# Water heater sizing estimator

Add your estimated hot water use during the busiest hour. Enter hot water volume, not mixed tap water. Use this demand worksheet with an installer; it does not choose a tank or tankless unit.

Use the tool on the [web page](https://plumberpros.fyi/tools/water-heater-sizing). It works without JavaScript.

## How to use it

1. Enter hot water for showers and baths.
2. Enter hot water for taps.
3. Enter hot water for appliances.
4. Enter other hot water use.
5. Read the result.

## How it works

US Department of Energy, Energy Saver Guide, July 2022, water heating section, fetched 2026-11-10: https://www.osti.gov/servlets/purl/1909356 . DOE explains that simultaneous use can exceed tankless delivery and recommends comparing equipment labels.

The calculation adds only the hot water volumes you enter for the same busiest hour. It uses no assumed household size, fixture flow, temperature or appliance consumption. Input limits are interface bounds, not expected use or sizing guidance.

The sum is a planning input for the installer. It does not calculate recovery, usable storage, incoming water temperature, mixing, temperature rise or fuel capacity. Do not select equipment from this number alone.

For a tankless system, ask the installer to check simultaneous flow and temperature rise. An hourly total cannot establish a tankless flow rating. For a storage unit, ask how the model rating and recovery cover your pattern of use.

> A demand worksheet for planning. It does not size equipment for installation or replace a site assessment.

## Questions

### Can I use mixed shower water volume?

No. This worksheet asks for hot water drawn from the heater. If you cannot separate it from mixed water, ask the installer to estimate demand instead.

### Does this result tell me which tank to buy?

No. It totals your entries for a discussion with the installer. Equipment rating, recovery and site conditions still need review.