What the Water Tank Capacity Calculator does
This calculator gives the capacity of a tank in litres, cubic metres, US and imperial gallons and cubic feet, for rectangular, vertical and horizontal cylindrical, conical and capsule-shaped tanks. Enter a liquid depth and it also tells you how much is in the tank now, and it prints a dipstick chart of volume at every tenth of the depth.
The geometry is exact rather than approximated, which matters most for horizontal tanks: a horizontal cylinder filled to a quarter of its depth holds only about 20% of its volume, not 25%.
How to use it
- Choose the tank shape. Vertical and horizontal cylinders are the usual plastic, steel and fuel tanks; the capsule shapes have rounded (hemispherical) ends; the cone covers hoppers and tapered tanks.
- Choose the unit you measured in and enter the internal dimensions. For a horizontal cylinder, length is measured end to end along the axis.
- Optionally enter the depth of liquid from the bottom, measured in the same unit, to see the volume at that level and how full the tank is.
- Read the capacity in each unit and use the dipstick chart to turn future depth readings into volumes.
Reading the results
Full capacity is the geometric volume of the space inside. Tanks are rarely filled to the brim - an overflow or inlet usually sits below the top - so the usable volume is somewhat less.
In a vertical tank, volume rises in step with depth. In horizontal cylinders, capsules and cones it does not, which is why the dipstick chart shows both depth % and volume %.
US and imperial gallons differ by about 20%. Check which one a pump, supplier or regulation uses before comparing.
Worked example: a rooftop cylinder and a horizontal fuel tank
A vertical rooftop tank measures 110 cm across inside and 135 cm tall. Its volume is π x 0.55² x 1.35 = 1.283 m³, or 1,283 litres. With water 80 cm deep it holds π x 0.55² x 0.80 = 0.760 m³, which is 760 litres.
A horizontal fuel tank is 1.2 m in diameter and 2.5 m long, so it holds π x 0.6² x 2.5 = 2.827 m³ (2,827 litres) when full. A dipstick shows 45 cm of fuel. The wetted cross-section is the circular segment 0.6² x acos(0.15 / 0.6) - 0.15 x √(2 x 0.6 x 0.45 - 0.45²) = 0.3874 m², so the tank holds 0.3874 x 2.5 = 0.968 m³, or 968 litres.
The depth is 37.5% of the diameter but the tank is only 34.3% full, because the narrow bottom of a cylinder holds less than the wide middle.
Formulas and scoring rules
- Rectangular
V = length x width x height; at depth h: length x width x h- Vertical cylinder
V = π x r² x height; at depth h: π x r² x h- Horizontal cylinder
V(h) = length x (r² x acos((r - h) / r) - (r - h) x √(2rh - h²))The circular segment area times the length.- Cone or frustum (upright)
V(h) = π x h / 3 x (r1² + r1 x r + r²), where r = r1 + (r2 - r1) x h / Hr1 bottom radius, r2 top radius, H height; set one diameter to 0 for a pointed cone.- Capsule
full V = 4/3 x π x r³ + π x r² x straight length; partial fills use spherical caps π x h² x (3r - h) / 3- Units
1 m³ = 1,000 L; 1 US gal = 3.785411784 L; 1 imperial gal = 4.54609 L; 1 ft³ = 28.3168 L
Limitations: what the result does not prove
- Measure inside the tank. Outside measurements include wall thickness, which on a concrete sump or a thick plastic tank can overstate capacity noticeably.
- Real tanks are not perfect shapes. Moulded plastic tanks have ribs, domed tops and sloping bottoms, and elliptical ends on steel tanks hold less than the hemispheres assumed here; the maker's stated capacity wins where it exists.
- A tank on a slope has a different level-to-volume relationship; the calculator assumes it is level.
- Volumes of liquids other than water are the same, but their weight differs - check that a roof or stand can carry the full load (water weighs about 1 kg per litre).
Privacy: where your data goes
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Standards and sources
- NIST - SI units and conversion factors (litre, gallon)
- NIST Handbook 44 - Specifications for weighing and measuring devices
Frequently asked questions
How do I calculate the capacity of a cylindrical water tank?
Multiply π by the radius squared and by the height, using internal measurements in metres, then multiply by 1,000 for litres. A tank 1 m across and 1.5 m tall holds π x 0.5² x 1.5 = 1.178 m³, or 1,178 litres.
How much is left in a horizontal tank at a given depth?
It is not proportional to depth, because the cross-section is a circle. Enter the diameter, length and dipstick depth: the calculator works out the circular segment the liquid fills and multiplies by the length. At half depth it is exactly half full; at a quarter depth it is about 20% full.
How many litres are in a cubic metre?
Exactly 1,000 litres. A cubic metre of water weighs about 1,000 kg, so a 5,000-litre tank on a roof puts roughly five tonnes on its supports when full.
What is the difference between US and imperial gallons?
A US gallon is 3.785 litres; an imperial (UK) gallon is 4.546 litres, about 20% larger. A 1,000-litre tank holds 264 US gallons or 220 imperial gallons, so always check which gallon a figure uses.
How do I make a dipstick chart for my tank?
Enter the tank's internal dimensions and read the dipstick table, which lists the volume at each tenth of the depth. Download it as CSV and mark a stick at those depths, or enter any other depth in the fill field to get its volume.
Why is my tank's label capacity different from the calculation?
Makers often quote nominal capacity, or measure to the overflow rather than the brim, and moulded tanks have curves and ribs that simple shapes ignore. If the difference is large, check you used internal dimensions and the right shape.
Last reviewed by the A2Z.Tools team against the sources listed above.
