Heating and Cooling BTU Calculator

How to use the BTU calculator

1. Choose Metric (m, m², kW) or Imperial (ft, sq ft, BTU/h).
2. Under What are you sizing?, pick Cooling (air conditioner or heat pump) or Heating (heater or heat pump).
3. Enter the Room length, Room width and Ceiling height.
4. Choose Insulation and windows: Good (recent, well sealed), Average or Poor (old, drafty, large windows).
5. For cooling, set the Sun exposure, the People who usually use the room and tick The room is a kitchen if it applies. For heating, pick a Climate or type your own Heating load.
6. Read the Estimated capacity in BTU/h, the same value In kilowatts, and In tons of cooling (12,000 BTU/h). The steps show every factor applied.

All calculations run in your browser.

How the estimate is made

Cooling follows the ENERGY STAR sizing chart for room air conditioners, which gives a capacity for each floor area (for example 5,000 BTU/h up to 150 sq ft, 6,000 up to 250, 12,000 from 450 to 550, 34,000 up to 2,500 sq ft). ENERGY STAR then recommends these adjustments, all applied here:

• heavily shaded room: −10%; very sunny room: +10%;
• more than two people regularly in the room: +600 BTU/h per extra person;
• kitchen: +4,000 BTU/h.

This tool adds two rule-of-thumb factors: the ceiling height divided by 8 ft (the chart assumes 8 ft ceilings), and an insulation factor of 0.9 (good), 1 (average) or 1.2 (poor).

Heating uses floor area × a load per square foot: 25, 35, 45 or 55 BTU/h per sq ft (about 79, 110, 142 and 173 W/m²) from mild to very cold winters, then the same ceiling and insulation factors. These are common rules of thumb, not a standard: an old, drafty room on a corner can need more, a new airtight one far less.

Finally, kW = BTU/h × 0.29307 ÷ 1,000 and tons = BTU/h ÷ 12,000.

Examples

• A 16 × 13 ft bedroom (208 sq ft) with 8 ft ceilings, normal sun and two people: 6,000 BTU/h, about 1.76 kW.
• A 5 × 4 m living room (20 m², 215 sq ft) facing south, very sunny, used by four people: 6,000 × 0.984 (2.4 m ceiling) × 1.1 + 2 × 600 = 7,696 BTU/h (2.26 kW).
• The same 5 × 4 m room heated in a cold climate: 20 m² × 142 W/m² × 0.984 ≈ 2.79 kW (9,538 BTU/h).

Choosing a unit

Buy the closest size, not the biggest. An oversized air conditioner cools the room quickly then stops, which leaves the air damp and clammy and wears the compressor with short cycles. An undersized one runs constantly and never reaches the set point on hot days. For heat pumps in Canada, also check the capacity at low outdoor temperatures (for example −15 °C or 5 °F) in the manufacturer’s data, since output drops in the cold.

This is a rough sizing aid for one room. A load calculation by a professional, CSA F280 in Canada or ACCA Manual J in the United States, prevails: it measures walls, windows, air leakage and orientation against your local design temperature.