Gallons
Formula for Gallons
Turn tank or container dimensions into gallons in one tap — with a clear US vs Imperial gallon choice so you never mix the two up.
Most people reach for a formula for gallons because they want a real-world capacity: how many gallons will this tank, drum or pipe hold? The tool above answers that directly. Enter the diameter and height in inches or feet, pick US or Imperial gallons, and you have your figure.
For anything bought or sold by volume, the gallon figure is the one that matters, and small measurement errors scale up fast: a couple of missed inches on a large tank can be dozens of gallons. Measuring the inside carefully, and reading both gallon types, keeps you honest.
How the volume is worked out
The volume of a cylinder is:
V = π r² hA worked example
Worked example. Take a cylinder with a diameter of 24 in (so a radius of 12 in) and a height of 36 in. The base area is π × 12² = 452.39 in², and multiplying by the height gives a volume of about 70.5 US gallons (16,286 in³ · 266.9 litres).
| Unit | Equal to |
|---|---|
| 1 US gallon | 231 in³ · 3.785 L |
| 1 Imperial gallon | 277.42 in³ · 4.546 L |
| 1 cubic foot | 7.481 US gal · 28.32 L |
| 1 litre | 0.264 US gal · 0.220 Imp gal |
| 1 US gallon | 128 US fl oz |
These are geometric gallons from the dimensions you enter. A real tank holds slightly less once wall thickness, fittings and headspace are taken into account — and always confirm whether you need US or Imperial gallons.
Conversions use standard factors: 1 US gallon = 231 in³, 1 Imperial gallon = 4.54609 L, 1 litre = 1000 cm³.
How to work it out
- Measure across the circular end — the diameter or radius.
- If you measured the diameter, halve it to get the radius r.
- Square the radius and multiply by π to get the base area.
- Multiply the base area by the height h: V = π r² h.
- 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.
Once the result is in
To save you a second search, here are the related answers people most often want straight after this calculation:
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.
How much does a full tank weigh?
Water is about 8.34 lb per US gallon, or 1 kg per litre. Tick estimate weight, choose water, and the calculator multiplies the volume by density to show the load a full tank puts on its stand or floor.
What to watch out for
Watch three things with gallons: which gallon you actually mean, inside versus outside dimensions, and whether the tank is truly full to the top when you picture it. Each one on its own can throw a delivery out by a noticeable margin.
One result, every unit
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.
Related things you might need
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:
When this comes in handy
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
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.