What size tankless water heater do I need?

Add up the flow of the fixtures that run at the same time, work out the temperature rise, and size the unit to deliver at least that flow at that rise. Two showers (2.0 GPM each) and a kitchen faucet (2.2 GPM) are 6.2 GPM; from 50°F to 120°F that needs 228,600 BTU/h gas input at 95% efficiency, or 63.6 kW electric. A family running two showers, a bathroom faucet, the kitchen faucet, the dishwasher and the washer together comes to 11.2 GPM and 413,000 BTU/h gas input.

How do I calculate BTU/h for a tankless water heater?

BTU/h into the water = 500.4 × GPM × temperature rise. The 500.4 is 60 minutes × 8.34 lb of water per gallon, and each pound takes 1 BTU per °F. For 6.2 GPM and a 70°F rise that is 217,200 BTU/h. A gas unit’s input is that divided by its efficiency: at 95% it is 228,600 BTU/h gas input, and at 82% (non-condensing) it is 264,800 BTU/h gas input.

How do I convert GPM and temperature rise to kW?

kW = 500.4 × GPM × rise ÷ 3,412.14, which is about GPM × rise ÷ 6.82. Electric resistance heating is about 100% efficient, so there is no efficiency term. For 6.2 GPM at 70°F the answer is 63.6 kW electric. That is about 265 A at 240 V, far more than one household circuit carries, which is why whole-house electric units are wired on several circuits.

How many circuits does a whole-house electric tankless heater need?

Several. A whole-house electric unit draws tens of kW, and one 240 V breaker is limited, so the load is split over dedicated circuits. One manufacturer’s installation manual lists an 11 kW unit on one 50 A breaker, an 18 kW unit on two 40 A breakers and a 27 kW unit on three 40 A breakers, and says these apply only to its listed models. Units of 36 kW and up need more again. Your unit’s manual gives the breaker sizes and wire, and the main panel may need an upgrade; have an electrician check this before you choose. The default load here is 63.6 kW electric, which is above any one of those example units.

What groundwater temperature should I use?

Use the temperature of the incoming cold water in your coldest month, because that is the hardest case for the heater. It follows shallow groundwater temperature, which runs from the 30s and 40s in the far north to the 70s in the south of the United States; the map on the Building America Solution Center page for gas-fired tankless water heaters shows it by region, and your water utility can tell you. The 50°F default is a placeholder. The effect is large: the default load at 40°F is 261,300 BTU/h gas input, and at 70°F it is 163,300 BTU/h gas input.

What temperature rise do I need?

The rise is the hot water temperature you want minus the incoming water temperature. At the usual 120°F setpoint and 50°F groundwater it is 70°F. DOE notes that a 70°F rise is typically possible at about 5 GPM from a gas-fired unit and about 2 GPM from an electric one, so flow and rise trade off: the same unit gives less flow at a higher rise.

How many GPM do showers and faucets use?

EPA WaterSense showerheads are 2.0 GPM or less, and the federal maximum is 2.5 GPM. A WaterSense bathroom faucet is 1.5 GPM or less against a federal 2.2 GPM, and 2.2 GPM is also the federal maximum for a kitchen faucet. Dishwasher and clothes washer fill rates range widely (1.0 to 3.0 and 1.0 to 6.0 GPM in the ASHRAE ranges), so this tool uses typical values of 1.5 and 2.0 GPM; check your appliance manuals. A fixture’s hot flow is lower than its rating whenever it mixes in cold water.

How much flow does a unit I am considering give?

Enter its gas input in BTU/h or its power in kW, and the tool shows the most flow it gives at your temperature rise: (input × efficiency) ÷ (500.4 × rise) for gas, (kW × 3,412.14) ÷ (500.4 × rise) for electric. A 199,000 BTU/h unit at 95% gives 5.4 GPM at a 70°F rise, short of the 6.2 GPM the default load needs; a 27 kW electric unit gives 2.63 GPM. A real unit’s data sheet shows its tested flow by rise and is the figure to trust.

Gas or electric tankless: which size should I use?

Use the line for your fuel. The gas figure is the burner’s input, the number on the unit’s data plate, and it already allows for the efficiency you enter. The electric figure is the power drawn. The same load, 6.2 GPM at 70°F, is 228,600 BTU/h gas input for gas and 63.6 kW electric for electric. A gas unit also needs venting and enough gas supply, and an electric one needs the circuits and panel capacity above.

Should I count every fixture in the house?

No. Count only the fixtures that really run together at your busiest moment. One shower and nothing else is 2 GPM and 73,700 BTU/h gas input; a unit sized for every fixture at once would be larger and cost more for a peak that rarely happens. The cost of counting too few is lukewarm water or a unit that cannot keep up when two fixtures run, so count the realistic worst case, such as two showers and the kitchen faucet. With federal-maximum 2.5 GPM showerheads the default load becomes 7.2 GPM and 265,500 BTU/h gas input.

What efficiency should I enter for a gas unit?

The tool defaults to 95% for a condensing unit and offers 82% as the figure for a non-condensing unit. These are typical estimates, so use the UEF or thermal efficiency on the data sheet of the unit you are considering. The default load needs 228,600 BTU/h gas input at 95% and 264,800 BTU/h gas input at 82%. Electric units are taken as 100% efficient.

How is this different from the water heater size calculator?

The water heater size calculator covers both a storage tank, sized by First Hour Rating from your busiest hour, and a basic tankless estimate. This page goes further for tankless: more fixtures including a dishwasher and a washer, the 500.4 constant and 3,412.14 BTU per kWh, a condensing or non-condensing efficiency, and the most flow a unit you choose gives at your rise.

Tankless Water Heater Size Calculator: GPM, Temperature Rise, BTU/h or kW

Two showers at 2.0 GPM and a kitchen faucet at 2.2 GPM running together need 6.2 GPM. Heating that from 50°F groundwater to 120°F is a 70°F rise, which takes 217,200 BTU/h into the water: a gas tankless unit with 228,600 BTU/h gas input at a typical 95% condensing efficiency, or about 63.6 kW for an electric one. Change the fixtures, the temperatures or the efficiency to match your home. This is sizing guidance from a heat-balance formula; the unit’s data sheet shows the flow it delivers at your rise.

Figures based on the sources listed belowUpdated 2026-10-17How we calculate

Showers running at the same time

Count only the fixtures that really run together at your busiest moment, not every fixture in the house.

Showerhead flowThe flow is stamped on the showerhead or in its manual. WaterSense showerheads are 2.0 GPM or less; the federal limit is 2.5 GPM. A shower mixes in cold water, so the hot flow is really less than this and the result is on the safe side.

Bathroom faucets running at the same time

Counted at 1.5 GPM each, the WaterSense maximum for a bathroom faucet (the federal maximum is 2.2 GPM).

Kitchen faucets running at the same time

Counted at 2.2 GPM each, the federal maximum for a faucet. Many kitchen faucets run less, so this is on the safe side.

Dishwashers running at the same time

Counted at 1.5 GPM, a typical figure inside the 1.0 to 3.0 GPM fill-rate range. Check the appliance manual.

Clothes washers running at the same time

Counted at 2.0 GPM, a typical figure inside the wide 1.0 to 6.0 GPM fill-rate range. Check the appliance manual.

Hot water temperature you want (°F)

120°F is the usual setpoint. Water much hotter than that scalds quickly, so go higher only for a reason.

Incoming groundwater temperature (°F)

Use the coldest water of the year, because that sets the size. It follows shallow groundwater temperature, from the 30s and 40s in the far north to the 70s in the south; the groundwater map on the Building America Solution Center page gives it by region. 50°F is a placeholder, not your figure.

Tankless fuelBoth are worked out below; this picks which one is the headline.

Gas unit efficiency (UEF or thermal efficiency) (%)

Gas only. 95% is a typical figure for a condensing unit and 82% for a non-condensing one; these are typical estimates, so use the rating on the data sheet of the unit you are considering. Electric units are close to 100% efficient, so this does not change the kW.

Gas unit input you are considering (optional) (BTU/h)

The maximum input rating of the unit, in BTU/h. Enter it to see the most flow it gives at this temperature rise.

228,600 BTU/h gas input

Method: tankless, gas
Flow needed: 6.2 GPM (Showers 2 × 2, Kitchen faucets 1 × 2.2)
Temperature rise: 70°F (120°F outlet − 50°F incoming)
Heat into the water: 217,200 BTU/h (500.4 × 6.2 GPM × 70°F)
Gas unit input: 228,600 BTU/h (output ÷ 0.95 efficiency)
Electric unit power: 63.6 kW (output ÷ 3,412.14), about 265 A at 240 V
Most flow from your unit: enter a unit size to see the most flow it gives at this rise

Shopping list

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Fixture flow rates used in this calculator

Showerhead flows are options in the calculator; the other fixtures are counted at the flow shown. Rows marked typical are estimates inside the published range, not measured values: check your own fixtures and appliances.
Fixture	Published flow	Source	In this calculator
Showerhead, WaterSense	2.0 GPM or less	EPA WaterSense	option: 2.0 GPM (typical)
Showerhead, federal maximum	2.5 GPM at 80 psi	US federal standard, 10 CFR 430.32(p)	option: 2.5 GPM
Bathroom faucet, WaterSense	1.5 GPM or less	EPA WaterSense (federal standard 2.2 GPM)	used: 1.5 GPM
Kitchen faucet, federal maximum	2.2 GPM at 60 psi	US federal standard, 10 CFR 430.32(o)	used: 2.2 GPM
Dishwasher	1.0 to 3.0 GPM fill rate	ASHRAE fill rates, as listed by the Building America Solution Center	typical, used: 1.5 GPM
Clothes washer	1.0 to 6.0 GPM fill rate	ASHRAE fill rates, as listed by the Building America Solution Center	typical, used: 2 GPM

Gas input and electric power by flow and temperature rise

Each cell is gas input in BTU/h at 95% efficiency (500.4 × GPM × rise ÷ 0.95), rounded to the nearest 100, then electric power in kW (output ÷ 3,412.14). Columns are the temperature rise in °F.
Flow	40°F rise	50°F rise	60°F rise	70°F rise	80°F rise
2 GPM	42,100 / 11.7	52,700 / 14.7	63,200 / 17.6	73,700 / 20.5	84,300 / 23.5
3 GPM	63,200 / 17.6	79,000 / 22	94,800 / 26.4	110,600 / 30.8	126,400 / 35.2
4 GPM	84,300 / 23.5	105,300 / 29.3	126,400 / 35.2	147,500 / 41.1	168,600 / 46.9
5 GPM	105,300 / 29.3	131,700 / 36.7	158,000 / 44	184,400 / 51.3	210,700 / 58.7
6 GPM	126,400 / 35.2	158,000 / 44	189,600 / 52.8	221,200 / 61.6	252,800 / 70.4
8 GPM	168,600 / 46.9	210,700 / 58.7	252,800 / 70.4	295,000 / 82.1	337,100 / 93.9

Temperature rise by incoming water temperature

Worked for 6.2 GPM (the default load) and a 120°F outlet; gas at 95% efficiency. Colder incoming water raises the rise and so the size.
Incoming water	Temperature rise	Heat into water (BTU/h)	Gas input (BTU/h)	Electric (kW)
35°F	85°F	263,700	277,600	77.3
40°F	80°F	248,200	261,300	72.7
45°F	75°F	232,700	244,900	68.2
50°F	70°F	217,200	228,600	63.6
55°F	65°F	201,700	212,300	59.1
60°F	60°F	186,100	195,900	54.6
70°F	50°F	155,100	163,300	45.5

Most flow from example unit sizes at each temperature rise

GPM a unit gives at the rise shown, from (input × efficiency) ÷ (500.4 × rise) for gas at 95% efficiency and (kW × 3,412.14) ÷ (500.4 × rise) for electric. The sizes are examples, not products. A unit’s own data sheet is the authority on its flow.
Unit size	40°F rise	50°F rise	60°F rise	70°F rise
Gas, 150,000 BTU/h input	7.12 GPM	5.7 GPM	4.75 GPM	4.07 GPM
Gas, 199,000 BTU/h input	9.44 GPM	7.56 GPM	6.3 GPM	5.4 GPM
Electric, 18 kW	3.07 GPM	2.45 GPM	2.05 GPM	1.75 GPM
Electric, 27 kW	4.6 GPM	3.68 GPM	3.07 GPM	2.63 GPM
Electric, 36 kW	6.14 GPM	4.91 GPM	4.09 GPM	3.51 GPM

Methodology & assumptions

A tankless (demand-type) water heater has no tank of stored hot water, so it is sized by two numbers: how much hot water you want at once (the flow in gallons per minute, GPM) and how far it has to heat the water (the temperature rise, in °F). 1) Flow = the fixtures that run at the same time, each at its flow: a showerhead at the flow you pick (1.5, 2.0 or 2.5 GPM), a bathroom faucet at 1.5 GPM, a kitchen faucet at 2.2 GPM, a dishwasher at 1.5 GPM and a clothes washer at 2 GPM. 2) Temperature rise = the hot water temperature you want minus the incoming groundwater temperature. 3) Heat into the water: water weighs 8.34 lb per gallon and one BTU raises a pound of water one °F, so 1 GPM is 60 gal/h × 8.34 lb/gal = 500.4 lb/h, and BTU/h = 500.4 × GPM × rise. 4) Gas: the burner input is the output divided by the efficiency (UEF or thermal efficiency): BTU/h input = output ÷ efficiency. The defaults are 95% for a condensing unit and 82% for a non-condensing one; these are typical estimates, not data-sheet values, so enter the rating of the unit you are considering. 5) Electric: resistance heating turns nearly all the power into heat, so it is taken as 100% efficient and kW = output ÷ 3,412.14 (BTU per kWh). 6) Most flow from a unit you choose: for gas, (input × efficiency) ÷ (500.4 × rise); for electric, (kW × 3,412.14) ÷ (500.4 × rise). Which flows are counted. Only count fixtures that really run together at your busiest moment: a household rarely runs every shower, the dishwasher and the washer at once, and sizing for everything at once buys a much bigger and more costly unit. The faucet, dishwasher and washer flows are the hot-water share of a typical flow, not a measured figure; a shower mixes cold water into the hot, so the hot flow is less than the showerhead’s rating and the result is on the safe side. The dishwasher and washer values are typical figures inside the published fill-rate ranges (1.0 to 3.0 and 1.0 to 6.0 GPM), so check your appliance manuals. Incoming temperature. The coldest water of the year sets the size. Shallow groundwater temperature runs from the 30s and 40s in the far north to the 70s in the south, and the Building America Solution Center page on gas-fired tankless water heaters has a map of it by region. 50°F is a placeholder you should replace with your own figure. What this is not. It is not a plumber’s design. It does not cover how long hot water takes to reach a distant tap, minimum flow to fire (about 0.5 GPM for many units), pipe sizing, venting and gas line capacity, electrical service, codes or permits. The data sheet of the unit gives its flow at several temperature rises; compare it to the flow and rise above.

How to size a tankless water heater

Add the flow. List the fixtures that run at the same time at your busiest moment and add their GPM: a showerhead is 1.5 to 2.5 GPM, a bathroom faucet 1.5 and a kitchen faucet up to 2.2.
Find the temperature rise. Subtract your coldest incoming water temperature from the hot water temperature you want (120°F is usual).
Convert to BTU/h. Multiply 500.4 by the GPM and by the rise to get the heat into the water in BTU/h.
Gas or electric. For gas divide by the unit’s efficiency to get the input in BTU/h; for electric divide by 3,412.14 to get kW.
Check the unit. Compare the unit’s data sheet flow at your temperature rise with the flow you need, and for electric check the circuits and panel with an electrician.

Worked examples

Project	Result
Two showers at 2.0 GPM and a kitchen faucet, 50°F to 120°F, condensing gas	228,600 BTU/h gas input (6.2 GPM × 500.4 × 70 ÷ 0.95)
The same load, non-condensing gas at 82%	264,800 BTU/h gas input
The same load, electric	63.6 kW electric (6.2 GPM × 500.4 × 70 ÷ 3,412.14)
A 199,000 BTU/h gas unit against the default load	228,600 BTU/h gas input needed; the unit gives 5.4 GPM at a 70°F rise, short of 6.2 GPM
A 27 kW electric unit against the default load	63.6 kW electric needed; the unit gives 2.63 GPM at a 70°F rise
A family: 2 showers, a bathroom faucet, the kitchen faucet, a dishwasher and a washer at once	413,000 BTU/h gas input (11.2 GPM); electric 115 kW electric
Cold climate: the default load with 40°F incoming water	261,300 BTU/h gas input (80°F rise)
Warm climate: the default load with 70°F incoming water	163,300 BTU/h gas input (50°F rise)
One shower only, gas	73,700 BTU/h gas input (2 GPM at a 70°F rise)

Frequently asked questions

What size tankless water heater do I need?: Add up the flow of the fixtures that run at the same time, work out the temperature rise, and size the unit to deliver at least that flow at that rise. Two showers (2.0 GPM each) and a kitchen faucet (2.2 GPM) are 6.2 GPM; from 50°F to 120°F that needs 228,600 BTU/h gas input at 95% efficiency, or 63.6 kW electric. A family running two showers, a bathroom faucet, the kitchen faucet, the dishwasher and the washer together comes to 11.2 GPM and 413,000 BTU/h gas input.
How do I calculate BTU/h for a tankless water heater?: BTU/h into the water = 500.4 × GPM × temperature rise. The 500.4 is 60 minutes × 8.34 lb of water per gallon, and each pound takes 1 BTU per °F. For 6.2 GPM and a 70°F rise that is 217,200 BTU/h. A gas unit’s input is that divided by its efficiency: at 95% it is 228,600 BTU/h gas input, and at 82% (non-condensing) it is 264,800 BTU/h gas input.
How do I convert GPM and temperature rise to kW?: kW = 500.4 × GPM × rise ÷ 3,412.14, which is about GPM × rise ÷ 6.82. Electric resistance heating is about 100% efficient, so there is no efficiency term. For 6.2 GPM at 70°F the answer is 63.6 kW electric. That is about 265 A at 240 V, far more than one household circuit carries, which is why whole-house electric units are wired on several circuits.
How many circuits does a whole-house electric tankless heater need?: Several. A whole-house electric unit draws tens of kW, and one 240 V breaker is limited, so the load is split over dedicated circuits. One manufacturer’s installation manual lists an 11 kW unit on one 50 A breaker, an 18 kW unit on two 40 A breakers and a 27 kW unit on three 40 A breakers, and says these apply only to its listed models. Units of 36 kW and up need more again. Your unit’s manual gives the breaker sizes and wire, and the main panel may need an upgrade; have an electrician check this before you choose. The default load here is 63.6 kW electric, which is above any one of those example units.
What groundwater temperature should I use?: Use the temperature of the incoming cold water in your coldest month, because that is the hardest case for the heater. It follows shallow groundwater temperature, which runs from the 30s and 40s in the far north to the 70s in the south of the United States; the map on the Building America Solution Center page for gas-fired tankless water heaters shows it by region, and your water utility can tell you. The 50°F default is a placeholder. The effect is large: the default load at 40°F is 261,300 BTU/h gas input, and at 70°F it is 163,300 BTU/h gas input.
What temperature rise do I need?: The rise is the hot water temperature you want minus the incoming water temperature. At the usual 120°F setpoint and 50°F groundwater it is 70°F. DOE notes that a 70°F rise is typically possible at about 5 GPM from a gas-fired unit and about 2 GPM from an electric one, so flow and rise trade off: the same unit gives less flow at a higher rise.
How many GPM do showers and faucets use?: EPA WaterSense showerheads are 2.0 GPM or less, and the federal maximum is 2.5 GPM. A WaterSense bathroom faucet is 1.5 GPM or less against a federal 2.2 GPM, and 2.2 GPM is also the federal maximum for a kitchen faucet. Dishwasher and clothes washer fill rates range widely (1.0 to 3.0 and 1.0 to 6.0 GPM in the ASHRAE ranges), so this tool uses typical values of 1.5 and 2.0 GPM; check your appliance manuals. A fixture’s hot flow is lower than its rating whenever it mixes in cold water.
How much flow does a unit I am considering give?: Enter its gas input in BTU/h or its power in kW, and the tool shows the most flow it gives at your temperature rise: (input × efficiency) ÷ (500.4 × rise) for gas, (kW × 3,412.14) ÷ (500.4 × rise) for electric. A 199,000 BTU/h unit at 95% gives 5.4 GPM at a 70°F rise, short of the 6.2 GPM the default load needs; a 27 kW electric unit gives 2.63 GPM. A real unit’s data sheet shows its tested flow by rise and is the figure to trust.
Gas or electric tankless: which size should I use?: Use the line for your fuel. The gas figure is the burner’s input, the number on the unit’s data plate, and it already allows for the efficiency you enter. The electric figure is the power drawn. The same load, 6.2 GPM at 70°F, is 228,600 BTU/h gas input for gas and 63.6 kW electric for electric. A gas unit also needs venting and enough gas supply, and an electric one needs the circuits and panel capacity above.
Should I count every fixture in the house?: No. Count only the fixtures that really run together at your busiest moment. One shower and nothing else is 2 GPM and 73,700 BTU/h gas input; a unit sized for every fixture at once would be larger and cost more for a peak that rarely happens. The cost of counting too few is lukewarm water or a unit that cannot keep up when two fixtures run, so count the realistic worst case, such as two showers and the kitchen faucet. With federal-maximum 2.5 GPM showerheads the default load becomes 7.2 GPM and 265,500 BTU/h gas input.
What efficiency should I enter for a gas unit?: The tool defaults to 95% for a condensing unit and offers 82% as the figure for a non-condensing unit. These are typical estimates, so use the UEF or thermal efficiency on the data sheet of the unit you are considering. The default load needs 228,600 BTU/h gas input at 95% and 264,800 BTU/h gas input at 82%. Electric units are taken as 100% efficient.
How is this different from the water heater size calculator?: The water heater size calculator covers both a storage tank, sized by First Hour Rating from your busiest hour, and a basic tankless estimate. This page goes further for tankless: more fixtures including a dishwasher and a washer, the 500.4 constant and 3,412.14 BTU per kWh, a condensing or non-condensing efficiency, and the most flow a unit you choose gives at your rise.