Tankless Water Heater Sizing Calculator

Estimate the required flow rate and BTU input for a tankless water heater based on fixture demand and incoming water temperature.

Desired output temperature (°F) Typically 120°F for domestic hot water

Incoming (groundwater) temperature (°F) Varies by region/season, typically 40-60°F

Simultaneous flow rate needed (GPM) Sum of fixtures running at once — e.g. 2 showers ≈ 5 GPM

Unit thermal efficiency (%) Typically 82-96% depending on model

Enter valid numbers for all fields.

Estimated tankless heater sizing

Required temperature rise

Output BTU/hr required

recommended minimum input BTU/hr rating

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Calculated using the standard formula: BTU/hr = GPM × 8.33 × temperature rise (°F) × 60, adjusted for unit thermal efficiency. Groundwater temperature varies significantly by region and season — use your local incoming water temperature for an accurate estimate. Always verify final unit selection against the manufacturer's published performance charts, which account for altitude and specific flow/rise combinations.