Volume formulas
Cylindrical: V = π × r² × h (where r = diameter ÷ 2)
Rectangular: V = L × W × H
Litres = V (m³) × 1,000
For a cylinder lying on its side (horizontal axis), use the rectangular formula with the cross-section area instead. Domestic tanks are almost always vertical, so the formulas above apply.
Recommended water demand
| Use type | L per person per day |
|---|---|
| Drinking + cooking only | 5 – 10 |
| Basic (no flush, no shower) | 40 – 60 |
| Indian residence (IS 1172) | 135 |
| Indian residence with garden / car wash | 180 – 220 |
| Hotel / hostel | 180 – 250 per occupant |
FAQ
Should overhead and underground tanks be the same size?
Typical practice: underground sump = 1.5 to 2 days of demand, overhead tank = 0.5 to 1 day. The sump receives the supply (which can be intermittent), the overhead provides head pressure for the taps.
Why use water depth instead of total tank height?
The top 100–150 mm of a tank is freeboard (air gap) for inlet splash and to prevent overflow. Use the actual water-fill depth, not the outer tank dimension, to avoid overstating capacity.
What about Sintex / plastic tanks?
Manufacturer-stated capacity (e.g. "1000 L") is the actual filled capacity, including the freeboard allowance. For those, just use the quoted figure rather than measuring.