AC Sizing Calculator
Size a window unit, portable, or mini-split zone with the standard rule of thumb: about 20 BTU per square foot, adjusted for sun, people, and cooking heat.
Your results
Worked example
A sunny 30 m² living room used by four people, not a kitchen:
- Area in square feet: 30 × 10.7639 = 322.9 sq ft
- Base load: 322.9 × 20 BTU = 6,458 BTU
- Sunny adjustment: 6,458 × 1.10 = 7,104 BTU
- People: 2 extra beyond the base × 600 = plus 1,200 BTU
- Recommended size: 7,104 + 1,200 = 8,304 BTU, or 0.69 tons
Units are sold in steps, so shop for the 8,000 to 9,000 BTU class here rather than jumping to 12,000. Oversized units cool fast, shut off early, and leave the room clammy.
How this calculator works
This is the long-standing rule-of-thumb method published by energy agencies: 20 BTU per square foot as the base, 10 percent down for heavy shade or up for strong sun, 600 BTU for each regular occupant past two, and 4,000 BTU for a kitchen's cooking heat. It sizes a single room or a single mini-split zone. Whole-home central systems need a proper heat loss and gain calculation (a Manual J or CSA F280 assessment in Canada), which accounts for insulation, windows, and duct runs this page cannot see.
Common questions
Is bigger always better for air conditioning?
No. An oversized unit short-cycles: it blasts cold air, hits the set point, and stops before it has pulled much humidity out. The room feels cold and damp, and the unit wears faster. The closest standard size at or just above this estimate is the right target.
What size do I need for a whole house?
Do not scale this page up by total house area and buy on the result. Canadian homes of similar size vary widely with insulation era and window area. Get a CSA F280 heat gain calculation from an HVAC contractor; it is the sizing basis Canadian codes expect.
Estimates only, using the rule-of-thumb method stated on this page. Results are provided for planning purposes for single rooms and zones. Whole-home systems require a professional heat gain calculation. Mississippi Maple accepts no liability for purchase decisions made using this calculator.