Water Tank Size Calculator
Find the right water tank size for your home from the number of people, daily usage and how many days of storage you want.
Inputs
IS 1172: 135 lpcd for homes with flush toilets
Formula
- Daily demand = people × litres per person + extra use
- Tank capacity = daily demand × days of storage
- Recommended size = next standard tank above the requirement
Indian standard IS 1172 allows 135 litres per person per day for a house with full plumbing (about 70–100 lpcd where usage is modest). Multiply by the number of people, add outdoor use, then multiply by how long you want to survive without supply. Most homes split storage: an underground sump for a day or two and a smaller overhead tank for daily use.
Step-by-step calculation
- 1
Daily demand
people × lpcd + extra
540 L/day
- 2
Storage
daily × days
540 L
- 3
Standard size
next size up
750 L
Worked example
- 1Family of 4 at 135 lpcd = 540 litres per day.
- 2Plus 60 litres for the garden = 600 litres per day.
- 3For 2 days of storage → 1,200 litres, so a 1,500 litre tank is chosen.
Tips
- Split the storage: sump for bulk, overhead tank for one day's use — smaller roof load.
- Keep 15–20% free space; tanks are rated to the brim, not to the working level.
Frequently asked questions
How many litres does a family of 4 need?
About 500–600 litres a day, so a 1,000 litre tank covers roughly two days of normal use.
Should the overhead tank hold everything?
No. Keep the overhead tank to about one day's demand and store the rest in an underground sump — it is cheaper and safer for the slab.
Does drinking water need separate storage?
Only 5–10 litres per person per day is for drinking and cooking; the rest is bathing, flushing and washing.
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