A tankless water heater is sized by two numbers: how many gallons per minute of hot water you use at the same moment, and how many degrees it has to heat the incoming water. A unit that is comfortable in Florida can run out of steam in Minnesota in January. Enter the fixtures you would realistically run at once and where you live, and the calculator works out the flow, the temperature rise, and the gas or electric capacity that covers both.
How many of each would run at the same time on your busiest morning? Flow rates are typical figures; your fixture’s rating is often stamped on it or listed in its manual.
| Fixture | At once | GPM each |
|---|
- Gas unit, condensing (input)—
- Gas unit, non-condensing (input)—
- Electric unit—
- What a 199,000 BTU condensing gas unit delivers at this rise—
- What a 27 kW electric unit delivers at this rise—
How the numbers work
Heating one gallon of water by one degree Fahrenheit takes about 8.33 BTU. So a flow in gallons per minute needs roughly GPM × 500 × temperature rise BTU per hour of heat output (8.33 × 60 minutes, rounded). Gas units are rated by their input, so the calculator divides by efficiency: about 95% for condensing units and 82% for non-condensing. Electric units convert nearly all their input to heat; one kilowatt is about 3,412 BTU per hour.
The temperature rise is your setpoint minus the coldest water entering the house. The regional figures are rough winter averages for groundwater-fed supplies. Shallow wells, long outdoor runs and very cold winters can bring it lower, so if you can measure the cold water at a tap in winter, use that.
Why this errs on the generous side
A shower is usually a mix of hot and cold water, so only part of its flow passes through the heater. Sizing every fixture as full-flow hot water, as this calculator does, is the conservative method most sizing guides use. It leaves margin for the coldest week of the year and for fixtures you add later.
Before you buy
- Gas units need a gas line and meter that can supply the full input rating, and proper venting. Undersized gas lines are a common reason a new tankless unit underperforms. A licensed plumber or gas fitter should confirm supply and venting, and most jurisdictions require a permit.
- Whole-house electric units draw very high current. A 27 kW unit pulls about 112 amps at 240 volts, often on multiple dedicated circuits. Because a water heater is a continuous load, the electrical code generally sizes its wiring and breakers at 125% of that draw. Many homes need a panel or service upgrade first, which a licensed electrician must assess.
- Hard water scales up tankless heat exchangers faster than tank heaters. Most makers call for periodic descaling, and a softener or scale-reduction system can be worth considering.
Related guides
- Best tankless water heaters, gas vs. electric
- Tankless vs. traditional water heater: real cost comparison
- Winterizing your water heater
- Water treatment decision matrix, for hard water
- The full Heating & Cooling guide
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For planning only, not plumbing, gas or electrical advice. Have a licensed professional confirm sizing, gas supply, venting and electrical capacity before installation.