Gallons
Gallons in a Cylinder
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.
Working in gallons is second nature for tanks, pools and fuel, and this gallons in a cylinder is built around that. Put in the dimensions, and it returns US gallons, Imperial gallons and litres together so you can hand the number to anyone.
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.
The formula behind it
The volume of a cylinder is:
V = π r² hA 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).
| 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.
After you have your answer
Most jobs need one or two more things once the volume lands. The common follow-ups, with the numbers you need, are below:
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.
Common mistakes to avoid
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.
Reading and converting your result
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:
Common uses
It comes in handy across quite different tasks:
- 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
To sum up, the whole job is two measurements and one formula. Enter them above, pick your unit, and you have an accurate figure plus every equivalent unit beside it. Keep a copy with the copy-result button, save the shareable link if someone else needs the same numbers, and use the history panel to compare a few options without starting from scratch.
Frequently asked questions
How do I convert cubic inches to gallons?
Does the gallon figure account for wall thickness?
What is the formula for the volume of a cylinder?
V = π r² h — the area of the circular base (π r²) multiplied by the height (h). If you have the diameter instead of the radius, halve it first: r = d ÷ 2.