Size a window AC, portable unit or mini-split for your room — cooling BTU/hr adjusted for ceiling height, insulation, sun and occupants, plus a heating BTU estimate by climate zone.
AC & heating sizing
Estimate only — for whole-home HVAC sizing, get a professional Manual J load calculation.
Window ACs and mini-splits come in standard capacities — the calculator maps your load to 5,000–36,000 BTU sizes you can actually buy.
Tall ceilings, poor insulation, afternoon sun, a crowd, or a hot kitchen all change the load — each factor is adjusted, not guessed.
One tool covers both: AC sizing for June and a climate-zone heating BTU range for furnace or heat-pump shopping.
The widely used baseline for cooling is about 20 BTU per square foot of floor area with standard 8-foot ceilings — so a 15 × 12 ft bedroom (180 sq ft) starts around 3,600 BTU/hr and lands on a 5,000 BTU window unit. That baseline then shifts with reality: add roughly 10% for poor insulation or strong afternoon sun, subtract 10% for a shaded room or a well-insulated newer build, scale up for ceilings above 8 ft, add about 600 BTU for each person beyond two, and add 4,000 BTU for kitchens where the range and fridge dump heat all day.
Bigger is not better. An oversized air conditioner cools the air fast but short-cycles — it shuts off before it has run long enough to pull humidity out, leaving the room cold and clammy, wearing out the compressor, and wasting electricity on constant restarts. An undersized unit runs nonstop and never catches up on a 95°F day. Sizing to the calculated load (or one standard size up when you’re between sizes in a hot climate) gives the best comfort and lowest bills.
| Room size (8 ft ceilings) | Cooling BTU/hr | Typical unit |
|---|---|---|
| 150–250 sq ft (bedroom) | 5,000–6,000 | Small window AC |
| 250–400 sq ft (master / office) | 6,000–9,000 | Window AC or 9k mini-split |
| 400–600 sq ft (living room) | 10,000–14,000 | Large window AC / 12k mini-split (1 ton) |
| 600–1,000 sq ft (open plan) | 14,000–24,000 | 18k–24k mini-split (1.5–2 ton) |
| 1,000–1,500 sq ft (small home) | 24,000–36,000 | 2–3 ton mini-split or central |
Window AC vs mini-split: a window unit is the cheap fix ($150–$550 installed in minutes) and fine for one room a few months a year. A ductless mini-split costs far more upfront ($2,000–$5,000+ installed per zone in 2026) but is 2–3× more efficient (SEER2 20+ vs ~11–15), far quieter, and most models also heat — in much of the US it pays for itself if it replaces both a window AC and electric heat. For heating, needs vary by climate: roughly 30–35 BTU per sq ft in the hot South up to 50–60 in Minnesota-cold winters. These per-square-foot rules are good for picking a room unit; for sizing a whole-home furnace or heat pump, have a contractor run an ACCA Manual J load calculation, which accounts for windows, ductwork, air leakage and local design temperatures.