What size air conditioner do I need? BTU, SACC and honest sizing
4 min read · updated August 31, 2026 · White Goods Verified
Air conditioner sizing is the difference between a machine that hums quietly all afternoon and one that cycles on and off while never quite drying the air. The math takes two minutes; here is the rule, the exceptions, and the 2025 rating change that made portable numbers confusing.
- The baseline rule is roughly 20 BTU per square foot — 8,000 BTU for a 350-400 sq ft room, 12,000 BTU for 550+.
- Portables changed rating systems (SACC under the 2025 DOE test) — a portable's SACC number is the honest comparison point, not the big BTU figure on the box.
- Upsize for sun, kitchens and high ceilings; downsize for shade and bedrooms — oversized units cool fast then leave the air clammy.
- Fit beats features: measure the window width or sleeve before falling in love with a unit.
The 20-BTU rule and its exceptions
Baseline: multiply room square footage by 20. A 12-by-16 bedroom (192 sq ft) lands near 4,000-5,000 BTU; a 400 sq ft living room wants about 8,000; a 550 sq ft open plan wants 10,000-12,000. Then adjust: add ~10% for rooms that bake in afternoon sun, ~10% for kitchens (appliance heat), and ~10% more for ceilings above 8 feet. Shade works the other way — subtract up to 10%.
More is not better. An oversized unit reaches the thermostat quickly and shuts off before it has dehumidified, leaving a cold-clammy room and a compressor that wears out from short-cycling. Slightly undersized beats oversized for comfort.
The portable AC rating change
In 2025 the DOE replaced the old ASHRAE test for portables with the SACC (Seasonally Adjusted Cooling Capacity) standard, which accounts for the energy penalty of exhaust hoses. The practical effect: a portable stamped 12,000 BTU (ASHRAE) may carry a SACC of only 6,000-7,000 — and SACC is the number that reflects real cooling.
When a listing shows both, compare SACC to SACC. When it shows only the ASHRAE-style number, mentally halve it before comparing against a window unit — which is the honest reason window units keep winning the value argument.
Fit and power, decided last
Two checks that strand returns: window compatibility (U-shaped units need a double-hung window of a stated width range; casement windows need special kits) and circuit voltage (units above roughly 12,000-15,000 BTU may need a 230V outlet). Both are on the listing spec sheet — measure before you order.
- 5,000-6,000 BTU: bedrooms and small offices up to ~250 sq ft.
- 8,000 BTU: the living-room standard for 300-400 sq ft.
- 10,000-12,000 BTU: open plans and big rooms, 450-650 sq ft.
- Through-the-wall replacements: match the sleeve and voltage first, BTU second.
Frequently asked questions
How many BTU is a bedroom?
Most bedrooms are comfortable with 5,000-8,000 BTU. A 150 sq ft bedroom needs about 5,000; a large 350 sq ft primary suite wants 7,000-8,000, more if it gets direct afternoon sun.
Are inverter air conditioners worth it?
Yes where noise and runtime matter: inverters modulate compressor speed instead of on/off cycling, so they hold temperature with far less sound fluctuation and typically use less energy over a season. The premium has shrunk to $50-150 in the current listings.
Machines this guide points to
Verified picks from our database mentioned in this guide — every price loads live from merchant listings.