Tankless Water Heater Sizing: GPM and Temperature Rise

To size a tankless water heater, add the GPM of fixtures likely to run together and calculate temperature rise by subtracting winter inlet-water temperature from the target outlet temperature. The correct model must deliver that combined flow at the calculated rise, not merely advertise a high maximum GPM.

This worksheet helps homeowners narrow capacity before comparing models. It does not replace the manufacturer’s installation manual, a plumbing assessment, or an electrical or gas-load calculation. After sizing demand, compare suitable models in our tankless water heater guide.

The Two Numbers That Control Capacity

Peak simultaneous flow is the total water used by fixtures that may operate together. Temperature rise is how many degrees the heater must add. A unit produces less hot-water flow when the required rise is larger, which is why one advertised GPM number cannot describe performance in every climate.

Step 1: Estimate Simultaneous Fixture Flow

List a realistic busy period rather than turning on every fixture in the house. Use measured flow where possible: collect water for a timed interval or check the fixture label. Published fixture estimates are planning values only because aerators, pressure, valves, and appliance cycles vary.

For example, one 2.0 GPM shower plus a 1.0 GPM faucet creates a 3.0 GPM planning load. If two showers commonly overlap, use their combined measured or rated flow instead.

Step 2: Calculate Temperature Rise

Use the coldest normal inlet-water temperature, not a summer reading. Subtract it from the desired outlet setting. With 50°F inlet water and a 120°F target, required rise is 70°F. The same heater will usually deliver more flow where inlet water is 70°F because the rise is only 50°F.

Step 3: Read the Performance Chart

Find the manufacturer’s table or curve showing GPM at multiple temperature rises. Locate your calculated rise, then verify that available flow meets or modestly exceeds peak demand. Do not substitute a retailer headline, BTU input, or electrical kW rating for the performance chart.

Step 4: Check the Utility Infrastructure

• Electric: confirm total amperage, dedicated breaker count, conductor size, panel spaces, and service capacity with a qualified electrician.
• Gas: confirm fuel type, meter and pipe capacity, combustion air, approved venting, condensate routing for condensing units, and clearances.
• All systems: plan isolation valves, flushing access, freeze protection where required, drain routing, permits, and service clearance.

Worked Sizing Examples

These examples illustrate the calculation; use actual fixture flow and local winter inlet conditions for a purchase decision.

Common Sizing Mistakes

• Buying from maximum GPM without noting the test temperature rise.
• Using annual-average inlet temperature instead of the coldest normal condition.
• Ignoring panel, gas-line, venting, or condensate upgrade costs.
• Oversizing for fixtures that never run together.
• Assuming a recirculation system has no effect on design or controls.

Allow for Real Operating Conditions

A small planning margin can account for normal variation in inlet temperature, fixture flow, and aging, but buying far beyond calculated demand can increase project cost without improving comfort. Discuss recirculation separately: a long return loop, integrated pump, buffer tank, crossover valve, and control schedule can change system design even when fixture GPM stays the same.

Tankless Sizing FAQ

Is a 7 GPM tankless heater enough for two showers?

It may be, but the label is incomplete. Add the actual shower flow rates and verify the heater’s output at your winter temperature rise. A model advertised at 7 GPM under warm-inlet conditions may deliver less where groundwater is cold.

Should every fixture be included?

Include fixtures and appliances that realistically overlap during the household’s busiest period. Designing for an impossible all-fixtures event can oversize the system; ignoring a routine shower-and-laundry overlap can undersize it.

Does BTU or kW replace the GPM chart?

No. Input capacity helps explain heating potential, but the model’s rated flow at a stated temperature rise is the more direct sizing check. Installation limits and minimum activation flow also matter.

Bottom Line

A useful tankless sizing statement includes both values: “We need 3.0 GPM at a 70°F rise.” Take that requirement to model performance charts and a qualified installer. Capacity, utility readiness, and installation quality should be settled before comparing apps or display features.

Sources

• U.S. Department of Energy: Sizing a New Water Heater
• U.S. Department of Energy: Tankless or Demand-Type Water Heaters