter: Use the inside diameter of the well casing. For drilled wells, common casing sizes are 4, 5, 6, and 8 inches. If you only know the casing size in inches, convert to feet by dividing by 12 (e.g., 6 in ÷ 12 = 0.5 ft).
• Water depth: This is the distance from the water surface to the bottom of the well. It is not the total well depth. You can measure it with a weighted tape or a water level meter. If you only know the total well depth and the depth to water from the surface, subtract: water depth = total depth – depth to water.

For example, if the total well depth is 120 ft and the water level is 40 ft below the ground surface, the water depth is 80 ft.

Common Mistakes and Misconceptions

A frequent error is using the total well depth instead of the water depth. The well may be 200 ft deep, but if the water level is at 100 ft, only the bottom 100 ft holds water. Using the full depth overestimates the volume by double.

Another mistake is mixing units. If you measure the diameter in inches and the depth in feet, you must convert one to match the other. The calculator here expects both in feet. For example, a 6-inch diameter is 0.5 ft, not 6 ft.

Also, this calculator assumes a perfectly cylindrical well with a constant diameter. In reality, some wells are dug wells with stone or concrete liners that are not perfectly round, and some drilled wells may have varying diameters. The result is an approximation, but for most purposes it is accurate enough.

Finally, remember that the volume is the static water in the casing, not the total water available from the aquifer. When you pump, the water level drops, and the well may produce much more water over time.

Units and Conversions

The calculator outputs in US gallons, cubic feet, and liters. Here are the standard conversions:

• 1 cubic foot = 7.48052 US gallons
• 1 cubic foot = 28.3168 liters
• 1 US gallon = 3.78541 liters
• 1 imperial gallon (UK) = 4.54609 liters

If you need imperial gallons, divide the US gallons by 1.20095. Most well volume calculators and water treatment guides in the US use US gallons.

Limitations of the Formula

The cylinder formula assumes the well is a straight, vertical pipe with a uniform cross-section. This is true for most drilled wells with steel or PVC casing. However, some wells have:

• Open borehole below the casing: The rock may be wider or narrower than the casing, so the actual volume may differ.
• Well screens: Screens allow water to enter, but they don’t change the internal volume significantly.
• Dug wells: These are often larger (3-5 ft diameter) and may have irregular walls made of brick or stone. The formula still works if you use an average diameter, but the result is less precise.

For most residential wells, the error is small enough that the calculation is useful for chlorination and treatment sizing.

Frequently Asked Questions

How do I convert well volume from gallons to liters?

Multiply the number of US gallons by 3.78541 to get liters. For example, 1175 gallons × 3.78541 = 4448 liters. The calculator already shows liters in the third output.

What if my well is not a perfect cylinder?

If your well has a varying diameter, measure the diameter at several points and use the average. For dug wells, measure the diameter at the top, middle, and bottom. The result will be an approximation, but it is usually good enough for practical purposes like chlorination.

Can I use this for a cistern or storage tank?

Yes, the same formula works for any vertical cylindrical tank. Just enter the inside diameter and the water depth. For horizontal tanks or other shapes, the formula is different.

How much bleach do I need to shock chlorinate my well?

After calculating the water volume in gallons, the typical recommendation is to use 1 quart (0.25 gallon) of household bleach (5.25-8.25% sodium hypochlorite) per 100 gallons of water. For 1175 gallons, that would be about 11.75 quarts, or roughly 3 gallons. However, always follow local health department guidelines and the bleach manufacturer’s instructions, as concentrations vary.

Why does the calculator ask for water depth, not total well depth?

Because only the part of the well that contains water contributes to the volume. The dry portion above the water level is empty space. Using total depth would overestimate the volume, which could lead to adding too much chlorine or undersizing treatment equipment.

Is the result in US gallons or imperial gallons?

US gallons. The calculator uses the US gallon (231 cubic inches). If you need imperial gallons, divide the US gallon result by 1.20095.

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in d | Well diameter | ft | 2 | 0
in h | Water depth | ft | 50 | 0
out gal | Volume | US gallons | (pi * (d/2)^2 * h) * 7.48052 | 1
out ft3 | Volume | cubic feet | pi * (d/2)^2 * h | 1
out lit | Volume | liters | (pi * (d/2)^2 * h) * 28.3168 | 1