About this AC Size (BTU)
Choosing the right size air conditioner is critical. An undersized unit won't cool the room, while an oversized unit will cycle on and off too frequently, failing to remove humidity. This calculator estimates the ideal BTU rating based on room size and sunlight.
Who this calculator is for
- Homeowners comparing window or portable AC sizes for a bedroom, office, or living room.
- Renters who want a quick cooling-capacity estimate before buying an air conditioner.
- Contractors or property managers doing early room-by-room HVAC planning.
When to use it
- Estimate a starting BTU target from room square footage and sun exposure.
- Compare whether a 5,000, 8,000, or 12,000 BTU unit is a closer fit for the space.
- Sense-check AC sizing before layering in occupancy, kitchen heat, insulation, or climate adjustments.
How to use this calculator
- Enter the Room Area (Square Feet).
- Select the Sun Exposure level (Shaded, Normal, Sunny).
- The result is the recommended BTU capacity.
Key assumptions
- The rule of thumb assumes a standard residential room with typical 8-foot ceilings.
- The calculator adjusts only for room area and sunlight, not occupancy, kitchens, or poor insulation.
- Room area must be greater than zero for the estimate to be meaningful.
Accuracy and testing
Review formula sources, default test cases, assumptions, unit handling, rounding rules, and changelog for this calculator.
How this calculator works
The base calculation assumes ~20 BTU per square foot for a standard room with 8ft ceilings.
* Sunny Room: Adds 10% to the requirement.
* Shaded Room: Subtracts 10% from the requirement.
* Kitchen: Typically adds 4,000 BTU (heat from appliances).
Inputs explained
Room Area
Length x Width of the room in feet.
Sun Exposure
How much direct sunlight enters the room? South/West facing windows usually mean 'Sunny'.
Understanding your results
Buy an AC unit within roughly 10% of the recommended BTU. Common window unit sizes are 5,000, 6,000, 8,000, 10,000, and 12,000 BTU.
Example scenarios
Small Bedroom
150 sq ft (10x15). Normal sun. Needs ~3,000-4,000 BTU (5,000 BTU unit works).
Large Living Room
400 sq ft (20x20). Sunny. Needs ~9,000 BTU (8,000 or 10,000 BTU unit).
Key Features
- Square Footage Logic
- Sunlight Adjustment
- Standard Sizing
Tips & Best Practices
- If the room is a kitchen, add 4,000 BTU to the total.
- If more than two people occupy the room regularly, add 600 BTU per person.
- High ceilings (>8ft) increase volume, so increase BTU by ~10%.
Glossary of Terms
- BTU
- British Thermal Unit. A measure of heat energy. One BTU is the amount of heat needed to raise 1 lb of water by 1°F.
- Tonnage
- Used for central AC. 1 Ton = 12,000 BTU/hr.