Tank Volume Calculator
Capacity in litres of round and rectangular tanks.
Your Tank
Use internal measurements. Results update as you type.
Measure how deep the liquid is from the tank floor to see how much is in it and how full it is.
Tank capacity
9,500 litres
(9.5 kL)
9.5 m³ · 2,510 US gal · 2,090 UK gal
61.9% full
5,881 L in the tank
3,619 L to fill
About 4.52 L per mm of depth.
Water weighs about 1 kg per litre. A full tank needs a level, compacted base or a stand rated for that weight, plus the weight of the tank itself.
Rainwater & tank sizing
- A 25 mm rain event on 150 m² at 80% collects 3,000 L.
- About 3.2 events like that fill your tank.
How to Use the Tank Volume Calculator
Choose a round upright tank, a horizontal cylinder or a rectangular (including slimline) tank, pick millimetres, centimetres or metres, and enter the internal measurements. The tank volume calculator shows the capacity in litres, kilolitres, cubic metres and US and UK gallons, plus the weight of the water when the tank is full. Add the current depth of the liquid and it works out how many litres are in the tank, the percentage full and how much is needed to top it up.
You might call it a tank size calculator, a water tank capacity calculator or a cistern size calculator, but the maths only depends on the shape. That means it also works as a heating liquid volume calculator for oil or diesel tanks: the litres are the same for any liquid, only the weight differs with the liquid's density. The optional rainwater panel estimates how much your roof collects and what size tank suits your water use.
How to Calculate Tank Volume
Horizontal cylinder = π × (diameter ÷ 2)² × length
Rectangular tank = length × width × height
Work in metres to get cubic metres, then multiply by 1,000 for litres (1 m³ = 1,000 L).
Partly full, horizontal: the liquid cross-section is a circular segment, with area r² × arccos((r − h) ÷ r) − (r − h) × √(2rh − h²), multiplied by the tank length. Here r is the radius and h is the liquid depth.
A horizontal tank is not linear: it is exactly half full at half the diameter, but the first and last 10% of depth hold far less than the middle.
Tank size = daily use (L) × days of storage
One millimetre of rain on one square metre is one litre. Efficiency of about 80% is a typical allowance for first flush, evaporation and overflow.
Worked Example: A 2,400 mm Round Tank
A round upright tank has an internal diameter of 2,400 mm and a height of 2,100 mm. Its capacity is π × 1.2² × 2.1 = 9,500 litres (9.50 kL, or about 2,510 US gallons and 2,090 UK gallons). Full, the water weighs about 9.50 tonnes.
With the water 1,300 mm deep, the tank holds 5,881 litres, is 61.9% full and needs 3,619 litres to fill. Each millimetre of depth is worth about 4.52 litres, which is handy for marking a depth gauge.
For rainwater, a 150 m² roof in a 25 mm rain event at 80% efficiency collects 3,000 litres, so it takes about 3.2 events like that to fill the tank from empty. If the household uses 300 L/day and you want 14 days of storage, the target is 4,200 litres, and this 9,500 litre tank would last about 31.7 days when full.
Capacities of Round Tanks
These are the geometric capacities worked out by the calculator for upright round tanks. They are not retail sizes, and a manufacturer's nominal capacity can differ.
| Diameter × height | Capacity | Litres per mm depth |
|---|---|---|
| 1,000 × 1,500 mm | 1,178 L | 0.79 |
| 2,000 × 2,000 mm | 6,283 L | 3.14 |
| 3,000 × 2,200 mm | 15,551 L | 7.07 |
For other shapes, a slimline rectangular tank 2,000 mm long, 500 mm wide and 1,500 mm high holds 1,500 litres, and a horizontal cylinder 1,200 mm in diameter and 3,000 mm long holds about 3,393 litres.
Measuring Your Tank
Use the internal dimensions, because wall thickness, ribs and the base all reduce the space inside. Measure the height to the overflow or the highest level the water can reach, not the top of the lid. Manufacturers' nominal capacity may include space above the outlet or overflow, so the real working volume can be lower than the label.
Sizing a Rainwater Tank
Start with how much water you use and how many days you want to be covered between rain events, then check what your roof can realistically collect. A larger roof or a wetter climate fills a tank faster, while heavy use needs more storage. Allow for first-flush diversion, filters and overflow, and note that only roof area that drains to the gutters counts. Some councils and water authorities offer rebates or have rules for rainwater tanks, including plumbing requirements, so check with yours before you buy.
A full tank is heavy: every litre of water weighs about a kilogram, so a 9,500 litre tank holds about 9.5 tonnes. It needs a level, compacted base or a stand rated for the load.
Frequently Asked Questions
- How do I calculate tank volume?
- Measure the inside of the tank, work out the volume in cubic metres and multiply by 1,000 to get litres. A round upright tank is π × radius² × height, a horizontal cylinder is π × radius² × length, and a rectangular tank is length × width × height. The calculator does the conversion for you in millimetres, centimetres or metres.
- What is the formula for the volume of a round water tank?
- Volume = π × (diameter ÷ 2)² × height. A tank 2,000 mm across and 2,000 mm high is π × 1² × 2 = 6.283 m³, which is about 6,283 litres. Always use the internal diameter and the height up to the overflow or the top of the water, not the overall outside height.
- How do I work out the capacity of a tank from its size?
- Enter the shape and dimensions into the tank capacity calculator and it returns litres, kilolitres, cubic metres and gallons. Remember that 1 m³ equals 1,000 litres, so a tank of 2.4 m diameter and 2.1 m high holds about 9,500 litres. Treat the answer as the geometric maximum, because the usable volume is normally lower.
- How do I find out how much water is in my tank?
- Measure the depth of the water from the tank floor and enter it in the optional depth field. For an upright or rectangular tank the volume is proportional to depth, so the calculator also shows litres per millimetre, handy if you mark a depth gauge. A horizontal tank is not linear: a depth of 25% of the diameter holds only about 19.6% of the volume in a 1,200 × 3,000 mm tank.
- Does this tank calculator work for heating oil, diesel or other liquids?
- Yes. Volume depends only on the shape and size, so the litres are the same whatever is inside, whether that is water, heating oil or diesel. Only the weight changes: the calculator's weight figure assumes water at 1 kg per litre, and most fuels are lighter, so work out the weight from the liquid's density if it matters.
- Can I use it as a cistern size calculator?
- Yes. A rectangular underground cistern uses the same length × width × height maths, and a round one uses the cylinder formula. Use the internal dimensions of the water space, and check how much of that volume sits below the inlet, outlet and overflow, since that is the part you can actually use.
- What size water tank do I need for rainwater harvesting?
- Work backwards from how much you use and how long you want to stay supplied. Multiply daily use in litres by the days of storage you want: 300 L/day for 14 days is 4,200 L. Then compare that with what your roof collects, which is roof area in m² × rainfall in mm × collection efficiency, because every millimetre of rain on one square metre is one litre.
- How much water will my roof collect in a rain event?
- Roof area × rainfall × efficiency. A 150 m² roof in a 25 mm rain event at 80% efficiency collects 3,000 litres. The 80% allows for first-flush diversion, evaporation, splash and overflow, and is a typical allowance rather than a fixed rule; the real figure depends on your roof, gutters and filters.
- What is the difference between tank capacity and usable volume?
- Capacity is the total volume of the shape, while usable volume is what you can draw off in practice. The outlet is usually above the floor, a sludge zone is best left undisturbed, and there is air space above the overflow. Manufacturers' nominal capacities may also include some of that space, so check the specification for your model.