Working out a water tank’s capacity is really just calculating its volume and converting the answer into litres, because one cubic metre of water is exactly 1,000 litres. For a rectangular tank you multiply length by width by height; for a cylindrical one you use pi times the radius squared times the height. The only things people trip on are measuring the inside rather than the outside, and keeping every dimension in the same unit before multiplying. This guide gives you both formulas, worked examples, the litre and gallon conversions, and the difference between a tank’s total and its usable capacity. Once you have the dimensions, the CalcRange Volume Calculator does the arithmetic and the unit conversion for you.
Capacity Is Just Volume in Litres
A tank’s capacity is the volume of water it can hold, and the whole job is to find that volume and express it in litres.
1 cubic metre (m³) = 1,000 litres
So capacity in litres = volume in cubic metres × 1,000
That single conversion links the geometry to the number you actually want. Calculate the volume in cubic metres using the right formula for the tank’s shape, multiply by 1,000, and you have the capacity in litres. If you prefer to work in centimetres, one litre is 1,000 cubic centimetres, so a volume in cubic centimetres divided by 1,000 also gives litres. The important habit is to pick one unit, metres or centimetres, and keep every measurement in it, because mixing units is the single biggest source of wildly wrong answers here.
Rectangular and Square Tanks
For a rectangular or square tank, multiply the three internal dimensions together.
The formula is length × width × height, all measured inside the tank, which gives the volume; multiply by 1,000 for litres. Take a tank that is 1.5 metres long, 1 metre wide, and 1 metre deep. The volume is 1.5 × 1 × 1 = 1.5 cubic metres, and the capacity is 1.5 × 1,000 = 1,500 litres. A smaller tank of 0.8 × 0.6 × 0.5 metres holds 0.24 cubic metres, or 240 litres. The measurements must be the internal ones, because the walls take up space the water cannot occupy. On a big tank the wall thickness is a small error, but on a small one it matters, so measure the inside where you can.
Cylindrical Tanks
Round tanks, the common overhead and storage type, use the circle-area formula for the base multiplied by the height.
Volume = π × r² × h, where r is the internal radius and h is the internal height
The radius is half the internal diameter. Say a cylindrical tank has an internal diameter of 1 metre, so a radius of 0.5 metres, and a height of 1.2 metres. The volume is π × 0.5² × 1.2 = 3.1416 × 0.25 × 1.2 = 0.942 cubic metres, which is 942 litres. If you only know the diameter, remember to halve it before squaring; using the full diameter as the radius is a classic mistake that quadruples the answer. As with rectangular tanks, use the internal measurements and multiply the cubic-metre result by 1,000 for litres.
Calculate Your Tank Capacity
Enter your tank’s dimensions into the CalcRange Volume Calculator to get the capacity in litres or gallons without doing the arithmetic. For the geometry behind it, including cones and spheres, our guide on volume formulas explained covers every common shape.
Converting to Gallons
If you need the answer in gallons, the conversion depends on which gallon you mean, and the two differ enough to matter.
One cubic metre, or 1,000 litres, is about 264.2 US gallons or 220 imperial (UK) gallons. So our 1,500-litre rectangular tank is roughly 396 US gallons or 330 imperial gallons, and the 942-litre cylindrical tank is about 249 US gallons or 207 imperial. The US gallon (3.785 litres) is smaller than the imperial gallon (4.546 litres), which is why the US figure is always the larger count for the same volume. When a tank or pump is rated in gallons, check which standard is being used, especially on imported equipment, or your capacity estimate can be out by around 20 percent.
Total vs Usable Capacity
The number you calculate is the tank’s total volume, but the water you can actually draw is usually a little less.
Two things reduce usable capacity. First, tanks are rarely filled to the absolute brim; a small gap at the top, called freeboard, allows for movement, inlets, and overflow, so a few centimetres of height at the top hold no usable water. Second, the outlet pipe often sits slightly above the very bottom, so a thin layer of water beneath it, along with any sediment, never comes out. For a rough tank that is filled near the top and drained near the bottom, the calculated volume is close enough for planning. For precise work, measure the height only between the usable water levels, from just below the overflow down to the outlet, and use that in the formula instead of the full internal height.
Common Mistakes
- Mixing units, such as length in metres and height in centimetres. Convert everything to one unit before multiplying.
- Using the diameter as the radius on a cylindrical tank. Halve the diameter first; forgetting quadruples the result.
- Measuring the outside of the tank. Walls displace water, so internal dimensions are the ones that count, especially on small tanks.
- Forgetting the 1,000 multiplier and reporting cubic metres as if they were litres, understating capacity a thousandfold.
- Assuming gallons are gallons. US and imperial gallons differ by about 20 percent, so state which you mean.
Frequently Asked Questions
How do I calculate the capacity of a water tank?
Find the tank’s internal volume and convert it to litres. For a rectangular tank, multiply length by width by height in metres, then multiply by 1,000. For a cylindrical tank, use pi times the radius squared times the height, then multiply by 1,000. One cubic metre of water equals 1,000 litres, which is the conversion that turns volume into capacity.
How many litres is a 1x1x1 metre tank?
Exactly 1,000 litres. A tank measuring one metre in each direction has a volume of 1 cubic metre, and one cubic metre equals 1,000 litres. This makes a one-metre cube a handy mental benchmark: any rectangular tank’s capacity in litres is its volume in cubic metres times 1,000.
What is the formula for a cylindrical water tank?
Volume equals pi times the radius squared times the height, using internal measurements. The radius is half the internal diameter. For a tank 1 metre across and 1.2 metres tall, that is 3.1416 times 0.5 squared times 1.2, which is 0.942 cubic metres, or 942 litres. Always halve the diameter to get the radius before squaring.
Should I measure the inside or outside of the tank?
The inside. The tank walls take up space that water cannot fill, so external measurements overstate capacity. On a large tank the wall thickness is a minor error, but on a small tank it is significant. Where you cannot reach inside, measure the outside and subtract roughly twice the wall thickness from each dimension.
How do I convert tank capacity to gallons?
Multiply litres by 0.264 for US gallons or by 0.220 for imperial gallons. So 1,000 litres is about 264 US gallons or 220 imperial gallons. The US gallon is smaller, so it gives a larger count for the same volume. Check which gallon a pump or tank rating uses, as the two differ by about 20 percent.
Why is my usable water less than the calculated capacity?
Because the calculation gives total volume, while real tanks are not filled to the brim and their outlets sit above the base. The gap at the top for overflow and the layer below the outlet both hold water you cannot draw. For usable capacity, measure the height only between the top water level and the outlet, and use that in the formula.
How big a water tank do I need for a household?
It depends on the number of people and daily use, which vary widely by region and lifestyle, so there is no single figure. A practical approach is to estimate daily consumption per person, multiply by the number of people and the days of storage you want, then size the tank to that total. Local water authorities often publish typical per-person usage figures to start from.
The Bottom Line
Tank capacity is volume expressed in litres, and one cubic metre is 1,000 litres. Use length times width times height for rectangular tanks and pi times radius squared times height for cylindrical ones, always with internal measurements in a single unit. Halve the diameter for the radius, mind which gallon you mean if you convert, and remember the usable water is a little less than the total. Feed your dimensions into the volume calculator and it handles the maths and the units together.
Last reviewed: August 2026. Recommended editorial review: every 24 months. Household water-need estimates vary by region; check local guidance for sizing.