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About this tool
Size an overhead tank and sump in litres from family size, IS 1172 per-capita demand, garden use and days of storage.
An overhead water tank size calculator converts your household's daily water demand into the number of litres the roof tank and the underground sump each need to hold. It multiplies residents by a litres-per-capita-per-day figure — 135 LPCD is the IS 1172:1993 minimum for a home with a full flushing system — adds garden and vehicle washing, multiplies by the days of storage you want in hand, then splits that total between overhead and sump and rounds up to the nearest tank sold off the shelf.
Open Overhead Water Tank Size Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Uses the IS 1172 and CPHEEO per-capita figures rather than a guess.
Shows both volumes so you can order the pair together.
Rounds the requirement up to the nearest tank actually on sale.
At the IS 1172:1993 minimum of 135 litres per person per day, four people use about 540 litres a day, so a 500-750 litre overhead tank covers roughly one day. Most Indian homes fit a 1,000 litre roof tank because garden watering, vehicle washing and guests push real demand past the minimum, and because supply cuts are common.
IS 1172:1993 sets a minimum of 135 litres per capita per day for residences with a full flushing system, split roughly as 55 L bathing, 30 L flushing, 20 L washing clothes, 10 L utensils, 10 L cleaning and 10 L drinking and cooking. The CPHEEO manual allows 70 LPCD where there is piped supply but no sewerage, and 150-200 LPCD in metropolitan cities.
Usually yes. Common practice is to hold about a third to a half of total storage overhead and the rest in the sump, because the sump is cheap to enlarge underground while every extra litre on the roof adds dead load to the slab. A full 1,000 litre tank weighs about a tonne once you include the tank and its stand.
One cubic foot holds 28.32 litres and one cubic metre holds 1,000 litres, so a 1,000 litre tank is 1 m³ or about 35.3 cubic feet. Add roughly 15 cm of freeboard above the design water level for the inlet, ball valve and air gap when you set the internal height.
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