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Calculation Machine

Water Heater Size Calculator

Tank or tankless size for your home.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
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.

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Household size4
  • Bathrooms2
  • Peak simultaneous fixtures2
  • Flow per fixture2.2 gpm
  • Temperature rise70 °F

Result

  • Recommended tank size70 gallons
  • Tankless flow needed4.4 gpm
  • Required heating capacity154,000 BTU/h
  • First hour rating target112 gallons
  • Peak demand264 gallons/hour

How to use the Water Heater Size Calculator

  1. 1Enter household size and bathroom count.
  2. 2Estimate how many fixtures run at once.
  3. 3Set the temperature rise for your climate.

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.

About this calculator