Free Tool
Free water heater size calculator
Enter the number of people and bathrooms to get a recommended tank size in gallons and the tankless flow you need in GPM. Free, no signup.
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Size it to the busy hour
A water heater fails the family at one moment: the morning everyone showers at once. Sizing it right means planning for that peak, not the average.
A tank is sized by storage. It has to hold enough hot water for your busiest hour, which tracks with the number of people in the home. Too small and it runs cold halfway through the morning; too big and you pay to keep water hot you never use.
A tankless unit is sized by flow instead, the gallons per minute it can heat while several fixtures run at the same time. That grows with the number of bathrooms, since more baths means more chance of simultaneous demand.
One thing to watch on tankless: cold incoming water cuts the real flow. A unit rated for plenty of GPM in a warm climate delivers less in a cold one, so size up in the north. Confirm the final model with a plumber, since fuel type, venting, and the incoming water temperature all matter.
Water Heater Size Calculator questions
What size water heater do I need?
For a tank, size by the people in the home so it covers the peak hour. For tankless, size by flow in GPM, which grows with the number of bathrooms. The calculator gives both.
How many gallons for a family of four?
Often around 50 gallons for a tank, but it depends on your usage patterns. The calculator suggests a starting size.
Does climate affect tankless sizing?
Yes. Colder incoming water lowers the real flow a tankless unit delivers, so size up in a cold climate.
Is this really free?
Yes. No email, no signup, and nothing you type leaves your browser.
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Disclaimer: These free tools and templates are provided for general information and estimating purposes only. All results — including quantities, material and cost estimates, load, sizing, and span figures, scores, and any guidance or warnings — are rough approximations that may not fit your specific project, site conditions, local building codes, or current prices. They are not engineering, architectural, structural, legal, financial, or other professional advice, and are not a substitute for a licensed engineer, architect, contractor, or other qualified professional. Always confirm quantities, sizing, and structural, electrical, mechanical, and code requirements with a licensed professional and your local authority having jurisdiction before purchasing materials or performing any work. Werx makes no warranty as to accuracy and accepts no liability for any decision made using these tools. Use at your own risk. See our Terms of Service.