Enter occupancy, institution type, and backup days to calculate the exact daily water demand and minimum tank capacity — in litres, US gallons, and UK gallons.
Select institution type, enter occupancy, set backup duration
1. Select Institution Type
School (day scholars): 45 litres per person per day — drinking, toilets and cleaning; the Indian design norm for day schools.
2. Enter Occupancy & Usage
Occupancy
persons
Litres / Person / Day
L/p/d
Backup / Storage Days
1 day
1 day234567 days
⚠ Please enter a valid occupancy (greater than zero) and a valid L/person/day value.
WaterTankCalculator.com — School/Institutional Tank Results
Your Results
Minimum Tank Capacity Required
—
Litres (L)
Daily Demand
— L/day
Backup Days
— days
US Gallons
— gal
UK Gallons
— gal
Cubic Metres
— m³
Water Weight
— kg
Capacity Visualisation
📖 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.
Select your institution type
Choose from school, college, hospital, hostel, office, or factory. Each type loads a default per-person rate from Indian design norms (the National Building Code and the CPHEEO manual). Other countries' codes use different figures — override it with your local code's value or your own metered data.
Enter the total occupancy
Count all regular occupants — students plus staff for schools and colleges, all employees for offices and factories. For hospitals, enter the number of beds: the hospital rate is per bed and already covers staff, kitchen and laundry. Use the busiest day's figure.
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.
Set backup days
The slider sets how many days of storage you need. Match it to the longest normal gap in supply — remember weekends and holidays when nobody is on site to manage refills — plus any outages you want to ride out.
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: The result is the minimum water to store, with no margin. The results note also shows the figure with a 15–20% margin, a common rule of thumb — a sports day at a school or a busy day at a hospital can push demand above the average, and the water below the outlet cannot be used.
📐 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
A day's supply is a common minimum, with more where supply is unreliable; check your local code, which may set storage requirements. The formula is the same for every institution type — only the per-person rate changes.
Standard Per-Capita Water Usage Rates
Institution Type
Default L/Person/Day
Basis
Notes
School (day scholars)
45 L
Indian norm for day schools (NBC / CPHEEO)
Drinking + sanitation + cleaning
College / University
135 L
Indian norm for boarding / hostels
Suits residential campuses; a day-only college is closer to 45 L
Hospital (general)
340 L
NBC India, hospitals up to 100 beds
Per bed, incl. laundry; 450 L per bed above 100 beds
Hostel / Boarding
135 L
Indian norm for hostels / boarding schools
Includes bathing, laundry
Office / Commercial
45 L
NBC India 2016
Sanitation + drinking only
Factory / Industrial
30 L
NBC India 2016, no bathrooms
45 L where workers shower; add process water separately
Note: These are design values from Indian norms, used here as defaults. Codes elsewhere (for example CIBSE Guide G in the UK) publish different figures, and actual consumption varies with climate, building age and fixture efficiency. Use your local code or metered data where available.
Common institution sizes and the minimum storage at the default rates with a 2-day backup (no margin added; the 200-bed hospital uses the 450 L rate for hospitals over 100 beds):
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
450
90,000
180,000
180
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.
🎯 When To Use
When to Use This Calculator
🏗️
New Building Projects
Determine tank capacity before construction begins so the tank platform and structural slab can be designed correctly from day one.
📈
Expansion Planning
When a school enrols more students or a hospital adds beds, check whether the existing tank can handle the increased daily demand.
⚡
Intermittent Supply Areas
In areas with unreliable municipal supply, increase the backup days to cover the longest gap you expect between supplies.
📋
Regulatory Compliance
Many building approvals require a documented water storage calculation. The results note shows the full working, which you can copy into your design record alongside your local code's rate.
🧮 Worked Example
Worked example: a day school with 650 students and 50 staff
A day school has 650 students and 50 teachers and staff. Mains water comes on weekdays only, and the school wants storage for 2 days at the default 45 litres per person.
Occupancy: 650 + 50 = 700 people.
Daily demand: 700 × 45 = 31,500 L/day.
Minimum storage: 31,500 × 2 = 63,000 L (63 m³, about 16,640 US gallons).
With a 20% margin: 63,000 × 1.2 = 75,600 L — for example two tanks of about 38,000 L, so one can be cleaned during the holidays while the other stays in use.
❓ FAQ
Frequently Asked Questions
How do I calculate water tank size for a school?
Multiply the total number of students and staff by the per-person rate — 45 litres is the Indian design norm for day schools; use your local code's figure if it differs — then multiply by the number of backup days you want, and add a margin. 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?
Indian design norms use 45 litres per student per day for day schools, and 135 litres for boarding schools and hostels because they include bathing, laundry and full domestic use. Other countries' codes use their own figures. For comparison, the WHO links about 50 litres per person per day with low health risk for whole-day household use, and WHO guidance for schools in low-resource settings sets far lower minimums for drinking and handwashing.
How many days of backup storage should a school tank hold?
With a reliable mains supply, about 1 day is a common minimum and 2 days a common choice. Where supply is intermittent, size for the longest gap you expect — and think about weekends and holidays, when nobody may be on site to manage refills. Keep water age in mind: a large tank that sits through a long holiday should be flushed or cleaned before the term starts.
How much water does a hospital need per bed per day?
India's National Building Code uses 340 litres per bed per day for hospitals with up to 100 beds and 450 litres per bed above 100 beds, including laundry. Specialised facilities such as dialysis units can need much more. Process water for sterilisation and HVAC, and fire water, must be calculated separately. In low-resource settings, the WHO's minimum standards for health-care facilities are far lower — those are survival minimums, not design figures.
Should I include staff in the occupancy count?
Yes for schools, colleges, offices and factories — count everyone who uses water on site on a normal day: students and teachers, administrative staff and support workers. Hospitals are the exception: their rate is per bed and already includes staff, so enter beds only.
What is the water requirement per person per day in an office?
India's National Building Code uses 45 litres per person per day for offices, covering toilets, washing and drinking. A canteen with cooking adds more, and water-efficient fittings can cut use well below the design figure — green building rating systems such as LEED and IGBC reward measured reductions.
How heavy will a full institutional water tank be?
Water weighs about 1 kg per litre. A 72,000-litre store for a medium school holds 72 tonnes of water, plus the weight of the tanks themselves. Volumes like this usually go in an underground or ground-level tank, with a smaller overhead tank; any rooftop tank must be checked by a structural engineer, since the load per square metre is far above what ordinary roof slabs are designed for.
Can I use this calculator for a university or large college campus?
Yes. Yes. For a residential campus, "College" loads 135 L/person/day (the hostel rate); for a day-only campus, a figure near the 45 L day-school rate is more realistic — type it in, or size day users and residents separately and add the results. Labs, pools, kitchens and irrigation should be added on top. 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.
✅ Sources & Review
Sources and review
The sizing formula is simple arithmetic; the default rates come from Indian design norms, and the comparisons from these references:
Bureau of Indian Standards — National Building Code of India 2016, Part 9 (Plumbing Services), and IS 1172: per-person water requirements for buildings other than residences (offices 45 L, day schools 45 L, boarding schools and hostels 135 L, hospitals 340 L per bed up to 100 beds and 450 L above, factories 30–45 L).
CPHEEO (Government of India) — Manual on Water Supply and Treatment: per-capita design norms for water supply.
Water weight and unit conversions: 1 L of water ≈ 1 kg; 1 US gallon = 3.785 L, 1 UK gallon = 4.546 L (NIST SP 811).
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.
