Tankless Water Heater Sizing: GPM, BTU and kW Explained

A tank water heater stores a batch of hot water in advance. A tankless unit heats water as you use it, so it has to deliver two things at once: enough flow (gallons per minute, GPM) and enough temperature rise (how many degrees it must add). Size it on those two numbers, and you will have hot water for the fixtures you run together, without paying for capacity you will not use.

Step 1: Add up the flow you need at once

Count the fixtures that might run at the same time on a busy morning, and add their flow rates. Typical figures (check yours; low-flow fixtures use less):

• Shower: 2.0 GPM (federal limit 2.5)
• Kitchen faucet: 1.5 GPM
• Bathroom faucet: 1.0 GPM
• Dishwasher: 1.5 GPM
• Washing machine: 2.0 GPM
• Bathtub filling: 4.0 GPM

A shower and a kitchen faucet together are 3.5 GPM. Two showers and a dishwasher are 5.5 GPM.

Step 2: Work out the temperature rise

The rise is the difference between the incoming (groundwater) temperature and the temperature you want at the tap, typically 120 °F. Groundwater is colder in the north and in winter:

• Cold climates: 40 to 50 °F in winter, so a rise of 70 to 80 °F.
• Moderate climates: 50 to 60 °F, so a rise of 60 to 70 °F.
• Warm climates: 60 to 75 °F, so a rise of 45 to 60 °F.

Always size for the coldest water you will see, since the same unit that is plenty in July can struggle in January.

Step 3: Convert to heating power

A gallon of water weighs 8.33 lb, and each pound needs 1 BTU to rise by 1 °F. Run at one gallon per minute for an hour, that is 8.33 × 60 ≈ 500 BTU per hour per degree.

Heating power needed (BTU/hr)

The tankless water heater calculator works this out for your own fixtures and temperatures.

Gas or electric?

Gas (natural gas or propane) tankless heaters are rated by their input in BTU per hour, with the largest around 199,000 BTU/hr, enough for about 5 to 9 GPM depending on the temperature rise. Gas units need a gas line of the right size, a vent (often stainless steel or special plastic), and sometimes a condensate drain, which adds to the installed cost. Divide your BTU requirement by the efficiency (roughly 0.9 for a high-efficiency unit) to find the input rating you need.

Electric tankless heaters are almost 100% efficient and need no venting, but they need a huge amount of current. At 240 V, a 5 GPM flow with a 70 °F rise needs about 51 kW, which is 214 amps, more than most homes' electrical service can spare. That is why whole-house electric units are rare, and why electric tankless heaters are mostly sized for a single bathroom or sink, or for warm climates with a small temperature rise.

Ways to cut the size you need

• Use low-flow fixtures. A 1.5 GPM shower head instead of 2.5 GPM trims the peak flow noticeably.
• Do not run everything at once. Starting the dishwasher after showers, instead of during them, lowers the peak.
• Use two smaller units (or point-of-use heaters for far-off bathrooms) in a big house, instead of one very large one.

Things to know before you buy

• "Endless" hot water has a limit. If the flow exceeds the unit's capacity, the water gets lukewarm, rather than running out.
• Maintenance. In hard water areas, scale builds up and needs flushing with vinegar or a descaler once a year.
• Cold water sandwich. There is a brief delay and a short burst of cold water if you turn the tap off and on quickly. Some units include a buffer to reduce it.
• Installation cost. A switch from a tank to a tankless unit often costs more than the unit itself, because of gas, venting or electrical upgrades. Get a quote before you decide.