Furnace and Heater Size Calculator
A first estimate of the heating output a home needs — from its floor area, the climate's heating factor, the ceiling height and the insulation standard — in kilowatts and BTU per hour, with a margin, and the furnace or heat-pump size to look for.
Heating load scales with floor area and the climate: a mild climate needs about 40 W per square metre of floor for a well-built house, a cold one 80 to 100.
How the furnace and heater size calculator works
Heating load scales with floor area and the climate: a mild climate needs about 40 W per square metre of floor for a well-built house, a cold one 80 to 100. Taller rooms hold more air and add load; good insulation and glazing take it away. A furnace should be sized close to the load with a modest margin — an oversized one short-cycles, wastes fuel and controls temperature badly. A full Manual J or equivalent calculation replaces this; it is the right ballpark for a quote.
Formula: load = area × factor × ceiling adjustment × insulation adjustment; furnace = load × (1 + margin); 1 kW = 3,412 BTU/h
Worked examples
| Inputs | Furnace output to look for | Note |
|---|---|---|
| A typical home | 10 kW (34000 BTU/h) | a whole-house system |
| Well insulated | 8.5 kW (29000 BTU/h) | 15% smaller |
| A cold climate | 22.3 kW (76000 BTU/h) | a large system |
FAQFrequently asked questions
How many kilowatts does a house need to heat?
Roughly 40 to 100 W per square metre of floor depending on climate and construction — 7 to 18 kW for a 180 m² home. In BTU per hour that is 25,000 to 60,000.
What heating factor should I use?
Mild coastal climates 35 to 50 W/m², temperate 55 to 75, cold continental 80 to 110. Older, leakier houses sit at the top of each range; the insulation setting adjusts within it.
Why not oversize to be safe?
An oversized furnace reaches temperature and shuts off within minutes, repeatedly — short cycling — which wastes fuel, wears the unit and leaves rooms uneven. Ten to twenty per cent margin is plenty.
Does this size a heat pump?
The heat output, yes. A heat pump's output falls as the outdoor temperature drops, so it is sized against its capacity at the design cold temperature, which the maker's tables give.
What is a proper load calculation?
Room-by-room heat loss through every wall, window, roof and floor at the design temperature, plus ventilation — Manual J in North America and similar methods elsewhere. Any installer should do one before quoting.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Cement Concrete & Aggregates Australia — concrete basics and yields — bag yields, density 2,400 kg/m³
- Australian Building Codes Board — the National Construction Code
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.