Will a small unit handle one shower?
Not quite. One 2.5 gallon per minute shower at a 78°F rise needs about 97,500 BTU per hour, just past the 90,000 ceiling of the calculator’s small class, so it lands in the standard whole-house class.
Tankless Flow Calculator
A tankless heater makes hot water as you use it, so its size comes down to flow and temperature rise. Choose the showers and fixtures that run at the same time to see what a unit has to deliver.
Pick your place and we set the incoming water temperature from its winter weather.
What runs at the same time
A tank stores hot water. A tankless unit makes it as you go, so it is sized in gallons per minute, not gallons. Add up what realistically runs together. We use 2.5 gallons per minute for a showerhead, which is the federal limit, about 1.8 for a faucet and about 2 for a dishwasher or washer. Two showers at once already means 5.
How cold the water comes in
Minneapolis has an average January low of 10°F, well under the 25°F cut-off the calculator uses for its coldest band. In that band it counts incoming water at 42°F, so a unit heating to the recommended 120°F setpoint has a 78°F rise to cover. Warmer places get 52°F or 65°F incoming water and a smaller rise.
How the size is worked out
The heat a unit has to deliver each hour is the flow in gallons per minute, times the rise in degrees, times 500. That figure comes from the weight of a gallon of water. We round it and place it in one of our own planning bands, based on that delivered heat. Up to 90,000 BTU per hour is a small unit. Up to 150,000 is a standard whole-house unit. Up to 190,000 is a large unit. Above that, two units are linked together.
A tankless unit trades flow for rise. The same unit gives more gallons per minute at a small rise than at a big one, which is why every tankless rating is stated at a rise.
Gas and electric limits
Gas units go up to about 199,000 BTU per hour of input, and two can be linked. Electric units top out near 36 kW, about 123,000 BTU per hour, and they need a very large electrical service. The result shows how many gallons per minute each one delivers at your rise. A plumber or an electrician checks the gas line, the electrical service and the venting before any unit is chosen.
Common questions
Can one tankless unit run two showers at once?
No, not at this rise. Two showers come to 5 gallons per minute, and at 78°F that is about 195,000 BTU per hour, past the 190,000 top of the calculator’s large class, so the result is two linked units.
Does the temperature of the incoming water change the size?
Yes. With water arriving at 42°F and leaving at 120°F the rise is 78°F, and every tankless flow rating is given at a stated rise. The bigger the rise, the fewer gallons per minute the same unit can deliver.
Is an electric tankless water heater enough for a whole house?
No, not for more than one shower at a time. At a 78°F rise the largest electric units, near 36 kW, deliver about 3.1 gallons per minute, and they need a very large electrical service that an electrician has to confirm.
Does a tankless water heater need a bigger gas line?
Often. A 199,000 BTU per hour unit draws several times what a tank heater does, and the existing gas line and venting may not be sized for it. A plumber checks both first.
Data: fixture flow rates from the federal showerhead limit and typical faucet and appliance flows, the 120°F setpoint recommended by the US Department of Energy, and incoming water bands set from Daymet winter data 2015 to 2024 for each place.
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