About the Water Heater Sizing Calculator
ASHRAE / DOE Standards
The Water Heater Sizing Calculator sizes both Tankless (On-Demand) and Storage Tank systems according to ASHRAE residential criteria and DOE energy standards.
Storage water heaters are sized by gallon capacity and First Hour Rating (FHR), while tankless systems are sized by peak simultaneous fixture flow rate (GPM) and groundwater temperature rise (ΔT = Desired Temp − Groundwater Temp).
Comparing dedicated technologies? Use our tankless sizing calculator, explore the sizing matrix chart, or verify copper supply velocity limits with the copper flow rate calculator.
Residential Water Heater Sizing Standards
DOE / ASHRAE Standards
Storage tanks are sized by First Hour Rating (FHR), while tankless systems are sized by peak simultaneous GPM and required temperature rise (ΔT = Target Temp − Groundwater Temp).
| Household Size | Storage Capacity | Target FHR | Tankless (Warm Climate) | Tankless (Cold Climate) |
| 1 – 2 People | 30 – 40 Gal | 45 – 55 GPH | 3.0 – 4.5 GPM | 4.5 – 6.0 GPM |
| 3 – 4 People | 40 – 50 Gal | 60 – 70 GPH | 5.0 – 7.0 GPM | 7.0 – 9.0 GPM |
| 5 – 6 People | 50 – 65 Gal | 75 – 85 GPH | 7.0 – 9.5 GPM | 9.0 – 11.0 GPM |
| 7+ People | 75 – 80+ Gal | 90+ GPH | 9.5 – 11.0+ GPM | 11.0+ GPM (Multi-unit) |
Dedicated Calculators by Home Layout
Real-World Calculation Example
Scenario: Sizing a Gas Tankless Water Heater (3-Bed Home in Chicago)
Running two 2.0 GPM showers and one 1.5 GPM kitchen faucet simultaneously in Chicago (winter groundwater = 45°F, target = 120°F).
Required Gas Power
199,000 BTU/hr
Recommended Class
9.0+ GPM Condensing
• Flow: 2.0 + 2.0 + 1.5 = 5.5 GPM peak load
• Heat Demand: BTU = (5.5 × 75 × 500) / 0.95 = 217,105 BTU/hr
• Selection: A 199,000 BTU/hr condensing tankless unit provides continuous high-demand supply.
