Tankless Water Heater Flow Rate Calculator
Determine the minimum flow rate (GPM) your tankless water heater must deliver to meet your household hot water demand.
Typical groundwater temperature varies by region (40–70 °F).
Recommended safe delivery temperature is 120 °F (OSHA / ASHRAE).
Check your unit's spec sheet. Common residential units: 120,000–199,000 BTU/hr.
Most condensing tankless heaters: 90–98 %. Non-condensing: 80–85 %.
Formula
Flow Rate (GPM) = (Qinput × η) / (500.4 × ΔT)
- Qinput — Heater BTU/hr input rating
- η — Thermal efficiency (decimal, e.g. 0.95)
- 500.4 — Unit conversion constant = 8.34 lb/gal × 60 min/hr × 1 BTU/(lb·°F)
- ΔT — Temperature rise = Tout − Tin (°F)
Derived from the steady-state heat transfer equation: Q = ṁ × Cp × ΔT, where water's specific heat Cp ≈ 1 BTU/(lb·°F) and density ≈ 8.34 lb/gal.
Assumptions & References
- Water specific heat capacity: 1.0 BTU/(lb·°F) — standard engineering value for liquid water.
- Water density: 8.34 lb/gal at typical domestic temperatures.
- Recommended hot water delivery temperature: 120 °F per OSHA 3261 and ASHRAE 188 to prevent scalding and Legionella growth.
- Groundwater inlet temperatures: 40–70 °F depending on geographic region (U.S. DOE data).
- Typical low-flow showerhead: 2.0 GPM (EPA WaterSense standard).
- Typical bathroom faucet: 1.5 GPM (EPA WaterSense standard).
- Condensing tankless heaters achieve 90–98 % efficiency; non-condensing 80–85 % (U.S. DOE ENERGY STAR).
- Formula source: ASHRAE Fundamentals Handbook, Chapter on Service Water Heating.
- This calculator assumes steady-state operation; startup lag and pipe heat loss are not modeled.