Water Heater Size Calculator
Estimate peak-hour hot water demand to help compare residential storage-tank water heaters by First-Hour Rating (FHR).
Estimate how many times each activity occurs during your busiest hour. Results update automatically; the per-use values are planning assumptions, not universal measurements.
Peak-Hour Hot Water Demand
Calculated from the activity counts and generalized gallons-per-use assumptions. FHR is not the tank's storage capacity.
How to Use the Calculator
- Enter the number of people in your household (1–30).
- Enter the number of each hot-water activity likely to happen during your busiest hour.
- Review the estimated peak-hour demand and compare it with the First-Hour Rating (FHR) on product labels.
- Use Reset Calculator to restore the example values.
Assumptions and Limitations
Illustrative values from the DOE Building America technical fact sheet: shower 15 US gal/use, bath 20, shaving 2, washing hands and face 2, shampooing hair 4, hand dishwashing 2, automatic dishwasher 14, preparing food 5, and clothes washing 32. This is an older example table, not a current universal standard. Actual hot-water draw varies with fixture flow, duration, appliance model and cycle, mixing temperature, and household habits.
How to interpret the result: the calculator adds the assumed hot-water use during one busy hour. The result is a preliminary demand estimate to compare against a product's First-Hour Rating (FHR), not the tank's storage capacity and not a guarantee of adequate supply. FHR is the maximum hot-water volume a storage water heater can supply during an hour beginning with the heater fully heated; ENERGY STAR describes its test-method outlet temperature as 125°F. See ENERGY STAR's FHR definition and the DOE Building America technical fact sheet for a separate, older example of peak-hour sizing assumptions. Published examples use different per-use values, so don't mix tables or treat these defaults as a code requirement.
This version covers residential storage-tank heaters only. It does not size tankless systems or model temperature rise, recovery rate, fuel type, local codes, or manufacturer-specific performance. Check the product's FHR and manufacturer's sizing guidance; consult a qualified plumber when needed.
