Find out exactly how much water your home needs every day, which tank size is right for your household, and how long your stored supply will last — all in one place with six purpose-built, free tools.
What Are Household Sizing & Water Requirement Calculators?
Household water planning is rarely guesswork-friendly. Too small a tank and you run dry during supply interruptions; too large and you pay for unused capacity while water sits too long. The six calculators in this category help with each decision — from finding your family's daily litre count to sizing a shared tank for a multi-story apartment block or a school campus.
These tools are designed for homeowners, building managers, property developers, water engineers, and anyone else who needs to answer the question: how much water do I need, and for how long? Each calculator shows its formula, a worked example and its sources. Benchmarks come from the WHO, US EPA WaterSense and published building design norms, and every rate can be replaced with your own metered figure or your local code's value. All six were reviewed by John Mac, PE.
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WHO Minimum
About 50 litres per person per day is the WHO level linked with low health risk; 100 L or more is "optimal access".
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Comfort Standard
Many piped homes use 100–200 L/person/day; US homes average about 310 L including garden watering (US EPA).
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Commercial Use
Office design figures are typically around 40–55 L per employee per day for washrooms and kitchens.
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Buffer Days
Match storage to the longest normal gap in supply — for example 2 days where water comes on alternate days.
🔍 How to Choose
How to Choose the Right Calculator
Start with daily usage, then work to tank size. The Daily Water Requirement Calculator should always be your first stop — it gives you the baseline consumption figure that every other calculator in this category depends on. Until you know your daily demand in litres, tank-size estimates are just guesses.
Sizing a tank for an existing home? Use the Water Tank Size for Home Calculator. It takes your daily figure and lets you dial in a refill interval (how often the mains or tanker refills your tank) to produce a recommended minimum capacity. Increase the buffer days if your area experiences frequent outages.
Trying to manage an existing supply? The How Long Will My Tank Last Calculator is your tool. Feed it your current fill volume and daily consumption, and it tells you exactly when you'll run out — useful for scheduling deliveries or managing usage during droughts.
Multi-unit residential or apartment buildings should use the Apartment Tank Size Calculator, which totals demand across all flats and lets you enter your own per-person figure. For offices, retail premises, hotels, or restaurants, the Commercial Water Tank Size Calculator applies per-person planning benchmarks for each building type, which you can replace with your own rate.
Schools, colleges, hospitals and hostels are sized per person (or per bed for hospitals) per day. The School & Institutional Calculator loads default rates from Indian design norms — 45 L for day schools, 135 L for hostels, 340–450 L per hospital bed — which you can change to your local code's figure. For a mosque or community hall, enter the daily attendance and a per-person rate from your local code.
Your Situation
Best Calculator
Key Inputs
Family home, unknown usage
Daily Water Requirement
People, lifestyle
New tank purchase for home
Tank Size for Home
Daily use, refill interval
Managing existing supply
How Long Will It Last?
Current volume, usage/day
Apartment / multi-unit block
Apartment Tank Size
Units, avg. occupancy
Office / hotel / restaurant
Commercial Tank Size
Staff count, property type
School / college / hospital
School & Institutional
Occupancy or beds, facility type
Tip: Leave a 15–20% margin above the bare storage figure — guests, dry-spell garden watering and leaks push demand above the average, and the water below the outlet can't be used. The home, apartment, commercial and daily-requirement calculators add a margin for you; the school calculator shows it in its results note.
✅ Sources & Review
How These Calculators Are Built and Reviewed
The sizing formulas are simple arithmetic — people × litres per person × days, plus a margin. What matters is the per-person figure, so each calculator shows where its defaults come from and lets you replace them with your own metered use or your local code's value. The main references are:
US EPA WaterSense — Statistics and Facts: 82 gallons (about 310 L) per person per day at home, outdoor use over 30%, and household leaks of about 180 gallons a week.
Bureau of Indian Standards — National Building Code of India 2016, Part 9, and the CPHEEO Manual on Water Supply and Treatment: per-person design norms for homes and institutions.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
Last updated
❓ FAQ
Frequently Asked Questions
How much water does a household of 4 need per day?
The WHO links about 50 litres per person per day with low health risk. Many piped homes use 100–200 litres per person, so a family of 4 typically uses 400–800 litres per day depending on lifestyle, climate and appliances; US homes average about 310 litres per person including garden watering. Use the Daily Water Requirement Calculator for a figure tailored to your household.
What size water tank do I need for my home?
Tank size depends on daily consumption, how often your tank gets refilled, and how many buffer days you want. Match the days of storage to the longest normal gap in your supply. For a family of 4 using 500 L/day, 2–3 days is 1,000–1,500 L of water, or about 1,200–1,800 L of tank with a 20% margin — in practice a 1,500 L or 2,000 L tank. Where outages are frequent, plan for more days. Use the Water Tank Size for Home Calculator to get a precise recommendation.
How do I calculate how long my water tank will last?
Divide the tank's current water volume (in litres) by your daily consumption. A 1,000 L tank used at 250 L/day lasts 4 days. If the tank is only 60% full, that's 600 L ÷ 250 L/day = 2.4 days. Use the How Long Will My Tank Last Calculator to get this instantly without the arithmetic.
Is a rooftop tank or underground tank better for a home?
Rooftop tanks deliver water by gravity — no pump needed — but they add structural load to your building and water can heat up in exposed tanks. Underground tanks stay cool (better for water quality and taste), have no height restrictions, and keep rooftops clear, but require a pump and are harder to inspect. The right choice depends on your building structure, budget and pressure needs. A full 500 L tank already weighs over half a tonne, so have the roof or stand checked before placing a tank up high.
How much water does an apartment building need?
Multiply the number of residents by per-capita daily usage (typically 150–200 L/person for urban apartments), then factor in your autonomy target. A 10-unit building with 2 residents each at 160 L/day needs 3,200 L for one day, or 9,600 L for three days. Add a margin (the Apartment Water Tank Size Calculator adds 15%) and use it to try different occupancy and usage figures.
What daily water allowance should I use for planning?
Use 80–100 L/person/day for conservative planning (basic domestic use: drinking, cooking, sanitation). Use 150–200 L/person/day for full comfort, including laundry, dishwasher, and garden. Non-residential buildings use design figures from local codes — for example India's National Building Code uses 45 L per person for offices and day schools, 135 L for hostels and 340–450 L per hospital bed. The Daily Water Requirement Calculator estimates household use by comfort level, activity and climate.
