How to Use the Home Boiler Size Calculator
Furnace and boiler capacity is measured in BTU/h (British Thermal Units per hour), and the amount you need depends on your home's square footage and climate. This calculator uses a common HVAC rule of thumb: roughly 30 BTU per square foot in warm southern climates, 45 BTU/sq ft in moderate climates, and up to 60 BTU/sq ft in cold northern climates, since colder regions need to overcome a bigger indoor-outdoor temperature gap.
The result is rounded up to the nearest common residential furnace size so you have a realistic model to shop for. It's best to size up slightly rather than down — an undersized unit will run at full output constantly during cold snaps, hurting both efficiency and equipment lifespan. For a precise sizing, a licensed HVAC contractor can run a full Manual J load calculation based on your home's insulation, windows, and layout.
Frequently Asked Questions
An undersized furnace will run constantly at full output during cold snaps, reducing efficiency and comfort. It's safer to round up to the next common furnace size.
Use roughly 30 BTU/sq ft for warm climates (southern US), 45 BTU/sq ft for moderate climates, and 60 BTU/sq ft for cold northern climates. Check your local IECC climate zone for precision.