Boiler Sizing Calculator

A boiler must cover the whole building, not the biggest room. The fabric losses add up, infiltration bleeds in, and in winter the domestic hot water is drawn at the same time the radiators are flat out.

Boiler Sizing Calculator sums building heat loss with hot-water demand and a pickup margin to give a boiler capacity in BTU/h and kW.

• Building heat loss: Volume, construction, climate.
• Hot water: DHW recovery added to the load.
• Pickup margin: Safety factor built in.
• Dual units: BTU/h and kW together.
• Live update: Results while you type.
• Export: Text, CSV or JSON report.

1. Enter floor area and average ceiling height.
2. Choose insulation and climate.
3. Add hot-water demand and the design temperature.
4. Read the boiler capacity in BTU/h and kW.
5. Export the result if needed.

Example 1. A 1,500 sq ft home with an 8 ft ceiling, average insulation in an average climate needs roughly 40,000-50,000 BTU/h - a 12-15 kW boiler before hot water.

Example 2. Adding a 30,000 BTU/h hot-water recovery and 15% margin pushes the same home past 80,000 BTU/h - the DHW demand is often the difference between a small and a mid-size boiler.

• Whole building: Sizes the boiler, not a room.
• DHW included: Hot water recovery added.
• Dual units: BTU/h, kW together.
• Margin built in: Pickup factor applied.
• Free and private: No uploads, no logging.