Gallons
Cylinder Volume 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.
The single most important thing a cylinder volume gallons has to get right is which gallon you mean. A US gallon and an Imperial gallon are not the same size, so this tool asks you to choose — and shows both, so there is never any doubt about the number you are reading.
Capacity questions almost always end in gallons — filling a stock trough, ordering heating oil, dosing a pool. The catch is that a gallon is not one fixed thing, so the same tank can be 200 gallons or 240 gallons depending on which side of the Atlantic you are standing on.
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 30 in (so a radius of 15 in) and a height of 48 in. The base area is π × 15² = 706.86 in², and multiplying by the height gives a volume of about 146.9 US gallons (33,929 in³ · 556.0 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.
Using the calculator
- 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.
What people work out next
The figure above is usually the start, not the finish. These are the questions that tend to come next, answered here so you do not have to run another search:
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.
Getting it right
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.
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.
What to work out next
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
People reach for this in a range of situations:
- 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.
Good to know
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.