Calculate hot water demand, number of people, or usage per person from any two values with liters/day and gallons/day conversions.
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Hot Water Demand Formula
The basic relationship used by the hot water demand calculator is:
The calculator can also rearrange the same formula to solve for the missing value:
For unit conversions, the calculator uses:
- HWD = total hot water demand per day
- N = number of people
- U = hot water usage per person per day
- Liters = volume in liters
- Gallons = volume in U.S. gallons
If you select Daily hot water demand, the calculator multiplies the number of people by the daily hot water usage per person.
If you select Equivalent number of people, it divides total daily demand by usage per person.
If you select Daily use per person, it divides total daily demand by the number of people.
When gallons are selected, the calculator converts the input to liters for the calculation, then converts the result back to the selected output unit.
Typical Daily Hot Water Use by Activity
The ranges below are illustrative planning assumptions, not measured requirements for every household. Hot-water volume depends on fixtures, behavior and the temperature basis. Daily totals do not replace peak-hour demand and recovery checks for heater sizing.
| Activity | Typical Hot Water Use | Notes |
|---|---|---|
| Shower | 25 to 60 liters per person | Depends on shower length and flow rate |
| Hand washing and bathroom sink use | 5 to 15 liters per person | Varies with frequency of use |
| Dishwashing by hand | 10 to 30 liters per use | Can be higher if water runs continuously |
| Clothes washing | 20 to 60 liters per load | Only applies if the washer uses hot water |
Common Hot Water Demand Estimates per Person
| Usage Level | Liters per Person per Day | Gallons per Person per Day |
|---|---|---|
| Low use | 30 to 40 L | 8 to 11 gal |
| Moderate use | 45 to 65 L | 12 to 17 gal |
| High use | 70 to 100 L | 18 to 26 gal |
Example Calculations
Example 1: Calculate hot water demand
You have 4 people, and each person uses 50 liters of hot water per day.
The total hot water demand is 200 liters per day.
Example 2: Calculate usage per person
A household uses 120 gallons of hot water per day, and there are 5 people.
The hot water usage per person is 24 gallons per day.
FAQ
What does hot water demand mean?
Hot water demand is the total amount of hot water needed over a set time period. In this calculator, it is measured per day. For example, if 3 people each use 50 liters of hot water per day, the total demand is 150 liters per day.
Should the number of people be a whole number?
Usually, yes. For a home or apartment, the number of people is normally a whole number. A decimal result represents equivalent average occupancy, useful for partial occupancy or averages across many days. It is not a rounded headcount or a complete water-heater sizing recommendation.
Why does the calculator convert gallons to liters?
The calculator uses liters as the base unit for the internal calculation. If you enter gallons, it converts gallons to liters first using 1 U.S. liquid gallon = exactly 3.785411784 liters. After calculating, it converts the result back to the unit you selected.
