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Calculator · Water heating
Tankless water heater size calculator
A tankless unit is sized by two numbers, how much hot water runs at once and how far it has to be heated. Two 2.0 gpm showers and a 1.5 gpm faucet make 5.5 gpm, and heating it from 50°F to 120°F takes 192,423 BTU/h into the water. That is 202,551 BTU/h of gas input at an efficiency of 0.95, or 56.4 kW electric. Enter your fixtures and water temperatures to get yours. The gas input is what you take to the gas pipe sizing calculator to size the line.
- Enter the showers and faucets that can run at the same time and the flow of each, plus any dishwasher or washer.
- Enter the cold water coming in and the hot water you want. DOE assumes 50°F and 120°F when you do not know.
- The result shows the peak flow, the temperature rise, and the gas input or electric kW the heater has to supply.
Example result for the default inputs
2 x 2.0 gpm showers, 1 x 1.5 gpm faucets at once. Water in at 50°F, out at 120°F.
Peak flow
5.5 gpm
Temperature rise
70 °F
Gas input needed
202,551 BTU/h
| Flow | 2 x 2.0 gpm showers + 1 x 1.5 gpm faucets = 5.5 gpm |
|---|---|
| Temperature rise | 120°F - 50°F = 70°F |
| Heat into the water | 499.8 x 5.5 gpm x 70°F = 192,423 BTU/h |
| Gas input | 192,423 BTU/h / 0.95 = 202,551 BTU/h. Size the gas line for this input and for every other appliance on it. |
| If one unit is not enough | A load above what one unit delivers at this rise can be split across two units linked together, or the peak can be cut by staggering fixtures. The manufacturer's data sheet gives each unit's rise at each flow. |
For estimation purposes only. Consult a licensed engineer for final designs.
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How the load is worked
A tankless heater has no stored hot water, so it has to heat the flow as it passes. Two numbers set the size, how much water runs at once and how many degrees it must rise. The heat follows from them.
- Peak flow
flow = sum of the fixtures that run together, gpmDOE's method is to list the hot water devices used at one time and add their flow rates. A 2.5 gpm shower and a 0.75 gpm faucet make 3.25 gpm, which is the figure DOE gives. Two 2.0 gpm showers and a 1.5 gpm faucet make 5.5 gpm.- Temperature rise
rise = hot water wanted - cold water coming in, °FDOE assumes 50°F cold water and 120°F hot water, a rise of 70°F, unless you know otherwise.- Heat into the water
BTU/h = 8.33 lb/gal x 60 min/h x flow x rise = 499.8 x gpm x °FA gallon of water weighs about 8.33 lb and takes 1 BTU per lb per °F. The worked example takes 192,423 BTU/h. The factor is often rounded to 500, which would give 192,500.- Gas input
input BTU/h = heat into the water / efficiencyAt an efficiency of 0.95 the worked example needs a gas input of 202,551 BTU/h.- Electric input
kW = heat into the water / 3,412.14The same load electric is 56.4 kW, which is 235 A at 240 V, taking resistance heating as 100 percent efficient.
Fixtures, water temperature and the load
Six cases run through the same function the form uses. The efficiency of 0.95 is an example, not the rating of any model.
| Case | Flow, gpm | Rise, °F | Into the water, BTU/h | Input |
|---|---|---|---|---|
| Two showers and a faucet, gas at 0.95 | 5.5 | 70 | 192,423 | 202,551 BTU/h |
| DOE's example, a 2.5 gpm shower and a 0.75 gpm faucet | 3.25 | 70 | 113,705 | 119,689 BTU/h |
| Two showers and a faucet, electric | 5.5 | 70 | 192,423 | 56.4 kW |
| Same, winter inlet at 40°F | 5.5 | 80 | 219,912 | 231,486 BTU/h |
| One shower and a faucet | 3.5 | 70 | 122,451 | 128,896 BTU/h |
| Two showers, a faucet and a dishwasher at 1.2 gpm | 6.7 | 70 | 234,406 | 246,743 BTU/h |
The DOE row is the example on its Energy Saver page, 3.25 gpm. Cold winter water is the biggest swing, since 40°F inlet water adds 14 percent to the load of the worked example. The fixture count matters as much, and one shower fewer takes the flow from 5.5 gpm to 3.5 gpm.
Standards and data sources
- U.S. Department of Energy, Energy Saver, Sizing a New Water Heater - the method of adding the flow of the devices used at one time, the 50°F cold water and 120°F hot water assumptions, the 70°F rise example, and the note that gas-fired demand units typically deliver that rise at 5 gpm and electric ones at 2 gpm. DOE's live page address returned an error on October 12, 2026, so the text was read from an archived copy of July 2026.
- 10 CFR 430.32(o) and (p), federal water use limits - 2.5 gpm for showerheads at 80 psi and 2.2 gpm for lavatory and kitchen faucets at 60 psi, used only to flag fixture flows above them.
- EPA WaterSense - the 2.0 gpm showerhead and 1.5 gpm bathroom faucet maximums of labeled products, shown as hints for typical low-flow fixtures.
- NIST SP 811, Appendix B - the 3,412.14 BTU/h per kW conversion, the same factor as the BTU to tons converter.
Limitations of this calculator
- This page works out the load. It does not pick a model, and it has no unit's rise-versus-flow table. A heater that cannot deliver the flow at your rise gives lukewarm water at the far fixture, so check the manufacturer's data sheet for the flow and the inlet temperature you entered.
- Peak flow is the fixtures you decide run together. The page does not guess how many people live in the house. A busy morning is the case to count.
- The page treats each fixture's full flow as hot water at the outlet temperature. A shower mixes in cold water, so it draws less from the heater than its flow suggests, which makes the result a little conservative.
- Cold water temperature is your input. There is no groundwater map here, and DOE's 50°F is a default for when you do not know. Winter inlet water is colder, and the colder the water, the more heat and the larger the unit.
- Gas efficiency is the rating on the unit you are considering. The 0.95 in the example is not the rating of any model. Electric resistance heating is taken as 100 percent efficient, which is this calculator's assumption.
- Gas input here is the heating load divided by efficiency. It is not a pipe size, a venting design or a combustion air calculation. Size the gas line for this input plus every other appliance on it, and follow the manufacturer's venting rules.
- The electric figure in amps is arithmetic at 240 V. It is not a circuit design and says nothing about whether the service can supply it. The unit's nameplate sets the circuits.
- Fixture flows are ratings at a test pressure. Real flow varies with the water pressure at the house. Units also have a minimum flow to fire, which a very low-flow fixture may not reach, so read the data sheet.
- Recirculation loops, mixing valves, solar preheat and combination space and water heating are not modeled. DOE tells you to ask a qualified contractor about combination systems.
- Plumbing, gas and electrical work is done by licensed tradespeople under the local authority having jurisdiction. Permits and codes vary by city and state, and nothing on this page is an approval of an installation.
Common tankless sizing mistakes
- Sizing by the number of bathrooms. Bathrooms do not set the load. What runs at the same moment does. A house with four bathrooms and two people can need less than a house with one bathroom and a crowded morning.
- Using 50°F water in a cold climate winter. DOE's 50°F is a fallback. In the worked example, 40°F inlet water adds 14 percent to the heat the unit has to deliver, from 192,423 to 219,912 BTU/h.
- Reading the gas input as the heat delivered. The input rating is what the burner uses. The water gets less, by the efficiency. This page shows both so the gas line is sized from the input, not from the heat in the water.
- Forgetting the gas line and the electrical side. A switch from a tank to a tankless unit can raise the gas input a great deal. The worked example needs 202,551 BTU/h of gas input. Electric, the same load is 56.4 kW.
- Counting every fixture in the house. Adding every fixture oversizes the unit. Count the ones that can really run together. Dropping one of the two showers in the worked example takes the flow from 5.5 gpm to 3.5 gpm.
- Setting the outlet above what the fixtures need. Higher outlet water means a bigger rise and a larger unit. DOE puts most uses at 120°F and warns about scalding near 140°F.
Common questions about tankless water heater sizing
How do I size a tankless water heater?
Add up the flow of the hot water devices that run at the same time, then find the temperature rise from the cold water temperature to the setting you want. That is the Department of Energy's method. Two 2.0 gpm showers and a 1.5 gpm faucet make 5.5 gpm. With 50°F water in and 120°F out the rise is 70°F, so the heater has to put 192,423 BTU/h into the water. Then compare the flow and rise with a manufacturer's data sheet.
How many gpm do I need for two showers?
Add the two shower flows, plus any faucet running at the time. Federal law caps showerheads at 2.5 gpm and WaterSense labeled heads use 2 gpm or less, so two showers are 4 to 5 gpm before a faucet. Measure yours, because older fixtures can run higher.
What temperature rise do I need?
The rise is the hot water temperature minus the cold water temperature coming in. DOE assumes 50°F cold water and 120°F hot water, a 70°F rise, unless you know otherwise. In a cold winter the inlet may be 40°F, and the worked example then needs 219,912 BTU/h instead of 192,423. Take a thermometer reading from your cold tap if you can.
Gas or electric tankless, which size do I need?
Heat into the water is the same either way. Fuel changes the input. The worked example needs 202,551 BTU/h of gas at an efficiency of 0.95, or 56.4 kW electric, which is 235 A at 240 V. DOE notes that gas-fired demand units typically deliver a 70°F rise at 5 gpm and electric ones at 2 gpm, which is why a whole-house electric unit is a big electrical job.
What size gas line does a tankless water heater need?
Start from the unit's gas input in BTU/h, which this page works out as the heat into the water divided by efficiency. The worked example needs 202,551 BTU/h. Then size the gas line for that input plus every other gas appliance on it, using the gas pipe sizing calculator on this site. The manufacturer's installation manual also sets venting and clearances.
What flow rates do showers and faucets use?
Federal rules cap showerheads at 2.5 gpm at 80 psi and lavatory and kitchen faucets at 2.2 gpm at 60 psi (10 CFR 430.32). WaterSense labeled showerheads use 2 gpm or less, and labeled bathroom faucets 1.5 gpm or less. Older fixtures and multi-head showers can run higher, so measure if you can.
What if one unit is not enough?
A load above what one unit delivers at your rise can be split across two units linked together, or the peak can be cut by staggering fixtures. This page does not name a unit size. The right input is the one that delivers your peak flow at your rise, from the manufacturer's data sheet.
Do I need a plumber or an electrician?
Yes. This page works out a load for estimating and for checking a quote. Licensed tradespeople install the unit, the gas line, the venting and the electrical circuits under a permit, and the local authority having jurisdiction inspects the work.
Used on the same job
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The chat does not size water heaters. It runs the free whole-house Manual J load for the heating and cooling side of the same house. For hot water, use this page and the manufacturer's data sheet.
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