There are two questions here. To size a tank, multiply occupants × per-capita demand (default 135 litres/person/day, the standard domestic norm) to get daily litres, then pick the next standard tank size. To find a tank's capacity, multiply length × width × height in metres and multiply by 1,000, because one cubic metre holds 1,000 litres.
Key takeaways
- Daily demand = occupants × 135 L (standard domestic norm).
- Family of 5 ≈ 675 L/day → a 1,000 L tank for one day plus buffer.
- Capacity (L) = L × W × H (m) × 1,000.
- 1 m³ = 1,000 litres exactly.
- Add a buffer of 1.5–2 days where supply is irregular.
How water tank sizing works
Household water storage is planned around daily consumption. Standard trade practice gives a per-capita figure — about 135 litres per person per day for a residence with full flushing systems — which you multiply by the number of occupants. That daily demand sets the minimum useful tank, and most homes round up to a standard moulded size (500, 750, 1,000, 1,500 or 2,000 litres) and often add a day of buffer.
The capacity side is pure geometry: a rectangular tank's volume in cubic metres times 1,000 gives litres. This is why a neat 1 m cube holds exactly 1,000 litres — a handy reference for overhead tanks.
Worked example: a family of five
Five occupants at 135 L/day need 5 × 135 = 675 litres a day. The next standard tank is 1,000 litres, which also leaves a comfortable buffer. To hold 1,000 litres (1 m³), a tank could be about 1 m × 1 m × 1 m, or a slimmer 1.25 m × 0.8 m × 1 m. If you already have a tank measuring 1 m × 1 m × 1 m, its capacity is 1 × 1 × 1 × 1,000 = 1,000 litres, which covers 1,000 ÷ 675 ≈ 1.5 days of demand.
Per-capita demand & standard tank sizes
| Occupants | Demand at 135 L/day | Suggested tank |
|---|---|---|
| 2 | 270 L | 500 L |
| 3 | 405 L | 500 L |
| 4 | 540 L | 750 L |
| 5 | 675 L | 1,000 L |
| 8 | 1,080 L | 1,500 L |
| 10 | 1,350 L | 1,500 – 2,000 L |
Suggestions cover roughly one day plus a small buffer; size up for gardens or irregular supply.
Build the structure to hold it
For an overhead tank slab or an underground sump, estimate concrete and steel with the RCC slab calculator, and size the room AC with the AC tonnage calculator.
Frequently asked questions
How do I calculate water tank size for a house?
Occupants × per-capita demand. At 135 L/day, a family of 5 needs 675 L/day, pointing to a 1,000 L tank for one day plus buffer.
How many litres does a family of 5 need per day?
About 675 litres at the standard figure of 135 L per person. Round up to a 1,000 L tank for comfort.
How do I calculate the capacity of a water tank in litres?
Volume in m³ = L × W × H (metres); litres = m³ × 1,000. A 1 m cube holds 1,000 litres.
What is the standard domestic water demand per person?
The standard domestic demand for residences with full flushing is about 135 litres per person per day. This tool uses 135 by default and lets you change it.
What size water tank do I need for 4 people?
About 540 L/day at 135 L each. A 500–750 L tank covers a day; many fit 1,000 L for a buffer.
Should the tank hold more than one day of water?
Often yes. With irregular supply, size for 1.5–2 days. Many homes split storage between an underground sump and an overhead tank.
The 135 litres-per-person-per-day domestic figure follows standard trade practice for water supply and sanitation. Capacity conversion uses the exact relation 1 cubic metre = 1,000 litres. Standard tank sizes reflect common moulded-tank capacities sold across Pakistan.
Last reviewed 2026-06-14