What size tankless water heater for 5 bathrooms?11.0 GPM at 50 F rise
Add simultaneous fixture flow and the rise from incoming water to the delivery temperature.
Your starting point
11.0 GPM at 50 F rise
Planning range
9.9–12.7 GPM
A screening estimate under the assumptions below. The range is a shopping guide, not a guarantee.
Planning estimate. A licensed contractor must confirm the final size, installation and local code requirements. Electrical work needs a licensed electrician.
Adjust your home
The calculation, step by step.
Simultaneous flow x temperature rise x 500 gives BTU/h delivered; divide by assumed thermal efficiency for input.
| Input or step | Calculated amount |
|---|---|
| Showers | 10.0 GPM |
| Faucets | 1.0 GPM |
| Dishwashers | 0.0 GPM |
| Temperature rise | 50 F |
| Delivered heat | 275,000 BTU/h |
| Estimated heater input | 289,474 BTU/h |
What moves this number?
Inlet temperature
Colder winter water increases required heating power.
Simultaneous use
Count fixtures running at once, not total bathrooms.
Flow rates
Measure fixture flow; the selected defaults are assumptions.
Your location and assumptions.
Harris, Texas is IECC 2021 zone 2A. The zone supplies an illustrative inlet-temperature preset; measured winter inlet temperature is more useful. Other counties in this state can have a different zone. Local code adoption may use another edition.
- Inlet 70 F, delivery 120 F; 95% assumed thermal efficiency.
- 500 = 8.33 lb per gallon x 60 minutes x about 1 BTU/lb/F.
- Default groundwater is an illustrative zone preset, not a measured local temperature.
- Showers 2 GPM; faucets 1 GPM; dishwasher 1.5 GPM assumed.
What might it cost to run?
For an electric system under the selected efficiency assumption: $12.67 for one equivalent hour at the full selected flow. Most uses are shorter; gas costs require a local fuel rate. Uses the published 2024 EIA Texas residential average of $0.1494/kWh, not your current tariff.
Before you choose equipment.
Choose a manufacturer flow chart at this exact temperature rise. Advertised maximum GPM is often at a much smaller rise. Verify fuel, electrical service and venting with the installer.
Bring this calculation to your installer. They will check the measured load, manufacturer performance, distribution or connections, installation conditions and local requirements.
A few more sizing questions.
What size tankless heater for 5 bathrooms?
The default screen gives 11.0 GPM at 50 F rise, with a planning range of 9.9–12.7 GPM. Change the inputs above to reflect your actual home. Bathroom count seeds a fixture example; simultaneous fixture use is what determines flow.
Why does incoming-water temperature matter?
At 70°F inlet and 120°F delivery, the required rise is 50°F. Colder inlet water increases the heating power needed for the same flow.
Can I use the advertised maximum GPM?
Only if its tested temperature rise matches your conditions. Use the manufacturer chart at 50°F rise, not the largest number on the product page.
How is heating power calculated?
Flow in GPM × temperature rise in °F × 500 estimates delivered BTU/h. This case needs 275,000 BTU/h delivered; the input estimate also accounts for the selected efficiency.
Can an electric tankless unit serve this demand?
The selected-efficiency input is equivalent to about 84.8 kW. Verify actual electric-heater efficiency, available service, dedicated circuits and winter flow with a licensed electrician and installer.
Is this a final equipment size?
No. A licensed contractor must measure your building or actual loads, check manufacturer performance and local code, and confirm the final selection.