📖 How To Use
How to Use This Calculator
Sizing a water tank for a school or institution is straightforward when you know three things: who uses it, how much they use per day, and how many days of supply you need in reserve.
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Select your institution type
Choose from school, college, hospital, hostel, office, or factory. Each type loads a standard per-capita usage rate based on WHO and CPHEEO (India) norms. You can override this with your own figure if you have measured data.
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Enter the total occupancy
Count all regular occupants — students plus staff for schools, patients plus staff for hospitals, all employees for offices. If occupancy varies, use the peak-day figure.
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Adjust the L/person/day if needed
The default fills automatically based on your institution type. Override it if your local standard, measured flow data, or regulatory requirement differs.
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Set backup days
The slider sets how many days of storage you need — useful for locations with intermittent municipal supply. Most urban institutions use 1–2 days; rural or unreliable-supply locations often need 3–5 days.
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Read your results
The primary result is the minimum tank capacity in litres. Secondary results show daily demand, US/UK gallons, cubic metres, and full-load water weight for structural planning.
Tip: Add a 15–20% safety buffer on top of the calculated minimum. Real consumption is never perfectly uniform — a sports day at a school or an emergency at a hospital can spike demand well above the daily average.
📐 The Formula
Institutional Water Tank Sizing Formula
The calculation is a straightforward two-step multiplication:
Daily Demand (L) = Occupancy × Litres per Person per Day
Tank Capacity (L) = Daily Demand × Backup Days
Example — 800-student school, 2-day backup:
Daily Demand = 800 × 45 = 36,000 L/day
Tank Capacity = 36,000 × 2 = 72,000 Litres
Most water supply guidelines recommend sizing for a minimum of 1 day's supply, with 2–3 days recommended for reliability. The formula is the same regardless of institution type — only the per-capita rate changes.
Standard Per-Capita Water Usage Rates
| Institution Type | Standard L/Person/Day | Source / Standard | Notes |
|---|
| School (day scholars) | 45 L | CPHEEO Manual | Drinking + sanitation + cleaning |
| College / University | 135 L | CPHEEO Manual | Higher lab and facility use |
| Hospital (general) | 340 L | WHO / NBC India | Per bed; includes clinical use |
| Hostel / Boarding | 135 L | CPHEEO Manual | Includes bathing, laundry |
| Office / Commercial | 45 L | NBC India 2016 | Sanitation + drinking only |
| Factory / Industrial | 30 L | NBC India 2016 | Minimum; add process water separately |
Note: These are minimum design values. Actual consumption varies with climate, building age, and fixture efficiency. Use metered data where available.
Volume Unit Conversions
US Gallons = Litres ÷ 3.78541
UK/Imperial Gallons = Litres ÷ 4.54609
Cubic Metres (m³) = Litres ÷ 1,000
Water Weight (kg) = Litres × 1
Water Weight (lbs) = Litres × 2.20462
📏 Reference Data
Quick-Reference: Typical Institutional Tank Sizes
Common institution sizes and the minimum tank capacity at standard usage rates with a 2-day backup:
| Institution | Occupancy | L/p/d | Daily (L) | 2-Day Tank (L) | m³ |
|---|
| Primary School | 300 students | 45 | 13,500 | 27,000 | 27 |
| Secondary School | 800 students | 45 | 36,000 | 72,000 | 72 |
| College | 1,500 students | 135 | 202,500 | 405,000 | 405 |
| Hospital (50 beds) | 50 beds | 340 | 17,000 | 34,000 | 34 |
| Hospital (200 beds) | 200 beds | 340 | 68,000 | 136,000 | 136 |
| Office (250 staff) | 250 persons | 45 | 11,250 | 22,500 | 22.5 |
| Factory (500 workers) | 500 persons | 30 | 15,000 | 30,000 | 30 |
Note: For hospitals and medical facilities, the above figures cover domestic use only. Process water, sterilisation, HVAC, and fire suppression systems add significantly to the total — always size these separately with a qualified MEP engineer.
❓ FAQ
Frequently Asked Questions
How do I calculate water tank size for a school?
Multiply the total number of students and staff by 45 litres (the CPHEEO standard for day-scholar schools), then multiply by the number of backup days you want. A 600-person school needing 2 days' backup needs 600 × 45 × 2 = 54,000 litres of storage.
What is the standard water requirement per student per day?
The CPHEEO Manual (India) specifies 45 litres per student per day for day-scholar schools. Residential schools and hostels use 135 L/student/day because they include bathing, laundry, and full domestic use. The WHO minimum for basic services is 50 litres per person per day.
How many days of backup storage should a school tank hold?
In urban areas with reliable municipal supply, 1 day is the practical minimum and 2 days is the typical recommendation. In peri-urban or rural areas with intermittent supply — or anywhere the mains supply is cut off on weekends — size for 3–5 days. Never go below 1 day regardless of how reliable the supply appears.
How much water does a hospital need per bed per day?
The National Building Code of India and WHO guidelines specify 340 litres per bed per day for general hospitals. This covers patient care, staff, kitchen, laundry, and cleaning. Specialised facilities (ICU-heavy, maternity, dialysis centres) may need 450–600 L/bed/day. Process water for sterilisation and HVAC must be calculated separately.
Should I include staff in the occupancy count?
Yes — always count everyone who uses water on-site during peak hours. For schools that means students and teachers, administrative staff, and support workers. The per-capita rates already account for a typical mix of users, so simply total all regular occupants and enter that number.
What is the water requirement per person per day in an office?
NBC India 2016 specifies 45 litres per employee per day for offices, covering sanitation and drinking. If the office has a cafeteria with cooking facilities, add 10–15 L/person/day on top of this base figure. Green Building (LEED/IGBC) credits are available for installations that reduce per-capita use below 35 L/day.
How heavy will a full institutional water tank be?
Water weighs exactly 1 kg per litre. A 72,000-litre tank for a medium school holds 72,000 kg of water plus the tank shell weight (typically 2–8 tonnes for concrete, 200–600 kg for polyethylene). Rooftop installations must be verified by a structural engineer — the load per square metre is almost always above residential slab ratings. Always use a load-distributing frame.
Can I use this calculator for a university or large college campus?
Yes. Select "College" for the 135 L/person/day rate (which covers labs, libraries, canteens, and sports facilities), enter the total peak-day headcount including residential students, and set backup days to at least 2. For very large campuses (5,000+ occupants), you may also want to distribute storage across multiple tanks at different locations rather than a single central tank.