Size an air conditioner, mini split or heater properly — room size plus the adjustments that
actually matter.
How AC sizing works
BTU = area × 25 × ceiling factor × sun factor
+ 600 × (occupants − 2)
+ kitchen allowance
This is the industry rule-of-thumb method (a simplified Manual J). It sizes correctly for
typical rooms; whole-house systems, poorly insulated spaces and extreme climates deserve a
contractor's full load calculation.
Quick sizing chart (cooling, standard ceilings)
| Room size | BTU | Typical unit |
|---|
| 150–250 sq ft | 5,000–6,500 | Small window unit |
| 250–350 sq ft | 7,000–8,500 | Window unit / 0.75-ton split |
| 350–450 sq ft | 9,000–10,500 | 9k mini split |
| 450–550 sq ft | 11,000–13,000 | 12k mini split (1 ton) |
| 550–800 sq ft | 14,000–18,000 | 18k mini split (1.5 ton) |
| 800–1,200 sq ft | 21,000–24,000 | 24k mini split (2 ton) |
Frequently asked questions
How many BTUs do I need per square foot?
The baseline for cooling is about 20–25 BTU per square foot with 8 ft ceilings. A 300 sq ft bedroom needs roughly 6,000–8,000 BTU; a 500 sq ft living area 10,000–12,500 BTU, adjusted for sun, ceiling height and occupancy.
What size mini split do I need for 500 square feet?
Around 12,000 BTU (a "1-ton" unit) for a typical 500 sq ft space with standard ceilings and average sun. Very sunny rooms or kitchens push it to 15,000–18,000 BTU.
Is it better to oversize or undersize an air conditioner?
Neither, but oversizing is the more common mistake — an oversized AC short-cycles: it cools the air fast but shuts off before dehumidifying, leaving the room cold and clammy while wearing out the compressor. Size to the calculation, not "bigger is safer".
How many BTUs for heating vs cooling?
Heating needs vary far more with climate and insulation: roughly 30–40 BTU/sq ft in mild climates, 45–60 in cold ones for older homes. This calculator focuses on cooling; use the heating factor option for a rough heating estimate.
What does BTU actually mean?
British Thermal Unit — the energy needed to heat one pound of water by 1°F. For HVAC, the rating is BTU per hour: how much heat the unit can move (cooling) or produce (heating) per hour.