Water Heater Size Calculator
Tank or tankless size for your home.
Enter your values
Enter the household size you are working with.
Enter the bathrooms you are working with.
Enter the peak simultaneous fixtures you are working with.
Enter the flow per fixture in gpm.
Enter the temperature rise in °F.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Recommended tank size
- 70 gallons
- Tankless flow needed
- 4.4 gpm
- Required heating capacity
- 154,000 BTU/h
- First hour rating target
- 112 gallons
- Peak demand
- 264 gallons/hour
Quick answer
Tank heaters are sized on first hour rating and household size; tankless units are sized on peak simultaneous flow.
At a glance
| What it does | Tank or tankless size for your home. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Household size, Bathrooms, Peak simultaneous fixtures, Flow per fixture, Temperature rise |
| Main output | Recommended tank size |
| Cost | Free — no sign-up, no download |
How to use the Water Heater Size Calculator
- 1Enter household size and bathroom count.
- 2Estimate how many fixtures run at once.
- 3Set the temperature rise for your climate.
Inputs explained
- Household size
- Enter the household size you are working with.
- Bathrooms
- Enter the bathrooms you are working with.
- Peak simultaneous fixtures
- Enter the peak simultaneous fixtures you are working with.
- Flow per fixture(gpm)
- Enter the flow per fixture in gpm.
- Temperature rise(°F)
- Enter the temperature rise in °F.
Worked example
Using the values the calculator loads with:
Inputs
- Household size4
- Bathrooms2
- Peak simultaneous fixtures2
- Flow per fixture2.2 gpm
- Temperature rise70 °F
Results
- Recommended tank size70 gallons
- Tankless flow needed4.4 gpm
- Required heating capacity154,000 BTU/h
- First hour rating target112 gallons
- Peak demand264 gallons/hour
Frequently asked questions
Tank or tankless?
Tankless suits steady demand and saves standby losses; tanks handle short heavy peaks better.
What is first hour rating?
The gallons a tank can deliver in one hour starting fully hot — the number that really matters.
What do I need to enter into the Water Heater Size Calculator?
Just 5 values: household size, bathrooms, peak simultaneous fixtures, flow per fixture, temperature rise. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Water Heater Size Calculator work out the answer?
Tank heaters are sized on first hour rating and household size; tankless units are sized on peak simultaneous flow. The working is shown step by step so you can verify the result by hand.
Is the Water Heater Size Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.