How to use the btu & heat pump sizing calculator
- Enter the floor area and ceiling height of the room, or of the whole house.
- Pick your climate, insulation quality and how much sun the space gets; add occupants, windows and a kitchen if relevant.
- Read the cooling load in BTU/h, tons and kW, the heating load, and the unit size to buy.
How the load is estimated
The cooling figure starts from a BTU-per-square-foot rate that depends on climate (15 in a cold region up to 30 in a hot one), then scales for ceiling height, insulation and sun. Each person adds about 400 BTU/h (roughly 250 sensible and 150 latent from breathing), a kitchen adds 4,000 BTU/h for the cooking appliances and fridge, and windows beyond about one per 150 ft² add 1,000 BTU/h each for solar gain. A whole-house figure is reduced by 15% because rooms do not all peak at the same time.
Cooling BTU/h ≈ area × rate × insulation × sun × (ceiling ÷ 8 ft) + 400 per person + 4,000 for a kitchen + 1,000 per extra window
Heating uses 12 (hot) to 40 (cold) BTU/h per square foot for average 1980–2010 insulation, scaled for insulation quality (×1.3 poor, ×0.75 good, ×0.5 high-performance) and ceiling height. This represents the heat the building loses on the coldest typical day (the 99% design temperature), which is what the heater must replace.
Reading the result
Air conditioners in North America are sold by BTU/h (window and portable units: 5,000–14,000; mini-splits: 9,000–36,000; central: 18,000–60,000, often quoted in tons where 1 ton = 12,000 BTU/h). In the UK, Australia, Singapore and most of the world they are sold by kW of cooling: 2.5, 3.5, 5, 7 kW and so on, where 3.5 kW is the same machine as a 12,000 BTU unit. The calculator shows both.
The "unit size to buy" is the first standard size at or above the load. If the load lands just above a size boundary, say 12,400 BTU/h, the 12,000 unit is usually the better choice than the 15,000: the rule-of-thumb has a ±15% band anyway, and a slightly small unit runs long and steady, which is exactly how modern inverter units are designed to work.
Heat pumps and cold climates
A heat pump's nameplate capacity is measured at 47 °F (8.3 °C) outdoors. At 17 °F (−8 °C) a standard unit delivers 60–70% of that; a cold-climate model (Mitsubishi Hyper-Heat, Daikin Aurora, Fujitsu XLTH, Bosch IDS and others) holds 80–100% down to −15 °F (−26 °C). In Canada, the northern US and inland UK the heating load is larger than the cooling load, so size on heating, check the manufacturer's capacity table at your design temperature, and decide whether electric resistance or a retained furnace covers the few coldest days. Natural Resources Canada and the NEEP cold-climate heat pump list publish those tables.
In the UK, the Microgeneration Certification Scheme requires a room-by-room heat-loss calculation for a grant-funded air-source heat pump; typical results are 40–70 W/m² for a 1990s house and 80–120 W/m² for a solid-wall Victorian one. The W/m² figure in the results lets you compare directly.
Why rule-of-thumb sizing goes wrong
The most common error is oversizing. Contractors do it to avoid callbacks, and it produces short cycling, poor humidity control and higher bills. The second is ignoring orientation: a west-facing room with big windows can need 30% more cooling than the same room facing north. The third is forgetting duct losses: a central system with ducts in a hot attic can lose 20–30% of its capacity before the air reaches the rooms. None of these are captured here, which is why the output is a class of unit, not a purchase order.
Worked example: 450 ft² open-plan living room and kitchen in Texas
Hot climate, 30 BTU/ft²; 9 ft ceilings (×1.125); average insulation; very sunny (×1.1): 450 × 30 × 1.125 × 1.1 = 16,700 BTU/h. Add four people (1,600), a kitchen (4,000) and one window beyond the normal ratio (1,000): 23,300 BTU/h, which is a 24,000 BTU (2-ton, 7 kW) mini-split. A ducted system serving this plus the bedrooms would be sized with Manual J.
Questions and answers
How many BTU do I need per square foot?
For cooling, 20 BTU per square foot is the usual starting point in a moderate climate with 8-foot ceilings and average insulation; hot, sunny climates need 25–30 and cool ones 15–17. A 300 ft² bedroom in a moderate climate needs about 6,000–7,000 BTU/h, so an 8,000 BTU window unit or a 9,000 BTU mini-split. For heating, 12–40 BTU per square foot for an average-insulated house depending on climate, up to about 50 for a poorly insulated one in a cold region.
What size air conditioner for a 12×12 room?
144 ft² at 20 BTU/ft² is 2,900 BTU/h, plus about 800 for two people, so around 3,700–4,500 BTU/h. The smallest common units are 5,000–6,000 BTU/h, which is fine; a short cycle on a slightly large unit matters less than an undersized one that never catches up.
Is a bigger unit better?
No. An oversized air conditioner cools the air quickly, shuts off before it has run long enough to pull moisture out, and leaves the room cold and clammy. It also cycles more, which wears the compressor. Size to the load and round up by one standard size at most.
How many kW is 12,000 BTU?
3.5 kW of cooling capacity (1 BTU/h = 0.293 W), which is also one ton of refrigeration. Note this is thermal output, not electrical input: a 12,000 BTU unit with a SEER2 of 16 draws roughly 1 kW.
How is a heat pump sized differently?
A heat pump does both jobs, so it has to meet whichever load is larger. In Florida or Queensland that is cooling; in Minnesota, Alberta or Scotland it is heating. Heat pump capacity drops as outdoor temperature falls, so in cold climates use the manufacturer's capacity at your winter design temperature, not the nameplate figure, which is rated at 47 °F (8 °C).
Why does my contractor talk about Manual J?
ACCA Manual J (in the US and Canada) is the room-by-room heat-load calculation that accounts for wall construction, window types, orientation, infiltration and local design temperatures. Equivalent methods are CIBSE in the UK and AIRAH in Australia. This calculator is a first-pass estimate for choosing a unit class or sanity-checking a quote; any ducted system or whole-house heat pump should be sized with a full load calculation.
Free to use, runs in your browser, nothing uploaded. Last updated 2026-10-07. Made by HandyToolkits; questions to [email protected].