Calculate the Volume of a Cylinder in Gallons

How the calculation works

1. Measure across the circular end — the diameter or radius.
2. If you measured the diameter, halve it to get the radius r.
3. Square the radius and multiply by π to get the base area.
4. Multiply the base area by the height h: V = π r² h.
5. Convert the result into your preferred unit above.

Getting an accurate measurement

Measure the inside diameter of a tank, not the outside — the wall does not hold liquid. Keep every measurement in the same system, and let the tool convert between units for you.

The formula behind it

The volume of a cylinder is:

A worked example

Worked example. Take a cylinder with a diameter of 18 in (so a radius of 9 in) and a height of 24 in. The base area is π × 9² = 254.47 in², and multiplying by the height gives a volume of about 26.4 US gallons (6,107 in³ · 100.1 litres).

Conversions use standard factors: 1 US gallon = 231 in³, 1 Imperial gallon = 4.54609 L, 1 litre = 1000 cm³.

After you have your answer

To save you a second search, here are the related answers people most often want straight after this calculation:

How do I read a tank that is only part full?

For an upright tank, enter the current liquid height instead of the full height. For a tank lying on its side, use the horizontal-tank mode, which turns a dip depth into gallons using the correct segment maths.

Is the answer US or Imperial?

Both are shown at once. In the United States use US gallons; most other English-speaking countries use the larger Imperial gallon. Mixing the two is the single most common tank-sizing error.

How many litres is that in metric?

Every gallons result on this page is shown next to its litre value in the breakdown, because 1 US gallon is about 3.785 litres and 1 Imperial gallon is about 4.546 litres. You can hand the same figure to anyone regardless of which system they use.

What if I only measured around the outside?

You do not need the diameter directly. Measure the circumference around the tank, switch the base input to circumference, and the tool divides by 2 pi to find the radius before working out the gallons.

Where people use this

A few of the jobs it gets used for:

• Sizing a water storage or rainwater tank
• Working out fuel or oil drum capacity
• Estimating how many gallons a pool or trough holds
• Checking a well or cistern volume

Getting it right

The mistakes that cost the most are unit mistakes. Reading a US figure as Imperial, or the reverse, is a 20% error before you start. Measuring the outside of a tank instead of the inside quietly overstates capacity, and forgetting the headspace at the top overstates it again. Measure inside, confirm which gallon, and treat the result as a full-to-the-brim maximum.

Getting the number in the unit you need

Showing every unit at once is about speed and safety: you see the answer in litres, gallons and cubic feet side by side, so there is no chance of applying the wrong conversion factor. Change the output unit at the top to make any of them the headline figure, switch the precision to exact if you want the value in terms of pi, and use copy-result to paste a tidy summary wherever you need it.

Keep going

Once you have your answer you may also want to work in tank capacity litres & liters cubic feet. These related calculators pick up where this one leaves off:

The bottom line: with the radius or diameter and the height, the volume follows immediately, and the breakdown lets you hand the same number to anyone in the unit they prefer. Measure inside dimensions for anything that holds liquid, leave a little headspace at the top of a real container, and treat the geometric figure as a clean maximum rather than the final word on usable capacity.

Frequently asked questions