⚡ Tankless Water Heater Size Calculator
🌍 Global sizing tool • Advanced flow & temperature analysis
Proper sizing of a tankless water heater depends on: inlet water temperature, desired output temperature, peak fixture demand (flow rate), and simultaneous usage. This calculator uses USA & international standards (ASHRAE, EU norms) to recommend the required flow capacity (GPM / L/min), heating power (kW / BTU/h), and electric/gas equivalents. Adjust climate region, fixtures, and diversity factor for real-world accuracy.
Proper sizing of a tankless water heater depends on: inlet water temperature, desired output temperature, peak fixture demand (flow rate), and simultaneous usage. This calculator uses USA & international standards (ASHRAE, EU norms) to recommend the required flow capacity (GPM / L/min), heating power (kW / BTU/h), and electric/gas equivalents. Adjust climate region, fixtures, and diversity factor for real-world accuracy.
📐 1. Global Settings & Climate
🌎 Region / Incoming Water Temp
💧 Inlet Water Temperature (°F)
🚿 Desired Output Temperature (°F)
⚙️ Peak Demand Diversity Factor
🚰 2. Fixture Flow Demand (Peak Simultaneous)
✅ Each fixture flow rate based on US/International typical values. Count = how many units may run simultaneously (max realistic). Diversity factor reduces total for real case.
📊 Recommended Sizing
🚀 Required Peak Flow Rate (at ΔT)
0.0 GPM
ΔT: 0°F (0°C)
⚡ Electric: — kW | 🔥 Gas: — BTU/h
📌 Min Tankless Capacity Required:
— GPM @ ΔT
🔍 Factors (USA & Global): Delta T = Desired – Inlet. Higher delta T reduces output flow of any tankless. Electric: kW = (GPM × ΔT°F) / 6.82 | Gas BTU = GPM × ΔT°F × 500. Always verify manufacturer charts.
📈 Fixture Flow Contribution (GPM / L/min)
Bar chart: each fixture group’s total flow (after diversity applied)
⚡ Sizing based on simultaneous peak flow & temperature rise. For whole-home tankless, compare results with manufacturer performance curve. Electric models require adequate amperage; gas units require proper venting.