Air Conditioner Room Size Calculator — BTU & Tonnage
Find the right air conditioner size for any room. Enter the floor area, ceiling height, sun exposure, number of occupants and whether it is a kitchen to get the required BTU/h and the nearest standard AC tonnage.
Unit system
sq ft
ft
Sun / heat exposure
Is this a kitchen?
Recommended unit size: 5,000 BTU
- 1
Base cooling load
200 × 25 = 5,00025 BTU/sq ft at standard 8 ft ceiling (ASHRAE simplified rule of thumb). - 2
Ceiling height adjustment
5,000 × (1 − 1) = 0 - 3
Sun exposure adjustment
5,000 × 0 = 0 - 4
Occupant & kitchen load
0Extra occupants × 600 BTU/h plus kitchen heat gain (4,000 BTU/h if kitchen). - 5
Total BTU/h required
5,000 + 0 + 0 + 0 = 5,000
How does this calculator work?
AC size (BTU/h) ≈ room area (sq ft) × 25, adjusted for ceiling height above 8 ft, sun exposure (−10% shaded to +20% very sunny), +600 BTU per occupant beyond 2, and +4,000 BTU for a kitchen. Divide by 12,000 to get tonnage. Round up to the next standard unit size.
Formula
How this is calculated
The 25 BTU per square foot rule of thumb is derived from the ASHRAE Manual J residential load calculation method, simplified for quick estimates. It assumes a standard 8-foot ceiling, average insulation, average occupancy and no unusual heat sources — a reasonable starting point for most residential rooms in a temperate climate.
Three adjustments refine the base figure. Ceiling height scales the load proportionally to room volume: a 10-foot ceiling adds 25% more air mass to cool. Sun exposure is a significant factor — south- or west-facing rooms with large windows can receive 10–20% more solar heat gain than a shaded room; heavily shaded rooms need 10% less. Each occupant beyond two adds about 600 BTU/h of sensible heat (based on a resting adult generating roughly 250–300 BTU/h sensible heat). A kitchen adds a fixed 4,000 BTU/h to account for cooking appliances and the extra heat they radiate.
Result should be treated as a starting point, not a precise engineering figure. Manual J does not produce simple rules — it accounts for wall and ceiling insulation R-values, window U-factors and SHGC, local design temperatures, infiltration and more. In high-heat climates (Texas, Arizona, the Middle East) or poorly-insulated older homes, real loads can run 20–40% higher than this estimate. For a new installation, an HVAC contractor should run a full load calculation.
Frequently asked questions
An oversized AC cools the room quickly but "short-cycles" — it turns off before running long enough to dehumidify the air. The room may feel cold but clammy. Short-cycling also stresses the compressor and wastes energy on frequent start-up spikes. A correctly sized unit runs longer cycles, removes more humidity and lasts longer.
One ton of cooling is 12,000 BTU per hour — a historical figure based on the rate of heat absorption by melting one short ton (2,000 lb) of ice in 24 hours. A 1.5-ton unit delivers 18,000 BTU/h; a 2-ton unit delivers 24,000 BTU/h. Most home window units are 0.5–1.5 tons; split systems typically start at 1 ton.
It gives a reasonable first estimate for commercial zones up to about 2,000 sq ft, but commercial spaces typically have higher internal heat loads (equipment, lighting, people), higher infiltration from door traffic, and stricter ventilation requirements. For whole-house systems or anything commercial, use ACCA Manual J software or hire a certified HVAC engineer.
TG we-Calculate Editorial Team. (2026). Air Conditioner Room Size Calculator — BTU & Tonnage [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/air-conditioner-room-size-calculator
TG we-Calculate Editorial Team. "Air Conditioner Room Size Calculator — BTU & Tonnage." TG we-Calculate. 2026. https://we-calculate.com/calculator/air-conditioner-room-size-calculator.
TG we-Calculate Editorial Team, "Air Conditioner Room Size Calculator — BTU & Tonnage," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/air-conditioner-room-size-calculator
@misc{wecalculate_air_conditioner_room_size_calculator, title = {Air Conditioner Room Size Calculator — BTU & Tonnage}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/air-conditioner-room-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
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