Water Tank Capacity Calculator

About the Water Tank Capacity Calculator

Work out how much water a tank or sump holds — full, or up to the water level you measured — in litres, cubic metres, US and imperial gallons and cubic feet. Rectangular tanks and sumps, round tanks standing or lying on their side, capsule tanks with rounded ends and cone-bottom tanks are all covered, and a depth chart lists the volume at 10 to 100 equal steps of the height, like a dipstick table.

To choose a size, switch to Size for a household: enter how many people use the water and how many days each tank should last, and the calculator applies the per-person figures of IS 1172:1993, the Indian Standard for water supply in buildings — 150 to 200 litres a day in cities with flush toilets, 135 litres for LIG and EWS homes, and the figures for offices, schools, hostels, hotels and hospitals. It also suggests the inside size of a rectangular sump.

Common uses

• Checking how many litres an existing sump or overhead tank holds before buying a pump or planning a refill.
• Choosing an overhead tank and sump size for a new house or a flat building.
• Reading how much is left in a lying diesel, rainwater or water tank from the depth on a dipstick.
• Comparing tank sizes quoted in litres, gallons or cubic feet.

Formulas

• Rectangular: length × width × water depth.
• Round, standing: π × r² × depth, with r half the diameter.
• Round, lying: the water’s end face is a circular segment of area r² × acos((r − h) ÷ r) − (r − h) × √(2rh − h²), multiplied by the length.
• Capsule, lying: the same segment over the straight part (overall length minus the diameter) plus a spherical cap π × h² × (3r − h) ÷ 3 for the two rounded ends, which together make a sphere.
• Capsule, standing: a hemisphere at the bottom, the cylinder, then the top hemisphere, each filled in turn.
• Cone bottom: while the water is in the cone, π × (r × h ÷ c)² × h ÷ 3, with c the cone depth; above it, the full cone π × r² × c ÷ 3 plus the cylinder above.

1 m³ = 1,000 litres. A US gallon is 231 cubic inches = 3.785411784 L and an imperial (UK) gallon is exactly 4.54609 L (NIST SP 811, Appendix B.9).

How much water a home needs (IS 1172:1993)

IS 1172:1993, Code of basic requirements for water supply, drainage and sanitation, clause 4.1, gives these minimum figures per person per day:

• Cities of over 1,00,000 people with full flushing: 150–200 L. Of this, about 45 L is for flushing (clause 4.1.1) — useful if the toilets have their own tank.
• Towns of 20,000–1,00,000 with full flushing: 100–150 L.
• Up to 20,000 people, no flushing system: 70–100 L through a house connection (40 L minimum at a public standpost).
• LIG and EWS housing: the 150–200 L may be reduced to 135 L (note to clause 4.1).

The standard assumes an average family of five (clause 1.2). The calculator uses the upper figure of each range — you can type any other value. Storage = people × litres per person per day × days the tank must last.

Buildings other than homes (IS 1172 Table 1)

• Offices, day schools and factories with bathrooms: 45 L per head; factories without bathrooms: 30 L.
• Hostels, boarding schools, nurses’ homes and medical quarters: 135 L per head.
• Hotels: 180 L per head.
• Hospitals including laundry: 340 L per head up to 100 beds, 450 L over 100 beds.
• Restaurants: 70 L per seat. Cinemas, concert halls and theatres: 15 L per seat.

Water for fire fighting is extra — Table 1 refers to IS 9668 for it, and it is not included here.

How many days of storage?

IS 1172 gives daily figures, not storage periods. The right number of days depends on how often water comes, for how long and how reliably, and how often the overhead tank can be pumped up. A tank filled every day needs at least a day’s water; with supply on alternate days, two. Many homes have a sump below ground that the supply fills, and an overhead tank that a pump fills from the sump. Your plumber, architect or the local water board can tell you what the supply in your area needs.

Sources

• IS 1172:1993, Code of basic requirements for water supply, drainage and sanitation (fourth revision), Bureau of Indian Standards — clauses 1.2, 4.1, 4.1.1, 4.2 and Table 1 (copy published by Public.Resource.Org).
• NIST SP 811, Appendix B.9 — gallon conversion factors.

Limitations

• Use inside sizes. Wall thickness, ribs and fittings of moulded plastic tanks make their real capacity a little smaller than their outside size suggests — the maker’s stated capacity is the one to trust for those.
• Shapes are ideal: perfectly flat bottoms, true cylinders and hemispherical ends. Dished or flat-ended tanks need the maker’s chart.
• The household figures are IS 1172 minimums for design. Real use varies with climate, gardens, washing machines and leaks; fire-fighting storage (IS 9668) is not included.
• The weight shown is the water only, at about 1 kg per litre — have a structural engineer check a roof or slab before placing a large overhead tank on it.

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