# Tankless water heater size calculator: GPM at the same time, temperature rise, and the BTU/h or kW that takes

> Add the gpm of fixtures running at once, subtract incoming from outlet temperature: BTU/h = GPM x 500 x rise, kW = GPM x rise / 6.83. 4.7 gpm at 73 F is 50.2 kW.

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| Source | https://ownerspec.com/plumbing/calculators/tankless-water-heater-size-gpm-temperature-rise/ |
| Publisher | OwnerSpec (https://ownerspec.com/) |
| Published | 2026-09-24 |
| Updated | 2026-09-24 |
| Facts verified | 2026-09-24 |
| Disclosure | Contains affiliate links. OwnerSpec may earn a commission at no cost to the reader. |

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## Short answer

A tankless water heater is sized by two numbers, not by the peak GPM printed on the box: the
flow of every fixture that will run at once, in gallons per minute, and the temperature rise,
which is the outlet setting minus the incoming water temperature. The heat input follows from
them. Gas: BTU/h = GPM x 500 x rise, divided by the burner's efficiency. Electric: kW = GPM x
rise / 6.83, the constant Stiebel Eltron prints on its Tempra spec sheet. Worked example: one
federal-maximum 2.5 gpm showerhead plus one 2.2 gpm kitchen faucet is 4.7 gpm. With 47 F
incoming water and a 120 F outlet the rise is 73 F. That is 171,550 BTU/h of heat, about
182,500 BTU/h of gas input at 94 percent efficiency, or 50.2 kW of electric element, more
than the largest 36 kW Tempra makes at 240 V. The 500 is 8.33 lb per gallon times 60 minutes;
6.83 is 3,412 BTU per kWh divided by 500.

## Fact sheet

- **Answer:** Add the flow of every fixture that runs at once, subtract the incoming water temperature from the outlet setting, then heat = GPM x 500 x rise in BTU/h and electric power = GPM x rise / 6.83 in kW. One 2.5 gpm showerhead plus one 2.2 gpm kitchen faucet with 47 F incoming water and a 120 F outlet is 4.7 gpm at a 73 F rise: 171,550 BTU/h of heat, or 50.2 kW.
- **Formula:** `Rise (F) = outlet temperature - incoming temperature; Heat (BTU/h) = GPM x 500 x rise; Gas input (BTU/h) = heat / efficiency; Electric (kW) = GPM x rise / 6.83; Amps at 240 V = kW x 1,000 / 240`
- **Inputs:** Showerheads running at once: 1 at 2.5 gpm (federal maximum); Kitchen faucets running at once: 1 at 2.2 gpm (federal maximum); Incoming water temperature: 47 F; Outlet temperature: 120 F; Gas efficiency: 0.94
- **Output:** 4.7 gpm at a 73 F rise: 171,550 BTU/h of heat, 182,500 BTU/h of gas input at 0.94, or 50.2 kW of electric element, which is 209 A at 240 V.
- **Assumptions:** 500 is 8.33 lb per gallon x 60 minutes per hour x 1 BTU per pound per degree F, which is 499.8; 6.83 is 3,412 BTU per kWh / 500, and is the constant Stiebel Eltron prints on its Tempra spec sheet; Fixture flows are the federal maximums (2.5 gpm showerheads, 2.2 gpm faucets) or the WaterSense maximums (2.0 gpm and 1.5 gpm); the reader's own fixtures may be lower; Incoming water temperature is read off the Rinnai residential ground water map, 35 F to 77 F across the US, or measured at the cold tap in the coldest month; Gas efficiency defaults to 0.94, which is what Rinnai's own RU130i table implies at 77 F incoming water: 5.7 gpm at a 43 F rise is 122,550 BTU/h from a 130,000 BTU/h input
- **Method:** The calculator sums the fixture flows the reader selects, subtracts the incoming temperature from the outlet temperature, applies the two constants above, and divides the electric kW by 240 V to show the amperage next to the manufacturers' own breaker tables.
- **Primary source:** [Stiebel Eltron, Tempra Series Tankless Electric Water Heaters engineering specification sheet (PDF)](https://www.stiebel-eltron-usa.com/sites/default/files/pdf/specification-sheet-tempra.pdf) (manufacturer or vendor document)
- **Facts verified:** 2026-09-24
- **Last updated:** 2026-09-24
- **Canonical URL (cite this):** https://ownerspec.com/plumbing/calculators/tankless-water-heater-size-gpm-temperature-rise/

If your number is the "up to 7 GPM" from a listing, put it down: that flow is measured at a rise most of the country never sees in January.

> Interactive calculator, "Flow at the same time, temperature rise, and the input it takes": use it on the HTML page at the URL above; the formula, constants and sources are written out below.

## What is the formula?

Two inputs from the house, then arithmetic.

| Step | Formula | What each term is |
| --- | --- | --- |
| Flow | GPM = sum of every fixture open at once | showerheads, faucets, appliances, in gallons per minute |
| Rise | rise = outlet temperature - incoming temperature | the outlet is the setting on the unit; the incoming is the cold tap in the coldest month |
| Heat | BTU/h = GPM x 500 x rise | the heat the water carries away, before any burner loss |
| Gas | input BTU/h = heat / efficiency | the burner rating on the gas unit's spec sheet |
| Electric | kW = GPM x rise / 6.83 | the element rating on the electric unit's spec sheet |

Temperature rise is the number the box hides: at a 73 F rise the electric formula gives a 27 kW unit 27 x 6.83 / 73 = 2.5 gpm, one showerhead.

The fixture flows are legal ceilings, so they are the safe default. DOE's FEMP Best Management Practice #7 lists the federal standards in force since 1 January 1994: showerheads no more than 2.5 gpm, lavatory and kitchen faucets no more than 2.2 gpm. WaterSense-labelled fixtures are lower, 2.0 gpm for showerheads and 1.5 gpm for residential lavatory faucets.

## What does a worked example look like?

The numbers the calculator opens on: a shower and the kitchen sink at once, with 47 F ground water.

1. Flow: 2.5 gpm showerhead + 2.2 gpm kitchen faucet = 4.7 gpm.
2. Rise: 120 F outlet - 47 F incoming = 73 F.
3. Heat: 4.7 x 500 x 73 = 171,550 BTU/h.
4. Gas input at 0.94 efficiency: 171,550 / 0.94 = 182,500 BTU/h.
5. Electric: 4.7 x 73 / 6.83 = 50.2 kW, which is 209 A at 240 V.

No residential unit on Stiebel Eltron's sheet reaches that; its biggest Tempra is 36 kW. The sheet's own table is the check: the 24 kW Tempra at 240 V gives a 54 F rise at 3.0 gpm, and 24 x 6.83 / 3.0 = 54.6 F.

## Where do the numbers come from?

| Constant | Value | Who prints it |
| --- | --- | --- |
| Weight of water | 8.33 lb per gallon | USGS, Facts About Water |
| One BTU | heat to raise 1 lb of water 1 F | EIA glossary |
| The 500 | 8.33 x 60 min/h x 1 = 499.8 | derived from the two lines above |
| One kWh | 3,412 BTU | EIA, Units and Calculators |
| The 6.83 | 3,412 / 500 = 6.82 | Stiebel Eltron's sheet prints gpm = kW x 6.83 / rise |
| Ground water | 35 F to 77 F across the US | Rinnai residential ground water map |
| Gas efficiency default | 0.94 | implied by Rinnai's RU130i table, below |

The gas default comes from a maker's table. Rinnai rates its 130,000 BTU/h RU130i at 5.7 gpm with 77 F ground water and a 120 F set point: a 43 F rise, 5.7 x 500 x 43 = 122,550 BTU/h delivered, 94 percent of the input. Enter your unit's own figure if its sheet gives one.

Incoming temperature is measured, not assumed. Rinnai's map bands the country from 35 F in the far north to 77 F in south Florida, with 75 F for Hawaii and Puerto Rico, and says inlet temperature varies locally and by season. DOE's Energy Saver page on demand water heaters returned a 404 to every fetch on 24 September 2026, so this page cites the manufacturers and the University of Georgia extension bulletin, which puts gas tankless heaters at about five gallons per minute and electric at two or three.

## How many amps does an electric unit draw?

Every figure is from the maker's own spec sheet or install guide, at 240 V single phase unless marked.

| Model | kW | Breakers | Maximum amps | Wire |
| --- | --- | --- | --- | --- |
| Rheem RTEX-04T (120 V) | 3.5 | 1 x 30 A single pole | 29 | 10 AWG |
| Rheem RTEX-06T | 6.5 | 1 x 30 A double pole | 27 | 10 AWG |
| Stiebel Eltron Tempra 12 | 12 | 1 x 50 A double pole | 50 | 6 AWG |
| EcoSmart ECO 18 | 18 | 2 x 40 A double pole | 75.0 | 8 AWG, 2 sets |
| Stiebel Eltron Tempra 20 | 19.2 | 2 x 40 A double pole | 80 | 8 AWG |
| Stiebel Eltron Tempra 24 | 24 | 2 x 50 A double pole | 100 | 6 AWG |
| EcoSmart ECO 27 | 27 | 3 x 40 A double pole | 112.5 | 8 AWG, 3 sets |
| Stiebel Eltron Tempra 36 | 36 | 3 x 50 A double pole | 150 | 6 AWG |

The arithmetic checks: 27 kW x 1,000 / 240 V is 112.5 A, exactly EcoSmart's figure. A 27 kW unit needs three double-pole slots and 112.5 A the panel does not already use, and the 50.2 kW in the worked example has no residential unit behind it at all.

An electrician confirms the panel, not this page. EcoSmart's guide says the unit must be installed by a licensed electrician and plumber, and Rheem notes 25 percent reduced output on 208 V. New breakers and conductors are licensed work with a permit in most jurisdictions; the adopted code varies.

The two Rheem point-of-use picks are the honest end of the range, one 30 A circuit each, and the formula reproduces Rheem's table: 6.5 x 6.83 / 48 F is 0.9 gpm, its zone 5 figure for the RTEX-06T at 105 F.

**Measure the cold tap before the cart**

The flow side is fixed by law; the rise side is your house in January. A thermometer under the cold tap on the coldest morning decides between a unit that works and one that runs lukewarm for four months.

If the answer is one remote sink, the Rheem RTEX-06T, 6.5 kW at 240 V fits on one 30 A circuit, and the RTEX-04T at 120 V does the same for one low-flow faucet in a warm zone. Skip both if the answer was a shower: 6.5 kW at a 73 F rise is 0.6 gpm. Past 36 kW the electric route is over, and the [first hour rating calculator](/plumbing/calculators/water-heater-size-first-hour-rating/) sizes a tank instead.

Hard water scales any tankless heat exchanger; the [hard water scale guide](/water/guides/hard-water-white-scale/) gives the grains-per-gallon threshold.

## The picks

- **Rheem RTEX-04T, 3.5 kW, 120 V point-of-use**: One low-flow faucet at a remote sink, 1 x 30 A single-pole circuit (Rheem's sheet lists 3.5 kW at 120 V, 29 A, a 1 x 30 A single-pole breaker and 10 AWG wire; 0.5 to 0.9 gpm at a 105 F set point in zones 5 to 9 and not recommended in zones 1 to 4; WQA tested to NSF/ANSI 372 for lead-free compliance) Amazon: https://www.amazon.com/dp/B0CJJY7RZZ/?tag=ownerspec-20
- **Rheem RTEX-06T, 6.5 kW, 240 V point-of-use**: One bathroom faucet, or two low-flow faucets at once, on a 1 x 30 A double-pole circuit (Rheem's sheet lists 6.5 kW at 240 V, 27 A, a 1 x 30 A double-pole breaker and 10 AWG wire; 0.7 gpm in zone 1 (37 F ground water) to 1.6 gpm in zone 9 (77 F) at a 105 F set point; WQA tested to NSF/ANSI 372) Amazon: https://www.amazon.com/dp/B0CJJXMCLJ/?tag=ownerspec-20

## Frequently asked questions

### What size tankless water heater do I need for a shower and a sink at the same time?

Add the two flows, then find the rise. A federal-maximum 2.5 gpm showerhead and a 2.2 gpm kitchen faucet are 4.7 gpm. With 47 F incoming water and a 120 F outlet, the rise is 73 F, which is 171,550 BTU/h of heat or 50.2 kW. Two WaterSense fixtures, 2.0 and 1.5 gpm, cut that to 3.5 gpm and 37.4 kW.

### What temperature rise should I use for a tankless water heater?

Outlet setting minus the coldest incoming water you will see. Rinnai's residential ground water map runs from 35 F in the far north to 77 F in south Florida (75 F for Hawaii and Puerto Rico), and it warns that actual inlet temperature varies locally and by season. Measure the cold tap in January if you can; a winter number sizes the heater, a summer number undersizes it.

### Can a 27 kW electric tankless water heater run a whole house?

Only at a small rise or a small flow. By the kW x 6.83 / rise formula, 27 kW delivers 2.5 gpm at a 73 F rise, one showerhead. EcoSmart's own install guide says its 27 kW unit draws 112.5 A on three 40 A double-pole breakers, and Stiebel Eltron lists its 36 kW Tempra at 150 A on three 50 A breakers.

### Where does the 500 in the BTU formula come from?

From water itself. USGS gives water a weight of 8.33 pounds per gallon, an hour has 60 minutes, and EIA defines a BTU as the heat that raises one pound of water one degree F. 8.33 x 60 x 1 is 499.8, rounded to 500. Divide EIA's 3,412 BTU per kWh by 500 and you get 6.82, the electric constant Stiebel Eltron prints as 6.83.

### Does a 120 V point-of-use tankless heater need an electrician?

Rheem's sheet for the 3.5 kW RTEX-04T lists 29 A on a 1 x 30 A single-pole breaker with 10 AWG wire, which is a dedicated circuit, not a wall outlet. Adding a breaker is licensed electrical work with a permit in most jurisdictions, and EcoSmart's guide says its whole-house units must be installed by a licensed electrician and plumber. Codes are adopted locally and vary.


## Sources

- [Stiebel Eltron, Tempra Series Tankless Electric Water Heaters engineering specification sheet (PDF)](https://www.stiebel-eltron-usa.com/sites/default/files/pdf/specification-sheet-tempra.pdf) (manufacturer or vendor document)
- [DOE Federal Energy Management Program, Best Management Practice #7: Faucets and Showerheads](https://www.energy.gov/cmei/femp/best-management-practice-7-faucets-and-showerheads) (government or regulatory source)
- [EPA WaterSense, Showerheads](https://www.epa.gov/watersense/showerheads) (government or regulatory source)
- [EcoSmart, Quick Install Guide, Models Eco 18-21-24-27 (PDF)](https://images.thdstatic.com/catalog/pdfImages/79/79ddcb47-8a11-4535-b0b8-a91762a07ac8.pdf) (retailer listing or retailer-hosted document)
- [Rheem, Professional Classic RTEX-04T and RTEX-06T point-of-use tankless electric specification sheet, form TKE-101-POUT (PDF)](https://files.myrheem.com/webpartners/ProductDocuments/4C4D1520-21FF-4814-B355-4E3DD7083F21.pdf) (manufacturer or vendor document)
- [Rinnai, United States Ground Water Temperature Map, Residential Use (PDF)](https://www.rinnai.us/download/download?path=%2Fs--kAc5SDo_--%2Futmb0tfm7jfcjbagjeti.pdf&filename=Ground+water+temperature+map+update.pdf) (manufacturer or vendor document)
- [USGS Water Science School, Facts About Water](https://www.usgs.gov/water-science-school/science/facts-about-water) (government or regulatory source)
- [EIA, Energy Explained: Units and Calculators](https://www.eia.gov/energyexplained/units-and-calculators/) (government or regulatory source)
- [EIA Glossary, British thermal unit](https://www.eia.gov/tools/glossary/index.php?id=British%20thermal%20unit) (government or regulatory source)
- [University of Georgia Cooperative Extension, Choosing a New Water Heater, HACE-E-60-06 (PDF)](https://www.fcs.uga.edu/docs/Energy_Water_Heater_HACE-E-60-06.pdf) (university or extension service)

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