Tankless GPM calculator
Domestic hot water · Planning tool
Change the example inputs to match your home. Results stay in this browser.
Tankless capacity depends on flow and the amount the heater must warm the water. A unit's headline maximum gallons per minute is insufficient without the temperature rise associated with that figure. Start with fixtures that can realistically run together during your busiest period.
Add simultaneous demand
The calculator multiplies the number of each running fixture by its entered GPM and adds the results. The example flows are assumptions: 2 GPM per shower, 0.75 for a bathroom faucet, 1.5 for a kitchen faucet, 1 for a dishwasher, and 2 for a clothes washer. Replace them with measured hot-side flow or model-specific data.
Appliance demand can be intermittent, and mixed shower water includes cold water. Treating the full fixture flow as heated water creates a conservative planning case. A more detailed design can account for the actual hot fraction, appliance intake, and realistic overlap. Do not subtract demand simply because you hope fixtures will never run together.
Calculate temperature rise
Temperature rise = requested heater outlet temperature − cold inlet temperature.
At a 45°F inlet and 120°F outlet, the required rise is 75°F. At a 60°F inlet, the same outlet requires 60°F. Ask for locally verified cold-season inlet temperature so a warm-weather estimate does not overstate winter capacity.
ENERGY STAR's water-heater criteria define GPM in relation to a specified temperature rise. Compare your result with the actual manufacturer's performance curve at your rise, rather than transferring a maximum-flow claim between conditions. Our tankless GPM guide explains that comparison.
Understand the heat result
The approximate water-side load is 500 × GPM × temperature rise, in BTU per hour. Dividing by 3,412.142 converts that heat rate to thermal kilowatts. The factor 500 approximates water's density, specific heat, and minutes per hour.
This is heat delivered to water. It is not a gas input rating, an electrical breaker recommendation, or annual energy use. Gas input must account for actual thermal efficiency. An electric unit requires a professional electrical-load and service assessment; a thermal-kW result alone does not size its circuits.
Turn the result into a design check
Ask the installer to verify the model's curve, pressure, activation flow, gas supply, venting, electrical capacity, and any recirculation strategy. For system tradeoffs, read tankless versus tank and tankless water heaters. Use the cost calculator after the installation scope is clear.