Gallons
How Many 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 how many 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.
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.
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.
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 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.
Follow-on questions, answered
To save you a second search, here are the related answers people most often want straight after this calculation:
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.
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.
Where people use this
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
Where people go wrong
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.
Reading and converting your result
One number, many units. Whatever you enter, the result lists the same volume as litres, US gallons, Imperial gallons, millilitres, cubic centimetres, cubic inches, cubic feet, cubic yards and cubic metres at the same time. You almost never have to convert by hand: read the figure in the unit your job uses and copy any of the others straight from the breakdown. If you need a shareable version, the copy-link button preserves your exact inputs so a colleague opens the page with the same calculation already filled in.
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:
In short: measure the width and the height, keep them in the same unit, and the tool turns them into a volume you can read in litres, gallons, cubic feet or anything else, with the working shown so you can trust it. If a result ever looks off, re-enter the measurement you are least sure of, or switch to the diameter input and let the tool halve it for you, which catches the most common error before it matters.
Good to know
Does this work for a tank or a pipe?
How many gallons are in a cylinder?
What is the difference between US and Imperial gallons?
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.