Water Tank Capacity Calculation: Litres from Size, and Sizing for a Family

Capacity in litres = length × width × water depth (all in metres) × 1000 for a rectangular tank, and π/4 × D² × water depth × 1000 for a round tank, because 1 m³ holds 1000 litres. For a house, size the tank from demand: the CPHEEO manual norm of 135 litres per person per day gives 675 litres/day for a family of five, so a 1000-litre overhead tank and a sump of about 1350–1500 litres (two days’ demand) are typical.

Unit conversions you need

The last line matters for structural design: a 1000-litre tank full of water puts about 1 tonne on the slab, plus the weight of the tank itself.

Formula 1: rectangular tank

Capacity (L) = L × B × H × 1000, with L, B and H in metres. Use the internal dimensions and the water depth (up to the overflow level), not the total wall height.

Example: internal size 2.0 m × 1.5 m, water depth 1.2 m.

• Volume = 2.0 × 1.5 × 1.2 = 3.6 m³
• Capacity = 3.6 × 1000 = 3600 litres

Same thing in feet

Capacity (L) = L × B × H (in feet) × 28.32.

Example: sump 6 ft × 4 ft × 4 ft water depth = 96 cft × 28.32 = 2718 litres.

Formula 2: cylindrical (round) tank

Capacity (L) = (π/4) × D² × H × 1000, with internal diameter D and water depth H in metres.

Example: D = 1.5 m, water depth 2.0 m.

• Area of base = 0.7854 × 1.5² = 1.767 m²
• Volume = 1.767 × 2.0 = 3.534 m³
• Capacity = 3534 litres

To find the water depth needed for a given capacity, rearrange: H = Volume / (0.7854 × D²). For 1000 L (1.0 m³) in a 1.1 m diameter tank: H = 1.0 / (0.7854 × 1.21) = 1.05 m.

How much water does a family use?

The CPHEEO Manual on Water Supply and Treatment (Ministry of Housing and Urban Affairs) recommends these per-capita supply norms:

For a house with flushing toilets, 135 lpcd is the standard design figure. A commonly quoted break-up of 135 lpcd (as given in the CPHEEO manual) is roughly: bathing 55, washing clothes 20, flushing WC 30, washing utensils 10, washing the house 10, cooking 5 and drinking 5 litres.

Worked example: tanks for a family of five

Data: 5 persons, city house with sewer connection, municipal supply once a day into an underground sump, pumped up to an overhead tank (OHT).

Step 1 – Daily demand

Demand = 5 × 135 = 675 litres/day.

Step 2 – Overhead tank

• Minimum = 1 day = 675 L.
• Common plastic tank sizes are 500, 750, 1000 and 1500 L. A 750 L tank just covers one day (750 / 675 = 1.11 days) with almost no margin, so adopt 1000 L: 1000 / 675 = 1.48 days of storage, enough for guests and a skipped pump cycle.
• Rated capacity of a plastic tank is usually its gross volume; the usable volume is a little less because the outlet sits above the bottom and the overflow sits below the lid.

Step 3 – Underground sump (RCC or masonry)

• Storage for 2 days = 2 × 675 = 1350 L = 1.35 m³.
• Try internal size 1.2 m × 1.0 m. Water depth required = 1.35 / (1.2 × 1.0) = 1.125 m. Adopt 1.2 m.
• Actual capacity = 1.2 × 1.0 × 1.2 × 1000 = 1440 L ≥ 1350 L. OK.
• Add freeboard of about 0.3 m above the overflow (assumed) for the inlet pipe and air gap: total internal depth = 1.2 + 0.3 = 1.5 m.
• Every 100 mm of water depth in this sump holds 1.2 × 1.0 × 0.1 × 1000 = 120 L – handy for marking a depth gauge.

Step 4 – Check the overhead tank against the sump

The sump (1440 L) refills the 1000 L OHT once and keeps a reserve of 440 L for the next fill. If the supply is erratic, increase the sump, not the OHT – water is heavy, and every extra 1000 L on the roof adds about a tonne of load on the slab and columns.

Quick sizing table (135 lpcd)

For apartments, multiply by the number of flats and design the storage with a plumbing consultant; fire-fighting reserve for taller buildings is a separate requirement.

Site tips and common mistakes

• Using outer dimensions. A sump measured outside the 230 mm walls overstates capacity by a lot. Always use internal size and water depth.
• Using total depth. Water stops at the overflow. Water below the outlet pipe (dead storage, often 50–100 mm) is also not usable.
• Ignoring the load on the roof. Place the OHT over columns or beams, on a raised platform, and include the full water weight in the slab design.
• Sump next to the septic tank. Keep the underground water tank well away from the septic tank and soak pit, and make it watertight. Size the septic tank with our septic tank size estimator.
• No cleaning access. Provide a manhole with a lockable cover and a sloped floor towards a scour point.

For concrete and steel quantities of an RCC sump, use our RCC estimator, or browse all free calculators.