What temperature rise should I size a tankless water heater for?

Subtract the incoming water temperature from the temperature you want. The Department of Energy says most uses want 120°F and to assume 50°F incoming water unless you know otherwise, which is a 70°F rise. For a dishwasher without its own heater it uses 140°F, a 90°F rise, and warns that 140°F raises the risk of scalding.

Free · No sign-up · Every step of the math shown

What size tankless water heater do I need?

Q: How many GPM do I need from a tankless water heater?

Add up the flow rates of everything that runs hot water at the same time. Federal law caps showerheads at 2.5 gallons per minute and bathroom and kitchen faucets at 2.2. WaterSense showerheads use no more than 2.0 and WaterSense bathroom faucets no more than 1.5. The Department of Energy's example: a 0.75 GPM faucet plus a 2.5 GPM shower head needs 3.25 GPM.

Q: Why does a tankless heater give less flow than the number on the box?

Makers publish flow at several temperature rises, and the flow drops as the rise goes up. Navien's NPE-240A2 table, for example, lists 11.2 GPM at a 35°F rise and 5.6 GPM at a 70°F rise.

Q: Gas or electric tankless water heater?

The Department of Energy says a 70°F rise is typically possible at 5 gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric ones. The whole-house electric units listed here need 75 to 150 amps and two to four double-pole breakers, per their makers.

Q: What if one tankless water heater isn't enough?

The Department of Energy says you can install two or more tankless water heaters, or separate tankless heaters for appliances that use a lot of hot water, such as a clothes washer or dishwasher.

Pick what runs hot water at the same time and enter how cold your water comes in. The size comes from the Department of Energy's method, and each heater below is checked against its maker's own flow table.

Shopping for a tank water heater instead? TankSizer sizes those.

01 What runs at the same time

Count the hot water uses you'd run at once on your busiest morning.

Showers2.5 GPM each

Bathroom faucets2.2 GPM each

Kitchen faucets2.2 GPM each

Anything else at once (tub, washer)GPM

Federal GPM limits cover showerheads and faucets, not tubs, washers or dishwashers. To measure one, hold a bucket under it for one minute and read the gallons.

02 Your fixtures

Flow rates to use

Standardfederal max WaterSenselabeled I measuredmy own

Federal maximums for anything made after 1994: showerheads 2.5 GPM, bathroom and kitchen faucets 2.2 GPM.

03 Water temperature

Incoming cold water°F

50°F is the Department of Energy's default. To measure yours, hold a thermometer under a cold-water faucet.

Hot water temperature

120°Fmost uses 140°Fdishwasher, no heater

04 Fuel

Heaters to show

Gasnatural gas or propane Electric240V Not sureshow both

Heaters that cover it

Makers publish how many gallons a minute a heater delivers at set temperature rises. Each card reads the maker's row at or above your rise, names it, and never guesses between rows. If a maker doesn't publish a number, it says so.

Where the numbers come from

The method is the U.S. Department of Energy's, from its page on sizing tankless (demand) water heaters:

"Tankless or demand-type water heaters are rated by the maximum temperature rise possible at a given flow rate. Therefore, to size a demand water heater, you need to determine the flow rate and the temperature rise you'll need."

List what you'd run at the same time and add up their flow rates in gallons per minute (GPM).
Subtract the incoming water temperature from the temperature you want. DOE: "Unless you know otherwise, assume that the incoming water temperature is 50ºF." And: "For most uses, you'll want your water heated to 120ºF."
Find a heater whose maker rates it for that flow at that rise.

DOE's own example: a 0.75 GPM faucet and a 2.5 GPM shower head at once need 3.25 GPM, and 120°F minus 50°F is a 70°F rise. Type those in under "I measured" and you get the same answer.

The "heat into the water" line uses physics only: 8.34 pounds per gallon of water, 1 BTU per pound per °F, 60 minutes an hour, and 3,412 BTU per kilowatt-hour. Heaters are matched on their makers' flow tables, never on that line.

Method: U.S. Department of Energy, Energy Saver, "Sizing a New Water Heater". DOE took its Energy Saver pages down in July 2026, so the link is the last live copy (saved July 2, 2026). Retrieved October 30, 2026.

Safety: gas heaters

"Proper installation depends on many factors. These factors include fuel type, climate, local building code requirements, and safety issues, especially concerning the combustion of gas-fired water heaters. Therefore, it's best to have a qualified plumbing and heating contractor install your demand water heater."

U.S. Department of Energy, "Tankless or Demand-Type Water Heaters" (last live copy, June 30, 2026).

"Most appliances should be installed by qualified professionals." "Install battery-operated CO alarms or CO alarms with battery backup on every level of the home and outside sleeping areas."

U.S. Consumer Product Safety Commission, Carbon Monoxide Questions and Answers. Retrieved October 30, 2026.

Questions

How many GPM do I need from a tankless water heater?

Add up the flow of everything that runs hot water at the same time. Federal law caps showerheads at 2.5 GPM and bathroom and kitchen faucets at 2.2 GPM. WaterSense showerheads use no more than 2.0 GPM and WaterSense bathroom faucets no more than 1.5. DOE's example, a 0.75 GPM faucet plus a 2.5 GPM shower, needs 3.25 GPM.

What temperature rise should I size for?

Your hot water temperature minus your incoming water temperature. DOE says most uses want 120°F, and to assume 50°F coming in unless you know otherwise: a 70°F rise. For a dishwasher without its own heater, DOE uses 140°F, a 90°F rise, and warns that 140°F increases the possibility of scalding.

Why does a heater give less flow than the number on the box?

Makers publish flow at several temperature rises, and flow drops as the rise goes up. Navien's NPE-240A2 table lists 11.2 GPM at a 35°F rise but 5.6 GPM at a 70°F rise. That's why the tool reads the row for your rise.

Gas or electric?

DOE: "Typically, a 70ºF (39ºC) water temperature rise is possible at a flow rate of 5 gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric ones." The whole-house electric units listed here need 75 to 150 amps and two to four double-pole breakers, per their makers. Gas units need a vent and a gas line sized for them, per their makers' manuals.

What if one heater isn't enough?

DOE says you can install two or more tankless water heaters, or separate tankless heaters for appliances that use a lot of hot water, such as a clothes washer or dishwasher. A plumber can tell you how to pipe them.

Federal maximums: 10 CFR 430.32(o) and (p), for fixtures made after January 1, 1994 (eCFR). WaterSense: EPA's showerhead and bathroom faucet pages; EPA has no WaterSense rating for kitchen faucets. Retrieved October 30, 2026.
Fixture	Federal max	WaterSense max
Showerhead	2.5 GPM	2.0 GPM
Bathroom faucet	2.2 GPM	1.5 GPM
Kitchen faucet	2.2 GPM	none

How the heaters are matched

Every spec comes from the maker's own product page, spec sheet, brochure or installation manual, retrieved October 30, 2026.
A heater covers your need only if the maker's published GPM, at the published rise at or above yours, is at least the flow you need. The card names the rise and the table.
If a maker stops its table below your rise, the card says so. It doesn't count as a fit.
Electric makers often publish flow by incoming water temperature for a 105°F, 113°F or 140°F output. The rise is their two numbers subtracted, shown on the card.