What size room will an 8,000 BTU air conditioner cool?

On the standard sizing chart, 8,000 BTU/hr covers roughly 300-350 ft² (about 28-33 m²) under average conditions — less if the room is sun-facing, has more than two regular occupants, is a kitchen, or has a ceiling taller than 8 ft (2.4 m), any of which pushes the required capacity higher for the same floor area.

How many BTU do I need per square foot?

A commonly quoted rule of thumb is about 20 BTU/hr per square foot (roughly 215 BTU/hr per square metre) for an average room, but the real sizing chart is not perfectly linear — it steps up in bands and the ratio shifts slightly at the small and large ends of the scale, so a lookup-table calculation like this one is more accurate than multiplying floor area by a flat rate.

What happens if I buy an air conditioner that is too big for the room?

An oversized unit cools the air quickly and switches off before it has run long enough to remove humidity properly, leaving the room feeling cold but damp. It also short-cycles — switching on and off more frequently than a correctly sized unit would — which increases electricity use from repeated startup surges and adds wear to the compressor over time.

Does ceiling height affect what size air conditioner I need?

Yes. The standard sizing chart assumes an 8 ft (2.4 m) ceiling; every foot above that adds roughly 1,000 BTU/hr to the recommended capacity, because a taller room has a larger volume of air to cool even though the floor area is unchanged.

How much does a correctly sized air conditioner cost to buy and install?

Costs vary widely by unit type (window, portable, or split system), capacity, and local installation rates — see the cost example further down this page for a typical figure in your country's currency.

Is a window unit or a split system better for sizing accuracy?

Both use the same BTU sizing chart, so the capacity you need does not change — but window units are typically sold in fixed steps (5,000, 8,000, 10,000, 12,000, 15,000 BTU/hr and so on), while split systems (mini-splits) are more often sized in half-ton or ton increments (1 ton ≈ 12,000 BTU/hr), so round up to the nearest size your preferred unit type is actually sold in.

Do I need a special outlet for a larger window air conditioner?

Many window units above roughly 12,000-15,000 BTU/hr require a dedicated 20-amp, 115V or 230V circuit rather than a standard shared household outlet — check the nameplate amperage of the unit you are considering against your existing circuit before you buy, since a new circuit is an added electrician cost.

BTU Calculator — Air Conditioner Room Size

What size air conditioner do I need for my room?

Buy an air conditioner that is too small and it will run flat out all summer without ever quite cooling the room. Buy one that is too big and it "short cycles" — blasting the room cold, switching off, then switching straight back on — which wastes electricity, wears the compressor out faster, and leaves the room clammy because the unit never runs long enough to properly dehumidify the air. Getting the size right, in BTU/hr (or the kW/hp equivalent), is the single most important decision you make before buying a window unit, portable unit, or split system.

This calculator uses the same room-area sizing chart that US Energy Star and Department of Energy guidance has published for decades — floor area in, a base BTU/hr rating out — and then layers on the adjustments that actually matter in the real world: whether the room gets direct sun or heavy shade, how many people normally use it, whether it is a heat-generating kitchen, and whether the ceiling is higher than a standard 8 ft (2.4 m). Enter your room's length and width, its ceiling height, its sunlight exposure, its room type, and how many people typically occupy it, and you get a recommended cooling capacity in BTU/hr, with kW and horsepower equivalents alongside it.

It works whether you are shopping for a single window unit for a bedroom, a portable unit for a home office, or a wall-mounted split system for a living room, and whether you think in square metres or square feet. The chart and the adjustment rules are the same industry-standard method HVAC retailers and installers use for a quick room-by-room sizing estimate — this is not a whole-house heat-load calculation (that requires a full ACCA Manual J-style survey of walls, windows, and insulation), but for sizing a single-room AC unit before you buy, it is the method that matters and the one every major manufacturer's own sizing guide is built on.

Below, we walk through exactly how the base chart and each adjustment factor work, with a fully worked example so you can check the calculator's output by hand, plus the reference chart itself, and the mistakes that most commonly lead people to buy the wrong size unit.

How to use the air conditioner room size calculator

Choose your room type — bedroom, living room, kitchen, basement, or attic. Kitchens get an automatic +4,000 BTU/hr allowance for cooking-appliance heat; basements and attics are included as options because both often have unusual temperature and insulation profiles worth double-checking with the sunlight setting.
Enter the room length and width in whichever unit you prefer (feet or metres) — the calculator converts and computes the floor area for you.
Enter the ceiling height. Standard 8 ft (2.4 m) ceilings need no adjustment; anything taller adds capacity, because there is simply more air volume to cool.
Select the sunlight exposure: sun-facing (a room that gets strong, direct sun for a large part of the day, typically south- or west-facing in the northern hemisphere), average, or heavily shaded.
Enter the number of people who typically occupy the room. The base chart already assumes one to two occupants; anyone beyond that adds load, since every person in a room adds body heat the AC has to remove.
Read off the recommended cooling capacity in BTU/hr, with kW and hp shown alongside — that is the minimum capacity to look for when comparing units, not an exact number you need to match precisely (round up to the nearest common unit size a retailer stocks).

The BTU sizing formula, explained

The starting point is a base capacity looked up from floor area on the Energy Star/DOE room-AC sizing chart — a table built from decades of real-world HVAC sizing data, not a simple linear formula. As a rough rule of thumb you will often see quoted, capacity works out to roughly 20 BTU/hr per square foot of floor area (about 215 BTU/hr per square metre) for a typical room, but the real chart is not perfectly linear — it steps up in bands, and the ratio changes slightly at both the very small and very large end of the scale, which is why a lookup table beats a single multiplier for an accurate answer.

total BTU/hr = base capacity (from floor area) + sun/shade adjustment + occupancy adjustment + kitchen adjustment + ceiling height adjustment

Each adjustment is applied on top of the base figure: a sun-facing room adds 10% to the base capacity, while a heavily shaded room subtracts 10% — because direct sunlight through windows and walls adds a real, measurable heat load that a north-facing or heavily shaded room simply does not have. Every occupant beyond the first two adds 600 BTU/hr, reflecting the roughly 400-600 BTU/hr of heat an adult body gives off at rest — a home office used by three or four people regularly needs meaningfully more capacity than the same room used by one. A kitchen gets a flat +4,000 BTU/hr on top, since cooking appliances (hobs, ovens) are a major and near-constant heat source that bedrooms and living rooms do not have. Finally, every foot of ceiling height above the standard 8 ft (2.4 m) adds 1,000 BTU/hr, because a taller room has more air volume to cool even at the same floor area.

Worked example: a 20 ft x 15 ft (300 ft², or about 27.9 m²) sun-facing living room with a standard 8 ft ceiling and three regular occupants. Base capacity from the chart for 300 ft² is 7,000 BTU/hr. Sun-facing adds 10%: +700 BTU/hr. Occupancy: one person beyond the base two, so +600 BTU/hr. No kitchen adjustment (it is a living room) and no ceiling adjustment (standard height). Total = 7,000 + 700 + 600 = 8,300 BTU/hr — so you would shop for the nearest common unit size at or above that, typically an 8,000 BTU unit rounded up to a 9,000-10,000 BTU model, or converted to roughly 2.4-2.9 kW / 0.7-0.8 hp.

One extra foot of ceiling height in that same room (9 ft instead of 8 ft) would add a further 1,000 BTU/hr, taking the total to 9,300 BTU/hr — a useful reminder that older homes and open-plan extensions with taller ceilings often need noticeably more capacity than the floor area alone would suggest.

BTU sizing chart and adjustment factors (reference)

The table below is the standard Energy Star/US DOE room-AC sizing chart this calculator is built on (floor area to base BTU/hr, before adjustments). It is the same reference table cited by ENERGY STAR's own room air conditioner guidance and used across the HVAC retail industry for quick single-room sizing.

Up to 150 ft² (≈14 m²): 5,000 BTU/hr
150-250 ft² (≈14-23 m²): 6,000 BTU/hr
250-300 ft² (≈23-28 m²): 7,000 BTU/hr
300-350 ft² (≈28-33 m²): 8,000 BTU/hr
350-400 ft² (≈33-37 m²): 9,000 BTU/hr
400-450 ft² (≈37-42 m²): 10,000 BTU/hr
450-550 ft² (≈42-51 m²): 12,000 BTU/hr
550-700 ft² (≈51-65 m²): 14,000 BTU/hr
700-1,000 ft² (≈65-93 m²): 18,000 BTU/hr
1,000-1,200 ft² (≈93-111 m²): 21,000 BTU/hr
1,200-1,400 ft² (≈111-130 m²): 23,000 BTU/hr
1,400-1,500 ft² (≈130-139 m²): 24,000 BTU/hr
1,500-2,000 ft² (≈139-186 m²): 30,000 BTU/hr
2,000-2,500 ft² (≈186-232 m²): 34,000 BTU/hr
Adjustments on top of the base figure: sun-facing room +10%; heavily shaded room -10%; each occupant beyond two, +600 BTU/hr; kitchen, flat +4,000 BTU/hr; each foot of ceiling height above 8 ft (2.4 m), +1,000 BTU/hr.

Pro tips and common mistakes

Bigger is not better. An oversized unit cools the room quickly, satisfies the thermostat, and switches off — before it has run long enough to properly remove humidity from the air. The result is a room that feels cold but clammy, an air conditioner that short-cycles (frequent on/off switching that wears out the compressor faster and wastes electricity on start-up surges), and a unit that cost more to buy than the correctly sized one would have.

Undersizing has the opposite problem: the unit runs continuously at full power, struggles to reach the target temperature on the hottest days, and uses more electricity over the season than a correctly sized unit running normal cycles — plus you feel the failure on exactly the days you need cooling most.

Multi-room and open-plan spaces need extra care. If a living room, dining area, and kitchen form one open-plan space, calculate the combined floor area as a single room (using the kitchen adjustment once for the whole space) rather than sizing each notional "room" separately and adding the results — the sizing chart already accounts for a single connected air volume, and separately-sized units in an open space are usually oversized in aggregate.

Windows and glazing matter more than the sun-exposure toggle alone captures. A room with floor-to-ceiling glass on a sun-facing wall will run hotter than this calculator's +10% adjustment assumes for a typical window count — if your room is unusually glazed, it is worth rounding up to the next unit size rather than the exact number shown.

Check the local voltage and circuit rating before buying a window or portable unit above roughly 12,000-15,000 BTU/hr — larger single-room units can require a dedicated higher-amperage circuit, which is a real installation cost that a straight unit-price comparison misses.

A common mistake is sizing a whole house by adding up individual per-room BTU figures for a central/ducted system — central air conditioning and heat pump sizing is a different, whole-building calculation (a proper ACCA Manual J-style load calculation accounting for shared walls, total glazing, insulation, and orientation), not simply the sum of each room's standalone sizing figure.

As a typical US cost example: a 250 sq ft bedroom sized to roughly 6,000-8,000 BTU/hr usually costs $300-$600 for a window unit bought and self-installed, or towards the top of a wider $300-$1,500 range once you factor in a wall-mounted split system with professional supply-and-install for a larger room or a whole-floor multi-zone setup.

Air Conditioner Room Size (BTU) Calculator