Water Heater Sizing Calculator (Peak-Hour Demand)
Enter how many times each fixture is used in the household’s busiest hour and get the peak-hour hot water demand — the DOE first-hour-rating (FHR) approach used to size a storage water heater. The result updates as you type.
Water Heater Sizing Calculator
Count expected uses in the single busiest hour of the day, not the whole day.
Choose a storage water heater with a First-Hour Rating (FHR) at or above this number. Tankless units are not rated by FHR — they are sized by peak simultaneous flow rate (GPM) and required temperature rise, so the average GPM above is a rough reference only, not a tankless sizing spec. This is a planning estimate; confirm the model’s FHR or GPM/temperature-rise rating with the manufacturer and your installer.
How peak-hour demand is calculated
The DOE first-hour-rating worksheet assigns a typical gallon draw to each fixture use, then sums the uses expected in the single busiest hour.
| Use | Gal per use |
|---|---|
| Shower | 10 |
| Bath | 20 |
| Shaving | 2 |
| Hand/face wash | 2 |
| Hair shampoo | 4 |
| Hand dishwashing | 4 |
| Automatic dishwasher | 6 |
| Food preparation | 5 |
| Clothes washer | 7 |
| Step | Example: 2 showers + 1 dishwasher + 1 clothes washer |
|---|---|
| Showers | 2 × 10 = 20 gal |
| Automatic dishwasher | 1 × 6 = 6 gal |
| Clothes washer | 1 × 7 = 7 gal |
| Peak-hour demand | 20 + 6 + 7 = 33 gal |
Storage vs. tankless
A storage water heater is matched to your peak-hour number directly — pick a model with an FHR at or above the total. A tankless unit has no tank to draw down, so it is sized differently: by the maximum GPM you will ever call for at once (all fixtures running together) and the temperature rise needed from your groundwater temperature to your setpoint. A high peak-hour gallon total does not automatically mean you need a bigger tankless unit — it means you need enough simultaneous GPM at the temperature rise your climate requires.
Plumbing Estimate & Invoice Templates
Turn the sizing decision into a priced, itemized proposal for the client — materials, labor, and markup, auto-totaled.
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Frequently asked questions
What is First-Hour Rating (FHR)?
FHR is the amount of hot water a storage water heater can deliver in one hour, starting full, factoring in tank size and recovery rate. It is printed on the Energy Guide label and is the number you match against your calculated peak-hour demand.
How do I estimate my peak hour of use?
Think about the busiest single hour in your household — typically a morning routine with overlapping showers, or an evening with dishwasher and clothes washer running alongside showers — and count fixture uses in that one hour, not the full day.
Does this work for sizing a tankless water heater?
Not directly. Tankless units are sized by peak simultaneous GPM and required temperature rise, not FHR. Use the peak-hour gallon total as a rough reference for how much simultaneous demand your household creates, then size the tankless unit on GPM and temperature rise with the manufacturer’s sizing chart.
Is this water heater calculator free?
Yes — free, no sign-up. Enter expected peak-hour fixture uses and read the total demand and a rough average GPM. Estimates only; confirm the unit’s FHR or GPM/temperature-rise rating before buying.