Hot Water Demand Calculator
Use this Hot Water Demand Calculator to estimate daily, peak and hourly hot water demand based on occupancy and usage per person.
Input Parameters
Persons
L/person/day
Multiplier
Hours
Calculation Results
Daily Hot Water Demand
—
L/day
Peak Demand
—
L
Peak Hourly Flow
—
L/h
Average Hourly Demand
—
L/h
Calculation Formulas
Daily Demand = Occupants × Hot Water RequirementPeak Demand = Daily Demand × Peak Demand FactorPeak Hourly Flow = Peak Demand ÷ Peak Usage PeriodAverage Hourly Demand = Daily Demand ÷ 24
Daily demand is the total usage over 24 hours. Peak demand and peak hourly flow describe the concentrated load during the busiest part of the day.
How to Use This Calculator
- Enter the number of occupants.
- Enter the hot water requirement per person per day.
- Set a peak demand factor for busy periods.
- Enter the peak usage period in hours.
- Review the demand figures and download a PDF report.
Frequently Asked Questions
How do I calculate hot water demand?
Multiply the number of occupants by the hot water used per person per day to get daily demand, then apply a peak factor to size for busy periods.
What is peak hourly flow?
It is the peak demand spread across the busiest usage period, giving an indication of the hourly hot water load a system must supply.
How much hot water does one person use?
A common planning figure is around 45–60 litres per person per day, depending on the building type and fittings.
What is a peak demand factor?
It scales the average daily demand up to reflect that usage is concentrated at certain times, such as mornings and evenings.
How does demand relate to calorifier sizing?
Peak demand drives the required storage capacity, while peak hourly flow helps assess whether storage and recovery together can meet the load.
Results are estimates based on the values entered and the calculation method shown. Actual system requirements may vary depending on operating conditions, demand patterns, equipment configuration and applicable project requirements.