How Many BTU Air Conditioner Do I Need? Room and Whole-Home Calculator

The formula behind the calculator

The calculator builds the cooling load from a floor-area baseline, scales it for the room and its surroundings, then adds the heat sources that a room-sizing chart usually lists separately.

Area = Length × Width (or the area you enter)
Base = Area × 20 × (Ceiling ÷ 8)
Load = Base × Room type × Insulation × Sun × Climate + (People − 2) × 600 + Cooking (4,000 if yes)
Power (kW) = Unit size ÷ 11 ÷ 1,000
Cost per hour = Power × Rate; Cost per month = Cost per hour × Run hours per day × 30

Twenty BTU/h per square foot is the long-standing starting figure for an 8-foot room in a typical US home. Taller ceilings enclose more air and more wall, so the baseline scales with height. Room type reflects when and how a room is used: bedrooms are cooled mostly in the evening and overnight (×0.95), home offices carry computer and monitor heat through the hottest part of the day (×1.1), kitchens have a refrigerator, dishwasher and lighting running all day (×1.15), and sunrooms are mostly glass (×1.4). Insulation moves the result 10–20% either way. A shaded room loses 10% and a sunny one gains 10%. Each person gives off roughly 400–600 BTU/h, so anyone beyond the two people already assumed adds 600, and active cooking adds about 4,000 BTU/h.

Equipment is sold in fixed steps, so the headline result is the smallest common size that covers the load. The running cost assumes an efficiency of EER 11 (11 BTU of cooling per watt-hour), which is typical of current window units rated by CEER; many mini-splits do better, so their real cost will be lower. Real power draw also rises somewhat on very hot afternoons. For a whole house, divide the load by 12,000 to get tons.

Worked example

The calculator's defaults: a 20 × 20 ft bedroom with 8-foot ceilings, average insulation and sun, a warm/mixed climate, two people, no cooking, electricity at $0.17 per kWh and 8 hours of compressor run time a day.

1. Area = 20 × 20 = 400 sq ft
2. Base = 400 × 20 × (8 ÷ 8) = 8,000 BTU/h
3. Room and conditions: 8,000 × 0.95 (bedroom) × 1.0 × 1.0 × 1.0 = 7,600 BTU/h
4. No extra occupants and no cooking: + 0
5. Load = 7,600 BTU/h, so the nearest standard size is 8,000 BTU/h
6. Power = 8,000 ÷ 11 ÷ 1,000 = 0.73 kW; cost = 0.73 × $0.17 = about $0.12 per hour
7. Monthly: $0.124 × 8 h × 30 days = about $30

Give the same room large west windows (sunny, ×1.1) and a third occupant and the load becomes 8,000 × 0.95 × 1.1 + 600 = 8,960 BTU/h. That calls for a 10,000 BTU/h unit, which draws about 0.91 kW and costs roughly $0.15 an hour or $37 a month on the same schedule.

BTU chart by room size

For average sun, 8-foot ceilings, two occupants and no kitchen:

Below about 150 sq ft even the smallest window unit is generous; pick one with an energy-saver mode so it cycles gently rather than blasting cold air for two minutes at a time.

Window, portable, mini-split or central?

• Window units are rated under a test that closely reflects real output, so the label figure can be used directly. See the window AC installation guide for fitting and sealing.
• Portable units lose part of their capacity to heat from their own hose and case. Compare them by the newer SACC number, not the older, larger figure; our SACC vs ASHRAE explainer shows how big the gap can be.
• Ductless mini-splits come in head sizes of 6,000, 9,000, 12,000, 18,000 and 24,000 BTU/h. Use the mini-split sizing calculator zone by zone.
• Central systems are sized in half-ton steps from 1.5 to 5 tons, and the ducts must match. Use the whole-home floor area here, then check the result with the AC tonnage calculator, which accounts for insulation and duct location.

Why bigger is not better

An air conditioner removes moisture only while its coil is cold and running. An oversized unit drops the temperature quickly, the thermostat is satisfied in a few minutes and the compressor stops before much water has dripped off the coil. The room ends up cool but damp, and the constant starts wear the compressor. This is the most common cause of short cycling and of an AC that does not remove humidity.

If the load lands just above a size step (for example 10,300 BTU/h), it is usually fine to choose the size below when the room is not sun-baked. Being 5–10% under on a few peak afternoons is far more comfortable than being 40% over all summer.

Measuring a room correctly

Measure the floor at its widest points and multiply length by width. For an L-shaped room, split it into two rectangles, add their areas and type the total into the floor-area box, which overrides length × width. Include any area that is open to the room with no door between them, such as a dining nook or a hallway, because the air conditioner will end up cooling it whether you intend it to or not. Leave out closets and spaces behind closed doors.

Vaulted or cathedral ceilings should use the average height: for a ceiling that rises from 8 to 14 feet, enter 11. Lofts open to the room below count toward the floor area. If you are sizing a whole floor with a single unit, remember that interior walls stop airflow from a window or wall unit, so rooms at the far end will run warmer than the calculator implies.

Adjustments the rule of thumb misses

The room-type and insulation selectors cover the most common differences, but some rooms need a manual nudge. Add capacity for a top-floor room under a dark roof, for a west-facing room with floor-to-ceiling glass, or for electronics such as a gaming PC or a server rack (each watt of continuous electrical load becomes 3.41 BTU/h of heat). Reduce it for a basement that stays cool from the surrounding soil. For whole-house equipment, ask the installer for an ACCA Manual J calculation; it models each wall, window and duct run instead of using a single per-square-foot figure. For a more detailed operating-cost estimate with your unit's actual EER or SEER2, try the AC running-cost guide, and see the EER rating explainer for what the number means.

Frequently asked questions