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Tankless water heater sizing worksheet
Work out the hot-water flow and temperature rise a heater must deliver before you compare models.
This worksheet does not choose a heater for you. It calculates the temperature rise and total hot-water flow your heater must deliver at the same time. Compare that result with the exact model’s performance table.
Enter only fixtures likely to run together. Use measured hot-water flow where possible, or the fixture maker’s stated gallons per minute. Do not enter the home’s total plumbing capacity.
Sizing target
3.5 GPM at a 70°F rise
Give this target to the installer. In the exact heater’s current performance table, find the closest temperature rise. The model’s available flow at that rise should meet or exceed your total.
How to use this result
Use it to screen models, then ask the installer to confirm the coldest incoming-water temperature, actual fixture flows and whether the home can supply the required gas or electricity. A product’s largest advertised GPM figure may be measured at a much smaller temperature rise than your home needs.
What this result does not cover
Have the contractor verify gas or electrical capacity, venting, minimum activation flow, available water pressure, hardness and other water-quality limits. Inlet temperature and simultaneous use should reflect the coldest relevant season and your actual fixtures.
Method: required temperature rise = desired outlet temperature − cold inlet temperature. Simultaneous flow is the sum of the hot-water flows entered. This follows the U.S. Department of Energy sizing method cited in the companion guide.
Read the complete sizing and private-well guide →Save the details for your next conversation.
Use the matching pack to collect the questions, model numbers and costs you still need to confirm.
Calculations checked October 1, 2026. These tools help you organize estimates and questions. They cannot inspect your home, diagnose water or approve a product. How we check our work.