Heat Load Rough Estimator
The Heat Load Rough Estimator gives a fast, rule-of-thumb heating or cooling load for a single room from its floor area, a base load factor, the number of people and any extra appliance load. It reports the total in watts, kilowatts and BTU/h, plus a suggested air-conditioner size rounded up to the next 0.5 kW. It is a first-pass sanity check only — never a substitute for a full heat-gain calculation.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the room's floor area in square metres.
- Optionally change the base load factor (W/m²) — the default 150 suits a typical insulated living space; raise it for lots of glass, poor insulation or hot climates.
- Optionally add the number of occupants (each counted as ~100 W) and any other/appliance load in watts, then read the total load and suggested AC size.
How it works
Total load (W) = floor area × base load factor + occupants × 100 W + other load. The result is divided by 1000 for kilowatts and multiplied by 3.412 for BTU/h. The suggested AC size rounds the kilowatt load up to the nearest 0.5 kW, reflecting the common step sizes of split-system units.
Worked example
Worked example. For a 30 m² living room at 150 W/m² with 2 occupants and 500 W of appliances: 30 × 150 = 4,500 W envelope, + 2 × 100 = 200 W people, + 500 W other = 5,200 W total. That is 5.2 kW, or 17,742.4 BTU/h, and rounds up to a 5.5 kW unit.
Common mistakes
- Treating the answer as a real design load — orientation, glazing, insulation and climate can move it dramatically, so always follow up with a proper heat-gain calculation before buying.
- Leaving the base factor at 150 W/m² for a west-facing, poorly insulated or tropical room, which will badly undersize the unit.
- Forgetting high-heat appliances (ovens, servers, many people) in the 'other load' field, so the estimate comes out too low.
Frequently asked questions
What base load factor should I use?
150 W/m² is a common starting point for a moderately glazed, insulated living space in a temperate climate. Bedrooms may need less (~100–130), while sun-exposed rooms, kitchens or hot climates can need 180–250 W/m² or more.
Can I use this to actually buy an air conditioner?
No. Use it only as a rough sanity check. Correct sizing needs a room-by-room heat-gain/heat-loss calculation (AS/NZS or ASHRAE method, or a manufacturer's tool) that accounts for orientation, glazing, insulation, infiltration, ceiling height and design temperatures.
Related tools
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- Air Changes Per Hour Calculator
- BTU to kW Converter
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