Heating, Cooling & Water · Calculator
Tankless Water Heater Size Calculator
This tankless water heater size calculator adds up the fixtures you run at once and your groundwater temperature to find the flow rate (GPM) and temperature rise you need, then turns that into a gas BTU rating or electric kW and amps.
Your hot water demand
VA, WV, MD, DE, KY, TN, MO, KS, NC, NM, NV, northern CA
Size for winter. Overwrite with your own reading.
120 F is the usual setting.
95 condensing, about 82 non-condensing.
| 30 F rise | 12.6 GPM |
| 40 F rise | 9.5 GPM |
| 50 F rise | 7.6 GPM |
| 60 F rise | 6.3 GPM |
| 65 F rise (yours) | 5.8 GPM |
| 70 F rise | 5.4 GPM |
| 80 F rise | 4.7 GPM |
How to size a tankless water heater
A tank heater is sized by gallons stored. A tankless heater is sized by two numbers: peak flow rate (how many gallons per minute you draw at your busiest moment) and temperature rise (how many degrees it has to heat the water). Get both right and you never run out.
The 500 in the gas formula is 8.33 pounds per gallon times 60 minutes. It converts GPM and degrees into BTU per hour. Divide by efficiency because a gas unit rated at 199,000 BTU input only puts about 95 percent of that into the water.
Not sure how the burner or heating elements keep up? Our explainer on how tankless water heaters work walks through the flow sensor and modulation.
Fixture flow rates
These are the defaults the calculator uses. They lean slightly high on purpose, because undersizing is the one tankless mistake you cannot fix later. Check the GPM stamped on your shower head or faucet aerator and edit the numbers above.
| Fixture | GPM used | Notes |
|---|---|---|
| Shower | 2.0 | WaterSense heads 2.0 or less, federal max 2.5 |
| Bathroom faucet | 1.5 | WaterSense max 1.5, many new faucets 1.2 |
| Kitchen faucet | 1.8 | Federal max 2.2, most new faucets 1.5 to 1.8 |
| Dishwasher | 1.5 | Fills briefly, 1.0 to 2.0 |
| Washing machine (hot fill) | 2.0 | 1.5 to 3.0 while filling |
| Bathtub | 4.0 | Tub spouts run 4 to 6 |
Groundwater temperature by region
Your incoming water temperature sets the rise, and it changes more than most people expect. Water in Minnesota can arrive at 40 F in January, while Florida water rarely drops below 70 F. That is the difference between an 80 F rise and a 50 F rise.
Always size for winter. A unit that looks perfect in July can turn a shower lukewarm in February. If you have a thermometer, run the cold tap for two minutes in winter and measure it. That beats any map.
| Region | Temp | States |
|---|---|---|
| Far north | 40 F | MN, WI, MI, ND, SD, MT, ME, VT, NH, upstate NY, AK |
| North | 50 F | PA, OH, IN, IL, IA, NE, NY, MA, CT, RI, NJ, CO, UT, ID, WY, WA, OR |
| Middle | 55 F | VA, WV, MD, DE, KY, TN, MO, KS, NC, NM, NV, northern CA |
| South | 65 F | GA, SC, AL, MS, AR, OK, north and central TX, AZ, southern CA |
| Gulf and Florida | 72 F | FL, south TX, south LA, HI |
Gas vs electric tankless
The table shows why electric whole-house units struggle in cold climates. At a 70 F rise, a 36 kW electric unit delivers about 3.5 GPM while a 199,000 BTU gas unit delivers about 5.4 GPM. Electric also needs 150 amps and several double-pole breakers, which most 200 amp panels cannot spare.
Electric tankless makes sense in warm regions, for a single bathroom, or as a point-of-use heater under a sink. For a whole house up north, go gas, or read our tankless vs tank comparison before you commit.
Venting and gas line
| Rise | 199k gas | 160k gas | 36 kW | 18 kW |
|---|---|---|---|---|
| 30 F rise | 12.6 GPM | 10.1 GPM | 8.2 GPM | 4.1 GPM |
| 50 F rise | 7.6 GPM | 6.1 GPM | 4.9 GPM | 2.5 GPM |
| 70 F rise | 5.4 GPM | 4.3 GPM | 3.5 GPM | 1.8 GPM |
Tankless sizing mistakes to avoid
- Sizing for every fixture in the house. You will never run every tap at once. Size for your realistic busiest moment, usually two showers plus one more fixture.
- Using the headline GPM. Box ratings often assume a 35 F or 45 F rise. Find the flow at your rise on the spec sheet, or use the max flow table in the calculator.
- Ignoring shower heads. Swapping 2.5 GPM heads for efficient 1.8 GPM shower heads cuts peak flow by 1.4 GPM with two showers running, which can drop you a full size.
- Forgetting power outages. Gas units need 120 V for the fan and controls, typically under 100 W running. A small generator keeps hot water flowing. Size one with our generator wattage calculator.
- Skipping descaling. Hard water scales the heat exchanger. Install isolation valves and flush the unit once a year.
Once the tankless water heater size calculator gives you a GPM and rise, compare models in our best tankless water heaters guide.
Frequently asked questions
What size tankless water heater do I need for a family of 4?
Most families of four need 5 to 6 GPM, which across the middle and northern US means a 180,000 to 199,000 BTU gas unit. That covers two showers and a faucet at once with a 65 to 70 F temperature rise. In Florida or the Gulf Coast, a 140,000 to 160,000 BTU unit often does the same job.
How do you calculate tankless water heater size?
Add up the flow rate (GPM) of every fixture you run at the same time, then subtract your incoming water temperature from your setpoint to get the temperature rise. Gas input needed is GPM x 500 x rise divided by efficiency. Electric kW needed is GPM x rise divided by 6.83.
Is a 199,000 BTU tankless water heater enough?
For most homes, yes. At 95 percent efficiency a 199,000 BTU unit delivers about 5.4 GPM at a 70 F rise and 7.6 GPM at a 50 F rise. That is two showers plus a faucet in a cold climate. If you need more, link two units together.
Can an electric tankless water heater run a whole house?
In warm climates, often yes. In cold climates, usually not. A 36 kW unit, the largest common residential size, delivers about 3.5 GPM at a 70 F rise, which is barely two low-flow showers, and it draws 150 amps. Most northern homes are better served by gas or a heat pump tank.
What temperature should a tankless water heater be set at?
120 F is the standard setting. It is hot enough for showers and dishwashers and low enough to limit scalding. Every degree you lower the setpoint reduces the rise the heater has to deliver, which raises its effective flow rate.
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