Tankless Water Heater Sizing Calculator
Size a tankless water heater by calculating simultaneous flow demand, temperature rise, and required BTU capacity.
Inputs
°F
°F
Results
Total flow demand (GPM)
10.5
Temperature rise (°F)
70
Required BTU/hr367,500
Condensing unit BTU/hr386,842
Electric equivalent (kW)107.7
Gas line size (in.)1
Vent diameter (in.)4
Units needed2
Equipment cost (low)$2,400
Equipment cost (high)$5,600
Installation cost$3,000
Annual savings vs tank$145
Non Condensing BTU448,170.73
Gas Model SizeMultiple units needed (>199k BTU)
How to Use This Calculator
- Enter the maximum number of simultaneous showers, sinks, and appliances that could run at once.
- Enter your groundwater temperature in °F — northern states average 40-50°F, southern states 60-72°F.
- Set your target hot water delivery temperature (120°F is the standard safe setting).
- Review Required BTU/hr and Electric Equivalent (kW) to match against available models.
- Check Units Required — if more than 1, you may need two units or a larger commercial-grade model.
- Use Annual Savings vs. Tank to estimate payback period against a traditional 50-gallon tank heater.
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