Daily Water Demand Calculator
How the formula works
The calculator multiplies the number of users by a typical water use rate, adds other demands and an allowance for losses.
Formulas
Base demand = Number of units × Water use per unit per day
Landscaping = Area (m²) × 5 L/m²/day
Total = (Base + Landscaping + Other) × (1 + Allowance %)
Flushing water = Total × Flushing share %
Domestic water = Total − Flushing water
Storage volume = Total × Days of storage
Where:
- Unit = person, student, bed, seat, worker or guest
- Allowance covers leakage, wastage and small unplanned uses
- 1 m³ = 1,000 litres; 1 US gallon = 3.785 litres
Typical values (L per unit per day)
Residential dwelling 135, large residential 200, hostel 135, office 45, day school 45, residential school 135, hospital up to 100 beds 340, hospital above 100 beds 450, hotel 180, restaurant 70, cinema 15, factory 45. These follow common NBC India (IS 1172) practice. Other countries use different values, and the local code is final. Use the Custom option to enter your own rate.
How to use the calculator: select the building type, enter the number of users, choose the allowance, and add the optional items if needed. Then click Calculate.
Important notes: fire water is not included. Cooling tower make-up, laundry and swimming pool top-up can be entered in “Other demand”.
Worked example
Example 1: Residential society
200 persons at 135 L/person/day, 500 m² garden, 10 % allowance, 33 % flushing, 1.5 days storage.
Base = 27,000 L. Landscaping = 2,500 L. Allowance = 2,950 L. Total = 32,450 L/day (32.45 m³/day). Flushing = 10,709 L. Domestic = 21,742 L. Storage = 48.68 m³.
Example 2: Office with cooling tower
350 persons at 45 L/person/day, 1,500 L/day other demand, 15 % allowance, 1 day storage.
Base = 15,750 L. Plus 1,500 L = 17,250 L. Allowance = 2,588 L. Total = 19,838 L/day (19.84 m³/day). Average flow = 13.8 L/min. Storage = 19.84 m³.
Example 3: Large hospital
250 beds at 450 L/bed/day, 10 % allowance, 25 % flushing, 1 day storage.
Base = 112,500 L. Allowance = 11,250 L. Total = 123,750 L/day (123.75 m³/day). Flushing = 30,938 L. Domestic = 92,813 L.
Example 4: Cinema with custom rate
800 seats at 15 L/seat/day, 5 % allowance.
Base = 12,000 L. Allowance = 600 L. Total = 12,600 L/day (12.6 m³/day), or 3,329 US gallons/day.
Common mistakes
- Using the total built-up area instead of the real number of users.
- Mixing up a per-person rate with a per-bed or per-seat rate.
- Forgetting garden watering, cooling tower make-up and laundry.
- Not adding an allowance for leakage and wastage.
- Using a single rate for all parts of a mixed-use building. Calculate each part separately.
- Counting fire water inside the daily demand. Fire reserve is a separate storage.
- Applying an Indian rate where the local code gives a different value.
- Ignoring visitors, staff and peak occupancy.
- Sizing the tank for one day only when the supply is unreliable.
Frequently asked questions
What is daily water demand?
It is the total water a building uses in one day for drinking, washing, flushing, cleaning and other uses.
How much water does one person use per day?
About 135 litres for a typical Indian home under NBC practice. Other countries and building types use different values.
What is the water demand of an office?
A typical value is 45 litres per person per day. Use your local code if it is different.
Why add an allowance?
Real systems lose water through leakage and wastage. A 5 to 15 % allowance is common.
What is the flushing share?
It is the part of the total used for toilet flushing. It matters when a separate flushing water line or recycled water is planned.
How do I convert litres to cubic metres?
Divide litres by 1,000. For example, 32,450 L is 32.45 m³.
How much storage do I need?
Many designs store one day of demand. The right value depends on supply reliability and local rules.
Is fire water included?
No. Fire water is a separate reserve and should be added to the tank design.
How do I size the tank after this?
Use the daily demand with the Water Tank Size Calculator to get tank dimensions.
Can I use my own water use rate?
Yes. Choose Custom in the building type list and enter litres per unit per day.
How is landscaping water counted?
This calculator uses 5 litres per square metre per day. Use “Other demand” for a different figure.
Is this a final design value?
No. It is a planning estimate. Final design should follow the local plumbing code.