How to use the AC BTU Calculator
Start with the room area and ceiling height. The calculator uses those values to estimate the base cooling load, then adjusts the result for climate, sun exposure, insulation, room type, people, and electronics.
Use square feet for US-style product listings or square meters for metric planning. The result shows both BTU/h and kW so you can compare window AC units, portable air conditioners, and mini-split specifications more easily.
If you already know a unit capacity and want to check room coverage, use the AC Room Size Calculator. For running cost, compare this sizing result with the AC Electricity Cost Calculator when it becomes available.
AC BTU formula
standard BTU = lookup from room size capacity tableheight factor = ceiling height ft / 8adjusted BTU = standard BTU x height factor x climate factor x sun factor x insulation factoradd-ons = people add-on + kitchen add-on + electronics watts x 3.412recommended BTU = adjusted BTU + add-ons, rounded to the nearest 500 BTU/hThe standard room-size capacity table gives a more realistic baseline for common window and portable AC sizes than a purely linear square-foot rule. The extra factors make the estimate more useful for real rooms, because a shaded bedroom and a sunny kitchen with electronics should not receive the same recommendation.
Assumptions and methodology
This AC BTU Calculator is designed for single-room planning. It starts with room area, then makes transparent adjustments for the conditions that users most often need to think about before buying an air conditioner.
- Room area uses a standard room air conditioner capacity table for a standard 8 ft ceiling.
- Higher ceilings increase the estimate because the room has more air volume.
- Hot climates, strong sunlight, and poor insulation increase the load; shaded rooms and good insulation reduce it.
- People beyond two add heat load, and kitchen mode adds capacity for cooking heat.
- Electronics are converted from watts to BTU/h using 1 W = 3.412 BTU/h.
Example calculations
250 sq ft bedroom or living room example
A 250 sq ft room with an 8 ft ceiling starts with a base estimate of 6,000 BTU/h. With average sun, average insulation, moderate climate, two people, and 150 W of electronics, the practical recommendation is about 6,500 BTU/h (1.90 kW).
A harder room changes quickly. For example, a 300 sq ft sunny kitchen with a 9 ft ceiling, hot climate, three people, and 500 W of heat-producing devices estimates about 16,000 BTU/h (4.69 kW). That is why room conditions matter more than floor area alone.
AC BTU chart by room size
These reference values assume an 8 ft ceiling, moderate climate, average sun exposure, average insulation, a living room, two people, and no extra electronics heat load.
| Room size | Metric | Estimated AC size | Cooling kW |
|---|---|---|---|
| 100 sq ft | 9.3 sq m | 5,000 BTU/h | 1.47 kW |
| 150 sq ft | 13.9 sq m | 5,000 BTU/h | 1.47 kW |
| 250 sq ft | 23.2 sq m | 6,000 BTU/h | 1.76 kW |
| 350 sq ft | 32.5 sq m | 8,000 BTU/h | 2.34 kW |
| 450 sq ft | 41.8 sq m | 10,000 BTU/h | 2.93 kW |
| 550 sq ft | 51 sq m | 12,000 BTU/h | 3.52 kW |
| 700 sq ft | 65 sq m | 14,000 BTU/h | 4.10 kW |
| 1,000 sq ft | 93 sq m | 18,000 BTU/h | 5.28 kW |
Popular AC size questions
These focused answers cover common room sizes and room types. Use them for a quick starting point, then return to the full calculator when sun, ceiling height, people, or electronics change the load.
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FAQ
How many BTU do I need per square foot?
A rough starting estimate is about 20 BTU per square foot, but this calculator uses a standard room-size capacity table as the baseline, then adjusts for ceiling height, sunlight, insulation, climate, people, and heat from appliances or electronics.
What size air conditioner do I need for a 250 sq ft room?
With an 8 ft ceiling, average sun, average insulation, moderate climate, two people, and 150 W of electronics, this AC BTU Calculator estimates about 6,500 BTU/h, or 1.90 kW.
Can an air conditioner be too large for a room?
Yes. An oversized air conditioner may cool the air quickly but run in short cycles, which can reduce humidity removal and comfort. It may also cost more upfront than a properly sized unit.
How do ceiling height and sunlight affect AC size?
Higher ceilings increase room volume, so there is more air to cool. Sunny rooms and west-facing windows also add heat load, so the calculator increases the BTU estimate when sun exposure is higher.
How do I convert BTU to kW for an air conditioner?
Use kW = BTU/h x 0.000293071. For example, 12,000 BTU/h is about 3.52 kW of cooling capacity. This is cooling capacity, not the electrical power draw from the wall.
Should I use this for whole-home HVAC sizing?
No. This calculator is meant for single-room or single-zone estimates. Whole-home HVAC sizing should account for ductwork, air leakage, orientation, insulation, windows, local design temperature, and should be checked by a qualified professional.