What size AC do I need? Get a number in 30 seconds
The sales rep asks: "how many square meters is the room?". Your neighbor says "1 kW per 10 square meters is enough". The internet says something different. Without a number, it is hard to commit a few thousand dollars on equipment.
You type in the room area, the ceiling height, how many people are usually in there, and how much sun hits the windows. The calculator shows the real cooling power you need: in kilowatts (kW) and in BTU/h (the unit printed on AC unit labels). It also suggests the closest off-the-shelf size: 2.5, 3.5, 5.0, 7.0 or 9.0 kW.
In advanced mode you add building insulation (old pre-1980 vs new build), heat-generating appliances (servers, gaming PCs), and the outdoor design temperature for the hottest day. The formulas are tuned for a temperate climate, like Western and Central Europe, the UK or the northern US.
How to use it
- Type the room area in square meters (m²). If you do not remember it, measure length and width and multiply. A 4 × 5 room is 20 m².
- Type the ceiling height. Apartments from the 1980s are typically 2.5 m, new builds 2.7 m, old townhouses 3.0 m and up. A taller ceiling means a bigger volume to cool.
- Pick how much sun hits the windows. Low (north-facing), medium (east or west), high (south, attic, large glazing). Sun can add 15-35% of the load.
- Type how many people are usually in the room. Each person adds about 100 watts of heat. The first 2 are baked into the baseline, so the calculator only adds extras above that.
- The result is the required power in kW and BTU/h plus the off-the-shelf size you would buy in a store (rounded up to the next standard).
- Switch to advanced mode to add building insulation (poor, normal, good), heat-generating appliances (gaming PC 400 W, server 300 W), and the outdoor design temperature for the hottest day of the year.
- Read the warnings under the result. The calculator will tell you when to add buffer (attic rooms, large south-facing windows) and when to avoid going bigger (oversized AC dehumidifies poorly, cycles on and off too fast, and wears out faster).
When this is useful
Six typical situations where this calculator saves you money and headaches:
- Picking AC for a bedroom. A 15 m² room, west-facing windows, two people sleeping there. The seller pushes a "mid-range model, 5 kW". You type the numbers in: the calculator says 1.9 kW, so a 2.5 kW unit is enough. You save $400-600 and avoid buying something too powerful.
- Cooling a living room with open kitchen. 40 m², south-facing window, 4 people plus a TV and a computer. Advanced mode shows 6.2 kW: you buy 7 kW, not 5. Without the calculator you might have saved on the purchase and then regretted it on hot July afternoons.
- Cooling an attic room. You type 30 m², ceiling 2.2 m (sloped), sunlight high, insulation poor. The calculator shows you need 5 kW, almost as much as a normal living room. A warning tells you: insulate the roof first, otherwise the AC will run non-stop.
- Planning a home office. You work on a gaming PC (400 W) plus two monitors. You type the appliance wattage in advanced mode. Result: a small 2.5 kW unit is enough, but without those appliances you would save 0.4 kW of demand.
- Comparing installer quotes. Three offers: 3.5, 5 and 7 kW, three different installers, each one says "this is perfect for you". The calculator gives you a number: you know who is lowballing or overselling. You negotiate on facts, not pitches.
- Checking if your old AC still cuts it. You have a 2.5 kW unit from 8 years ago, room is 25 m², and it cannot keep up in July. You type the numbers in: the calculator shows you need 3.5-4 kW. So it is not the equipment failing, it was always undersized.