Tank Volume Calculator
Capacity of a cylindrical, rectangular, or capsule tank in litres, gallons, and cubic metres.
Four tank shapes
Vertical cylinder, horizontal cylinder, rectangular, and capsule — each with its exact formula.
Every unit at once
Enter metres, centimetres, feet, or inches; read the result in litres, US and imperial gallons, and m³.
What is tank volume?
Capacity from a few measurements
Tank volume is the amount of liquid a tank holds when full — its capacity. You can find it from a handful of measurements and the right geometric formula: a circle's area for cylinders, a rectangle for box tanks, and a sphere's volume for the rounded ends of a capsule. The hard part is usually not the maths but keeping the units consistent and remembering to use the radius, not the diameter.
Every tank reduces to a basic solid. The volume of a circle-based tank uses π (3.14159…), the ratio of a circle's circumference to its diameter standardised in references such as NIST's mathematical constants.
V = π × r² × hFor a rectangular tank the volume is simply length × width × height. For a capsule — a cylinder capped by two hemispheres — add the straight section to one complete sphere: V = π r² L + 4⁄3 π r³. In each case r is the radius, which is half the diameter you measure across the tank.
Suppose you have a vertical cylindrical water tank measured in feet: 6 ft across and 8 ft tall.
Halve the diameter
A 6 ft diameter gives a 3 ft radius.Convert to one unit
In metres, r = 0.9144 m and h = 2.4384 m.Apply the cylinder formula
V = π × 0.9144² × 2.4384 = 6.405 m³.Convert to litres
6.405 m³ × 1,000 = 6,405 litres.Convert to gallons
6,405 ÷ 3.785 = 1,692 US gallons, or ÷ 4.546 = 1,409 imperial gallons.
Unit conversions are where most tank calculations go wrong. The factors below are exact, drawn from the standard definitions maintained by NIST and the international system of units.
1 m³ = 1,000 L
A cubic metre is exactly one thousand litres — the bridge between geometry and capacity.
1 US gal = 3.785 L
The US liquid gallon is defined as 3.785411784 litres exactly.
1 imp gal = 4.546 L
The UK imperial gallon is about 20 % larger at 4.54609 litres.
Because the imperial and US gallons differ by roughly a fifth, a "500 gallon" tank can mean 1,893 litres or 2,273 litres depending on the country — always confirm which gallon a specification uses. This calculator reports both so the ambiguity disappears.
The figure is the tank's full capacity. In practice you rarely fill a tank to the brim: liquids expand with temperature, and vented tanks need an air gap, so usable capacity is typically 80–90 % of the geometric volume. The litre figure is also a quick weight check — fresh water weighs almost exactly one kilogram per litre, so a 6,405-litre tank holds roughly 6.4 tonnes of water. That matters when a full tank sits on a structure, roof, or trailer.
The geometry is exact for an idealised tank, but real tanks deviate.
Real tanks differ from ideal shapes
Wall thickness, internal baffles and fittings, rounded internal corners, and dished (rather than flat or perfectly hemispherical) ends all shift the true capacity by a few percent. This tool also reports full volume only, not a partial fill level — that requires the area of a circular segment for horizontal tanks. For trade, custody-transfer, or regulatory measurement, use the manufacturer's certified capacity rather than a calculated estimate.