Interactive Calculator · 2026
Water Storage Days Calculator
Find out how many days your home's water will last if the supply is cut. Enter your total storage (sump plus overhead tank), the number of occupants and litres used per person — get your days of autonomy instantly.
Days of autonomy across storage sizes — your size highlighted
Your household
Sump plus overhead tank, filled to the brim.
People drawing on the storage each day.
Litres per capita per day — CPHEEO puts full domestic use around 135 LPCD; frugal households use less.
Days of autonomy by storage size
How the reserve stretches as tank capacity grows, at your current demand.
Your 3,000 L of storage covers a household of 5 for about 4.44 days at 135 LPCD. Autonomy scales linearly with capacity — double the tank, double the days.
This assumes normal usage. During an actual cut you'd stretch it further by rationing — flushing less, shorter showers, buckets over the hose.
How this is calculated
- Daily demand = occupants × LPCD = 5 × 135 = 675 L per day.
- Days of autonomy = total storage ÷ daily demand = 3,000 ÷ 675 = 4.44 days.
- Roughly in hours = days × 24 = 4.44 × 24 = 106.67 hours.
Indicative planning figure. Keep a 2–3 day buffer for erratic supply, and remember that oversizing tanks risks stagnation — verify against your area's typical supply gaps before sizing storage.
Frequently asked questions
- How does the water storage days calculator work?
- It divides your total stored water by your daily demand. Daily demand is the number of occupants multiplied by litres used per person per day (LPCD), so 5 people at 135 LPCD need 675 litres a day. A 3,000 litre reserve then lasts about 4.4 days. Multiply days by 24 to see it in hours. Autonomy scales linearly, so doubling the tank roughly doubles the days.
- What values should I enter for storage and litres per person?
- For storage, add your sump and overhead tank capacity filled to the brim, in litres. For occupants, count everyone drawing on the supply each day. For LPCD, CPHEEO norms put full domestic use around 135 litres per person per day, though frugal homes manage on 70 to 100 and homes with gardens or more fittings can exceed 200. Start at 135 and adjust to your habits.
- How many days of water reserve should a home keep?
- It depends on how reliable your supply is. Where municipal water arrives daily, one to two days of storage is usually enough. For erratic or tanker-dependent areas, a two to three day buffer is safer. Avoid oversizing, since water sitting too long can stagnate and grow biofilm. Treat this figure as an indicative planning number and verify against your area's typical supply gaps before sizing tanks.
Related Guides — Deep-dive reading
Municipal Water vs Borewell: Which to Rely On for an Indian Home?
A fair, India-first head-to-head: corporation (municipal) supply is treated and cheap to run but comes for a few hours a day, while a borewell gives you your own 24x7 groundwater at an upfront drilling cost — with hardness and a falling water table to manage. When to pick each, and why most homes end up keeping both.
PlumbingOverhead vs Underground Water Tank: Which Do You Need in India?
A fair, India-first head-to-head: an overhead tank gravity-feeds your taps but holds little; an underground sump stores huge volumes from erratic supply but needs a pump. Capacity, pressure, cost, cleaning, contamination — and why most Indian homes quietly run both.
PlumbingUnderground Water Sump Tanks in India: Sizing, Waterproofing and Safe Placement
The below-ground reservoir that catches municipal and borewell water at ground level before a pump lifts it to your overhead tank — how it fits the supply chain, why waterproofing is make-or-break, how big to build it, where to keep it away from the sewer, and how to keep the pump from running dry.
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