Water Tank Size Calculator
Find how many litres or gallons your water tank holds and holds now, or what size tank your home needs, with the time to fill it and how long it lasts.
Measure inside the tank if you can. Outside sizes include the walls and give a little more.
Depth of water from the bottom inside the tank.
Water below the outlet pipe can't be drawn off. Measure from the bottom inside.
Example: 4 people × 150 L. Use your own figure; your water bill shows it.
150 L is an example only. Use your own figure.
A hose often runs 10–20 L a minute.
Animals, car washing, cleaning, guests, a business.
For hot days, guests and water you can't draw off the bottom.
We work out the height for this width.
Time a 10 L bucket: 10 ÷ seconds × 60 = litres a minute.
How your tank is worked out
Volume from the tank's sizes, or the water you use × the days you want to last.
- 1
Sizes in metres
110 cm = 1.1 m wide120 cm = 1.2 m tallradius 0.55 m
- 2
Find the volume
π × 0.55² × 1.2= 1.1404 m³
- 3
Turn into litres
1.1404 m³ × 1,000= 1,140.4 L
- 4
Water in it now
π × 0.55² × 0.9= 855.3 L75% full
Read more: the formulas for every tank shape, a worked example, common sizes and water use
A tank's volume comes from its inside sizes. Work in metres, so the answer is in cubic metres (m³), then times 1,000 for litres. For a tank that is only part full, use the water depth in place of the full height. Round tanks lying on their side need the slice formula, because the water is wider in the middle than at the bottom.
The formula
radius r = diameter ÷ 2 · all sizes in metres
vertical round tank = π × r² × height
horizontal round tank = π × r² × length
rectangular tank = length × width × height
cone-bottom tank = π × r² × straight side height + π × r² × cone depth ÷ 3
sphere = 4 ÷ 3 × π × r³
capsule = π × r² × straight middle + 4 ÷ 3 × π × r³
litres = m³ × 1,000 · UK gallons = litres ÷ 4.54609 · US gallons = litres ÷ 3.785411784
water now (vertical or rectangular) = the same formula with the water depth for the height
water now (horizontal round, depth y) = (r² × acos((r − y) ÷ r) − (r − y) × √(2 × r × y − y²)) × length
water now (round ends and spheres) adds a cap = π × y² × (3 × r − y) ÷ 3
% full = water now ÷ tank holds × 100
time to fill = litres ÷ fill rate in litres a minute · days it lasts = litres you can use ÷ litres used a day
size needed = (people × litres each a day + other daily use) × days × (1 + spare % ÷ 100)
Worked example
What a tank holds. A vertical round tank 110 cm across and 120 cm tall, with water 90 cm deep. In metres that is 1.1 m and 1.2 m, so the radius is 1.1 ÷ 2 = 0.55 m. The tank holds π × 0.55² × 1.2 = π × 0.3025 × 1.2 = 1.1404 m³, which is 1,140.4 litres (250.9 UK gallons or 301.3 US gallons). The water now is π × 0.55² × 0.9 = 0.8553 m³, so 855.3 litres, and 855.3 ÷ 1,140.4 = 75% full, with 285.1 litres of room left. At 10 litres a minute it fills from empty in 1,140.4 ÷ 10 = 114 minutes, which is 1 h 54 min. At 600 litres a day, a full tank lasts 1,140.4 ÷ 600 = 1.9 days. Full, the water weighs about 1,140 kg.
What size you need. 4 people each using 150 litres a day use 4 × 150 = 600 litres a day. For 2 days that is 600 × 2 = 1,200 litres. With 10% spare, 1,200 × 1.1 = 1,320 litres, so the next common size up is a 1,500 litre tank. A vertical round tank 1.1 m across holding 1.5 m³ needs to be 1.5 ÷ (π × 0.55²) = 1.5 ÷ 0.9503 = 1.58 m tall inside. It fills in 1,500 ÷ 10 = 150 minutes (2 h 30 min) and lasts 1,500 ÷ 600 = 2.5 days.
Common tank sizes
| Litres | m³ | UK gal | US gal | Water weight |
|---|---|---|---|---|
| 500 | 0.5 | 110 | 132 | 500 kg |
| 1,000 | 1 | 220 | 264 | 1,000 kg |
| 1,500 | 1.5 | 330 | 396 | 1,500 kg |
| 2,000 | 2 | 440 | 528 | 2,000 kg |
| 3,000 | 3 | 660 | 793 | 3,000 kg |
| 5,000 | 5 | 1,100 | 1,321 | 5,000 kg |
| 10,000 | 10 | 2,200 | 2,642 | 10,000 kg |
These are generic sizes that many makers sell; a "1,000 litre" tank may hold a little more or less, and its real height and width vary by maker. Check the label or the maker's sheet.
Tips
- To find your own water use, take the m³ on your water bill, divide by the days and by the people at home, then times 1,000. That gives litres a person a day.
- Add spare for the water below the outlet pipe, sediment at the bottom, hot weather and guests. 10–20% is common.
- Water is heavy: each litre is about 1 kg, so 1,000 litres is about a tonne. Make sure a roof or stand can carry a full tank.
- A horizontal tank's level is not a straight guide to its litres: a quarter of the depth holds less than a quarter of the water. Use the water level box to read it properly.
Common questions
How do I work out how many litres a round water tank holds?
Measure the inside diameter and height in metres. Halve the diameter to get the radius, then work out π × radius × radius × height. That gives cubic metres; times 1,000 gives litres. A tank 1.1 m across and 1.2 m tall holds π × 0.55 × 0.55 × 1.2 = 1.1404 m³, which is about 1,140 litres.
How do I find the litres in a horizontal tank that is only part full?
Measure how deep the water is. The water fills a slice of the round end, so work out that slice's area with r² × acos((r − depth) ÷ r) − (r − depth) × √(2 × r × depth − depth²), then multiply by the tank's length. In a horizontal tank half the depth is half the water, but a quarter of the depth is much less than a quarter of the water, which is why a simple ruler reading is not enough.
What size water tank do I need for my home?
Multiply the people in the home by the litres each uses a day, add any garden or other daily use, then multiply by the number of days you want to store. Add 10–20% spare. 4 people using 150 litres each for 2 days is 1,200 litres; with 10% spare that is 1,320 litres, so a 1,500 litre tank is the common size to buy. 150 litres is only an example: use your own figure.
How much water does one person use a day?
It varies a lot by country, climate and habits, so use your own figure. A good way to find it is your water bill: take the cubic metres used, divide by the days in the bill and the people at home, then times 1,000 for litres a person a day. 15 m³ in 30 days for 4 people is 15 ÷ 30 ÷ 4 × 1,000 = 125 litres a person a day.
How long will it take to fill my water tank?
Divide the litres the tank holds by the flow rate in litres a minute. A 1,000 litre tank filled at 10 litres a minute takes 100 minutes, or 1 hour 40 minutes. To measure your flow rate, time how long a 10 litre bucket takes to fill: 10 ÷ seconds × 60 = litres a minute.
How much does a full water tank weigh?
Water weighs about 1 kg a litre, so a 1,000 litre tank holds about 1,000 kg (1 tonne) of water, plus the tank itself. Check that a roof, stand or slab can take the weight before you place a large tank.
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