Tankless Water Heater Sizing Calculator
Size a tankless water heater by calculating simultaneous flow demand, temperature rise, and required BTU capacity.
Inputs
Results
Total flow demand (GPM)
10.5
Temperature rise (°F)
70
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.
What each input means
- Simultaneous showers
- Max showers running at the same time (2.0 GPM each).
- Simultaneous sinks
- Hot water faucets running at once (1.5 GPM each).
- Dishwasher running? (0/1)
- Dishwasher uses ~1.5 GPM hot water.
- Washing machine running? (0/1)
- Hot/warm wash cycle uses ~2.0 GPM.
- Groundwater temp (°F)
- Incoming cold water temperature. North: 40-50°F, South: 60-72°F.
- Target hot water temp (°F)
- Desired hot water delivery temperature (120°F is standard).
What each result means
- Total flow demand (GPM)
- Peak simultaneous hot water flow rate.
- Temperature rise (°F)
- Degrees the water must be heated.
- Required BTU/hr
- Heat output needed (before efficiency losses).
- Condensing unit BTU/hr
- Gas input needed for a condensing unit (95% efficient).
- Electric equivalent (kW)
- Equivalent electric tankless size.
- Gas line size (in.)
- Minimum gas supply pipe diameter.
- Vent diameter (in.)
- Exhaust vent pipe diameter.
- Units needed
- Number of tankless units (max ~199k BTU each for gas).
- Equipment cost (low)
- Budget-end tankless unit(s).
- Equipment cost (high)
- Premium-end tankless unit(s).
- Installation cost
- Professional installation estimate.
- Annual savings vs tank
- Estimated yearly energy savings over a 50-gal tank heater.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersSimultaneous showers = 2, Simultaneous sinks = 2, Dishwasher running? (0/1) = 1, Washing machine running? (0/1) = 1 = 6 input(s) provided
- Calculate Total flow demandTotal flow demand = showers * showerFlowGPM +10.5 = 10.5
- Calculate Temperature riseTemperature rise = targetTempF - groundwaterTempF70 = 70
- Calculate Required BTU/hrRequired BTU/hr = totalFlowGPM * tempRiseF * 500367500 = 367500
- Calculate Condensing unit BTU/hrCondensing unit BTU/hr = requiredBTU / 0.95386842 = 386842
Engine last updated . Checked against 1 independently-derived test — how we verify calculators.
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