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:
HWD = N * U
The calculator can also rearrange the same formula to solve for the missing value:
N = HWD / U
U = HWD / N
For unit conversions, the calculator uses:
Liters = Gallons * 3.78541
Gallons = Liters / 3.78541
- 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 leave Hot Water Demand blank, the calculator multiplies the number of people by the daily hot water usage per person.
If you leave Number of People blank, it divides total daily demand by usage per person.
If you leave Hot Water Usage per Person per Day blank, 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
| 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.
HWD = 4 * 50 = 200 liters/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.
U = 120 / 5 = 24 gallons/person/day
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 can still be useful when you are estimating capacity for shared buildings, partial occupancy, or average use across many days.
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 gallon = 3.78541 liters. After calculating, it converts the result back to the unit you selected.
