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.
Sizing air conditioning by room area is a rule of thumb that gets you close, and the standard starting point is roughly 20–25 BTU per hour per square foot. Adjustments then account for the things area alone cannot see: sun exposure, ceiling height, insulation quality, the number of occupants and whether the room contains a kitchen. Oversizing is a real fault rather than a safe margin — an oversized unit cools the air quickly and shuts off before it has removed enough humidity, leaving a room that is cold and clammy while short-cycling wears the compressor. Correct sizing means the unit runs long, steady cycles.
Base BTU/h = Floor area (sq ft) × 25Adjustments: +10% very sunny · −10% heavily shaded · +600 per additional occupant · +4,000 for a kitchenTons of cooling = BTU/h ÷ 12,000where:
Assumptions: A rule of thumb for single rooms. Whole-house systems should be sized by a Manual J load calculation, which accounts for insulation, window performance, air leakage and local climate design temperatures.
Size a unit for a real room, applying the sun adjustment.
Result9,350 BTU/h — a 10,000 BTU unit, not 14,000
The instinct to "buy bigger to be safe" is exactly wrong here. The 14,000 BTU unit would cool the air faster but run in short bursts, leaving humidity high — the room would feel cold and damp rather than comfortable, and the compressor would wear out sooner.