How to Size a Tankless Water Heater
The most common reason a tankless water heater underperforms is being sized too small. Unlike a tank heater that stores pre-heated water, a tankless unit must heat water on demand. Getting the size right requires knowing your peak flow rate and your required temperature rise.
Updated 28 March 2026
Why Sizing Is Critical
A tank heater stores 40 to 80 gallons of preheated water. Even an undersized tank heater can deliver acceptable performance until the tank empties. A tankless heater has no stored reserve. If you draw more hot water than the unit can heat, the output temperature drops immediately. Two showers running simultaneously overwhelm an undersized unit.
The unit's capacity (BTU or kilowatts) determines how much water it can heat to the target temperature at a given flow rate. A larger temperature rise or higher flow rate both require more capacity. Cold-climate homes need significantly larger units than warm-climate homes because the incoming water starts much colder.
Step 1: Calculate Your Peak Flow Rate
Identify how many hot water fixtures will run simultaneously at your peak demand time. This is typically morning when multiple people are showering and the dishwasher or washing machine may also be running.
| Fixture | Typical GPM | Notes |
|---|---|---|
| Shower (standard head) | 1.5 to 2.5 | Low-flow: 1.5 GPM, standard: 2.0 GPM |
| Shower (rain or dual head) | 2.5 to 4.0 | High-flow heads add significantly |
| Kitchen faucet | 1.0 to 2.2 | Varies widely by fixture |
| Bathroom faucet | 0.5 to 1.5 | Low-flow codes: 0.5 to 1.0 GPM |
| Dishwasher | 1.0 to 2.0 | Modern dishwashers use less |
| Washing machine (hot) | 1.5 to 3.0 | Most modern washers use warm, not hot |
| Bathtub fill | 3.0 to 5.0 | High flow for tub fill time |
Example: Family of four morning peak
This household needs a tankless heater rated for at least 5.5 GPM at their required temperature rise. Many families find they need a 7 to 8 GPM unit to allow headroom for less predictable simultaneous use.
To measure your actual shower flow rate: hold a bucket under the showerhead for 15 seconds, measure the water collected in gallons, and multiply by 4. A reading of 0.5 gallons in 15 seconds equals 2.0 GPM.
Step 2: Calculate Required Temperature Rise
The temperature rise is the difference between the incoming ground water temperature and your desired output temperature. Most households target 120 degrees Fahrenheit at the heater, which is the recommended safe hot water temperature.
Formula
| Region | Winter ground water (°F) | Required temp rise |
|---|---|---|
| Southeast US (FL, GA, LA) | 65 to 75°F | 45 to 55°F rise |
| Mid-Atlantic (DC, VA, NC) | 55 to 65°F | 55 to 65°F rise |
| Midwest (OH, IL, IN) | 45 to 55°F | 65 to 75°F rise |
| Northern US (MN, WI, NY) | 35 to 45°F | 75 to 85°F rise |
| Mountain West (CO, UT, MT) | 40 to 55°F | 65 to 80°F rise |
Manufacturers publish sizing charts showing GPM output at various temperature rises for each model. Use these charts with your calculated flow rate and temperature rise to identify the minimum unit size needed. Always size up by 20 to 30% to allow for peak demand headroom.
Recommended Unit Size by Household
| Household | Warm climate | Cold climate | BTU (gas) |
|---|---|---|---|
| 1 to 2 people, 1 bathroom | 4 to 5 GPM | 6 to 7 GPM | 110,000 to 150,000 |
| 2 to 3 people, 1 to 2 bathrooms | 5 to 6 GPM | 7 to 9 GPM | 140,000 to 175,000 |
| 3 to 4 people, 2 to 3 bathrooms | 6 to 8 GPM | 8 to 10 GPM | 160,000 to 199,000 |
| 4+ people, 3+ bathrooms | 8 to 10 GPM | 2 units or 10+ GPM | 199,000+ or dual units |
Common Sizing Mistakes
Using summer flow rates for a year-round system
Ground water is coldest in winter, which is when your heater must work hardest. Size for your worst-case scenario (coldest months, peak simultaneous use), not average conditions. A unit that handles summer just fine may struggle in January.
Not accounting for low-pressure activation
Most tankless heaters require a minimum flow of 0.5 to 0.75 GPM to activate. Very low-flow fixtures (such as a bathroom faucet turned to a trickle) may not trigger the unit. This is a practical issue for some users. Check the minimum activation flow rate in the specifications.
Ignoring installation infrastructure needs
A correctly sized unit is useless if the gas line or electrical circuit cannot support it. Confirm with a plumber and electrician that the required infrastructure is in place before purchasing the unit. Undersized gas supply is a common cause of poor performance even with the right heater.
Flow rates and sizing guidance represent typical values for standard US residential fixtures. Actual sizing should always be confirmed by a licensed plumber familiar with tankless heater installation in your climate zone. Manufacturer sizing charts for your specific model are the definitive reference.