Tankless Water Heater Calculator

Enter the fixtures you want running at the same time and your water temperatures. You get the flow rate (GPM) and the heating power the unit must deliver, for gas or electric.

Results

Flow rate needed

3.5 GPM

At a 70 °F temperature rise

Heating power needed: 122,500 BTU/hr

If you buy a gas unit

Gas input rating (about 90% efficient): 136,111 BTU/hr

If you buy an electric unit

Electric power: 35.9 kW
Current at 240 V: 150 amps

Many homes cannot supply the amps a whole-house electric tankless heater needs, which is why they are often sized for a single bathroom, or gas is chosen instead. Fixture flows are typical; check yours. Winter groundwater is colder than summer groundwater, so size for the coldest month.

How this calculator works

A tankless heater has to raise the temperature of every gallon flowing through it, at the rate you demand. Water weighs 8.33 lb per gallon and takes 1 BTU to raise a pound by 1 °F, so a gallon per minute raised by one degree needs 8.33 × 60 = 500 BTU per hour.

BTU/hr = GPM × 500 × (hot temperature − cold temperature) kW = BTU/hr ÷ 3,412

Worked example: a shower (2.0 GPM) and a kitchen faucet (1.5 GPM) run at once: 3.5 GPM. With 50 °F groundwater heated to 120 °F, the rise is 70 °F, so the heater needs 3.5 × 500 × 70 = 122,500 BTU/hr. A gas unit rated at about 136,000 BTU/hr input covers it. An electric unit would need 35.9 kW, which is about 150 amps at 240 V.

Tips before you buy

Cold groundwater drives the size. The same flow in a northern winter (40 °F) needs roughly 40% more heating power than in a southern home (65 °F).
Electric tankless heaters need enormous current: a whole-house unit can need 100 to 200 amps, more than many panels can spare. Check your panel and get an electrician's opinion before you buy.
Gas tankless units need a proper gas line size and venting (often stainless steel), which can add a lot to the installed cost.
Lower the flow rather than raising the unit size where you can. Low-flow shower heads and aerators reduce the GPM you need to supply.
Hard water scales a tankless heater. Plan to flush it once a year, or add a softener.

Tools & materials for this project

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Gas tankless water heater Electric tankless (point of use)Low-flow shower head Tankless flush kit Water softener

Frequently asked questions

What size tankless water heater do I need?

Add up the flow of every fixture you want to run at the same time (GPM), work out the temperature rise, then use GPM × 500 × rise for BTU per hour. A typical household with two showers needs about 5 GPM and 150,000 to 199,000 BTU gas input.

How many GPM is a shower?

Modern showerheads are limited to 2.5 GPM by US federal standards, and many are 1.5 to 2.0 GPM. Kitchen faucets are about 1.5 GPM and bathroom faucets 1.0 GPM.

Can I run an electric tankless water heater on my panel?

Only if it has the spare capacity. A unit large enough for a whole house can draw more than 100 amps at 240 V. Smaller units, sized for a single sink or shower, are far easier to install.

Is a gas or electric tankless heater better?

Gas units deliver more hot water at a high flow rate, and cost less to run where gas is cheap, but need venting and a gas line. Electric units are cheaper to install and 99% efficient, but are limited by electrical capacity.

Related calculators & guides

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.

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