Tank Size Calculator
Choose the tank shape, type its inside measurements in centimetres and set how full it is. You get the total capacity and the volume of liquid in it right now.
- Round, horizontal and box tanks
- Partial fill levels
- Litres and gallons
For a vertical cylinder, length is ignored. For a horizontal cylinder, height is ignored and the fill level is the depth as a share of the diameter.
Capacity depends on shape as much as size. A round tank 100 cm across holds 785 litres for every metre of height, while a square tank 100 cm wide holds 1,000. Horizontal cylinders are harder still, because each centimetre of depth holds a different amount as the tank curves.
Standard tank sizes
Typical tanks for water, heating oil and rainwater. Search for a capacity or use type to find a close match.
| Use | Shape | Size (cm) | Capacity (L) | US gal |
|---|---|---|---|---|
| Rain barrel | Vertical | 58 Ø × 90 | 210 | 55 |
| Header tank | Box | 61 × 46 × 46 | 114 | 30 |
| Hot water cylinder | Vertical | 45 Ø × 105 | 150 | 40 |
| Heating oil | Box | 175 × 72 × 132 | 1200 | 317 |
| Heating oil | Horizontal | 130 Ø × 230 | 2500 | 660 |
| IBC tote | Box | 120 × 100 × 116 | 1000 | 264 |
| Water butt | Vertical | 70 Ø × 110 | 350 | 92 |
| Rainwater tank | Vertical | 150 Ø × 190 | 3000 | 793 |
| Farm water | Vertical | 245 Ø × 230 | 10000 | 2642 |
| Septic tank | Horizontal | 150 Ø × 280 | 4500 | 1189 |
Listed dimensions are external and approximate. Manufacturers usually quote nominal capacity, which can be 5% more than the usable volume once fittings and outlets are allowed for.
Round tank or square tank
Same width, same height, different capacity. Slide the width and compare a round and a square tank 1 metre tall.
A circle fills π/4 of the square around it, so a round tank always holds about 78.5% of a square one of equal width. Round tanks are still popular because they resist pressure evenly and need no internal bracing.
How to measure a tank
Measure inside if you can. Follow these steps for a reading you can trust.
Identify the shape
Check whether the tank stands upright, lies on its side, or has flat sides.
If you can only measure outside, subtract twice the wall thickness from each dimension. Plastic tanks are often 5 to 10 mm thick and steel tanks 3 to 6 mm.
Which tank shape suits you
Each shape has a reason to exist. Pick a shape to see where it works best.
Takes the least floor space for its volume and handles water pressure evenly. The most common shape for water storage.
Sits low and spreads its weight. Volume per centimetre of depth changes as the tank curves.
Fits against walls and into corners and stores the most in a given space. Large ones need internal bracing.
For a fixed amount of space, a box tank stores the most. For strength with thin walls, a cylinder wins. Horizontal tanks are chosen when height is limited, such as under a deck or on a trailer.
Before you buy or fill a tank
A full tank is heavy. Check these before installing.
Tick each item as you go.
Water weighs 1 kg per litre, so a 1,000 litre tank puts a tonne on its base. Diesel and heating oil weigh about 0.84 kg per litre.
What you get from the result
One measurement, several useful answers. Each tile explains one result.
Full volume in litres from the inside measurements.
What is in the tank at the fill level you set.
US and imperial gallons, which differ by about 20%.
Useful for ordering water deliveries or concrete pads.
The weight of water when full, for planning the base.
Tank gauges on horizontal fuel tanks are notoriously poor, because the tank holds more around the middle. Dipping the tank and using the calculator gives a better figure.
Frequently Asked Questions
No question matches that search.
How do I calculate the volume of a round tank?
Multiply π by the radius squared and by the height. With centimetres, divide the answer by 1,000 to get litres. A tank 100 cm across and 150 cm tall holds 3.1416 × 50² × 150 ÷ 1,000 = 1,178 litres.
How much is in a horizontal tank that is half full?
Exactly half the capacity, but only at the halfway mark. At a quarter of the depth, a horizontal cylinder holds about 19.6% of its capacity, not 25%.
How many litres are in a US gallon?
3.785 litres. An imperial gallon is larger at 4.546 litres.
How do I measure a tank I cannot get inside?
Measure outside and subtract twice the wall thickness. For a round tank, wrap a tape around it, divide by π for the outer diameter, then take off the walls.
Does the domed top of a tank add volume?
Yes, a little. A shallow dome adds roughly 2% to 5%. The calculator works on the straight section, which is usually the usable part.
How heavy is a 1,000 litre water tank when full?
The water alone weighs 1,000 kg. Add the tank itself, often 40 to 70 kg for plastic.
What size rainwater tank do I need?
A common guide is 5% of the yearly rainfall that lands on your roof. 100 m² of roof in an area with 800 mm of rain collects 80,000 litres a year, so a 4,000 litre tank.
Size it once, use it for years
Measure inside, choose the right shape and check the fill level by dipping rather than trusting a float gauge. With the capacity and weight in hand you can plan the base, the delivery and the space it will need.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.