How to Calculate Water Tank Capacity for a House

By Vystra · 29 September 2026

An undersized tank means running dry; an oversized one wastes money and roof load. A quick calculation gives you the right size.

Step 1: Estimate daily need

A common design figure is 135 litres per person per day.

For a family of 4:

4 × 135 = 540 litres per day

Step 2: Decide storage days

Most homes store 1–2 days of water as buffer for supply interruptions.

540 × 2 days = 1,080 litres

Round up for guests and safety — say 1,500 litres.

Step 3: Convert capacity to tank size

The formula:

Volume (litres) = Length × Width × Height (in metres) × 1000

So 1,000 litres = 1 cubic metre. A tank of 1m × 1m × 1.5m holds:

1 × 1 × 1.5 × 1000 = 1,500 litres

Step 4: Split overhead and underground

Many homes use an underground sump (larger, main storage) plus an overhead tank (smaller, for pressure), with a pump between them.

Don't forget

• Roof load — a full overhead tank is heavy; the structure must take it.
• Extra demand — gardens, car washing, more occupants later.

The takeaway

Sizing a tank is simple: daily need × storage days, then convert to dimensions. Getting it right avoids both shortage and wasted cost. With BuildVystra you can plan and budget plumbing and tank work as part of your overall project estimate.

Frequently asked questions

How much water does a person need per day?

A common design figure in India is about 135 litres per person per day for domestic use, covering drinking, cooking, bathing and washing. For a family of four that is roughly 540 litres a day, before adding any buffer for guests or gardens.

How do I convert tank size to litres?

Multiply length x width x height in metres to get cubic metres, then multiply by 1000 to get litres. For example, a tank 1m x 1m x 1m equals 1 cubic metre, which is 1000 litres.