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IPC Ch.5
Validated against the standard.
PDF export
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$2.99 Pro
Lifetime unlock. No subscriptions.
How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact IPC Ch.5 reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
Size a water heater too small and you run out of hot water; too big and you waste energy and money. This app sizes both storage (tank) and tankless heaters from real demand — peak-hour usage for tanks, and simultaneous flow plus temperature rise for tankless.
How it works
- Storage tanks — match the first-hour rating to the household's peak-hour hot-water demand.
- Tankless — size by the GPM of fixtures running at once and the required temperature rise from incoming cold to setpoint.
Worked example
A tankless unit must raise 40°F groundwater to 120°F — a 80°F rise. Two showers at 2 GPM each = 4 GPM. A unit rated for at least 4 GPM at an 80°F rise is required; smaller units would deliver less flow at that rise.
Related calculators
Confirm supply with the water pipe sizing calculator and totals with the fixture unit calculator.
Water heater sizing FAQ
What is the first-hour rating?
For storage heaters, the first-hour rating is how much hot water the unit can deliver in a busy hour, combining tank volume and recovery. Match it to the household's peak-hour demand.
How is a tankless heater sized?
By the simultaneous flow in GPM and the temperature rise needed. Colder incoming water means a bigger rise, which lowers the flow a given unit can supply.
Tank and tankless both?
Yes. The app sizes storage tanks by peak-hour demand and tankless units by flow and temperature rise, so you can compare options.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Water Heater Sizing on your phone
Free with rewarded ads · $2.99 lifetime Pro · iOS & Android. Works 100% offline on the job site.