Tankless water heater sizing: how many GPM do you need?

One GPM number on the box won't size a tankless heater. The unit has to deliver your flow and lift the water from its incoming temperature to shower temperature at the same time, and the colder the water arrives, the harder that gets. That's why the Department of Energy sizes demand heaters on simultaneous flow and temperature rise together. Get those two numbers for your house, read the maker's chart with them, and let a qualified installer confirm the pick and the install.

Affiliate disclosureAs an Amazon Associate we earn from qualifying purchases made through links on this page. Commissions never change our ratings.

Flow is an instantaneous demand

GPM means US gallons per minute. It describes flow at a moment, not the amount used over a day and not the volume stored in a tank. List the hot-water fixtures that realistically operate together and obtain their flow information. A succession of showers may create a different capacity problem from several showers running simultaneously, even if daily water use is similar.

Our calculator assigns illustrative flow values to selected fixtures so you can explore the arithmetic.

• Those defaults are not measurements of your plumbing and are not model-specific appliance requirements.
• Replace assumptions with actual information before buying.
• A dishwasher's intermittent demand also should not be confused with a continuous shower load.
• Tell the installer which combinations matter and which can be scheduled separately.
• Deliberately avoiding simultaneous high-demand uses may change a feasible shortlist, but the site cannot decide that compromise for your household.

Temperature rise connects climate to capacity

Temperature rise is the desired outlet temperature minus the inlet temperature. The Department of Energy gives examples using a 50 °F inlet and a 120 °F outlet, while emphasizing the need to determine the actual application. Our calculator instead fixes an illustrative 105 °F outlet and labels its regional inlet figures as assumptions. It is a scenario tool, not a recommended appliance setting or a sanitation specification.

This distinction matters: entering the same fixture count with colder inlet water produces a larger heating requirement. A regional label is not a measurement because season, local water source and distribution conditions vary. Obtain an appropriate winter design inlet temperature through your utility or installer. Requirements for particular appliances may differ from a shower scenario. The professional must choose the appropriate design conditions and safe settings for the entire installation rather than treating the calculator's default as a universal target.

Why the electric conversion has the units it does

The underlying heat balance uses water mass, temperature change and time. As an engineering approximation, one US gallon of water weighs about 8.34 pounds; one British thermal unit raises one pound of water by one degree Fahrenheit. Multiplying gallons per minute by pounds per gallon and minutes per hour gives approximately 500 times GPM times temperature rise in BTU per hour. The Energy Information Administration gives 3,412 BTU as the energy equivalent of one kilowatt-hour.

Consequently heat output in kW is approximately GPM multiplied by rise and 500, divided by 3,412. Rearranging gives GPM approximately equal to kW multiplied by 6.824 and divided by rise. The coefficient is not 0.0684 GPM per kW at a one-degree rise. This arithmetic describes ideal heat transfer using rounded physical constants; it is not a certified flow rating and does not account for every operating limit or control behavior of an actual heater.

Read the curve instead of the maximum

A listing's maximum flow normally belongs to a particular operating condition. It cannot establish the same flow at every temperature rise. Find the chart for the exact model, fuel and voltage, and compare it with the design demand. Do not copy a nearby model's chart or infer performance by scaling the enclosure size in a product photograph.

Stiebel Eltron's Tempra documentation illustrates another important distinction: a flow-control feature can reduce delivered flow when demand exceeds the unit's heating capacity. Maintaining temperature by reducing flow is not the same as meeting the original requested flow. Check whether a proposed model's controls and limits suit your expectations. Our gas-versus-electric guide compares the broader infrastructure choices, while the point-of-use guide explains why one remote sink presents a different sizing problem from a whole home. Both still require a model-specific check.

Turn the result into a question for an installer

Bring the simultaneous fixture list, winter inlet assumption and required outlet conditions to the professional preparing your quote. Ask where that demand lies on the manufacturer's flow-versus-rise chart and what happens if an additional fixture opens. Ask about minimum activation flow and the effect of the actual voltage where relevant. None of those answers can be replaced by an editorial star badge.

Keep heating output separate from purchased fuel input. A gas burner's BTU-per-hour input is not all delivered to the water; the exact unit's documentation is needed to relate the two. Likewise, a simplified electric heat calculation is not authority to select conductors, breakers or service capacity. Our electrical-requirements guide identifies the documents for that separate professional assessment. Use the calculator to expose assumptions and compare scenarios, then let the manufacturer documentation and qualified installer decide the actual equipment and all installation details.

Our rated picks

The three best-rated models of our ranking Best Whole-House Tankless Water Heaters in 2026: Gas and Electric, from the buyer review record and the listed specs.