AC BTU Sizing Calculator
The "square footage × 20" shortcut gets most rooms wrong. Sun, ceiling height, bodies, and the kitchen all move the number, buy the unit this lands on.
The method is the manual-J core: 200 BTU/ft² base, ×1.1–1.15 sun, +600 BTU per extra person, +4,000 for a kitchen, ceiling-height volume correction. Shaded and low-ceiling rooms land below the shortcut; sun and vaulted land above it.
How the math works
- Base = area × 200 BTU/ft² (the modern, insulation-aware base). Ceiling multipliers: 9 ft ×1.05, 10 ft ×1.1, vaulted ×1.25.
- Adders: +600 BTU per occupant beyond the first, +4,000 BTU for a kitchen, sun factor 1.10–1.15. Round up to the nearest retail size: 5,000 · 6,000 · 8,000 · 10,000 · 12,000 · 14,000 · 23,000 · 24,000.
- Tons = BTU ÷ 12,000 (central-air language: 1.5 tons ≈ 18,000 BTU for a ~1,000 ft² home, not a room).
- Amps = watts ÷ 120; an 8,000 BTU window unit draws ~650 W ≈ 5.5 A on a 15 A circuit, share it with nothing else.
Oversizing is the silent failure: a 14,000 BTU unit in a 6,000 BTU room short-cycles, runs humid, and dies early. The result rounds to a retail size, not above it.
How many BTUs for a 12×15 bedroom?
180 ft² standard room with two occupants: ~7,800 BTU → buy the 8,000 BTU (6,000 is too weak, 10,000 short-cycles). The same room under a hot roof with afternoon sun lands at 10,000.
What size AC for a 500 sq ft room?
Base 10,000 BTU; vaulted ceiling and sun push it to 12,000–14,000. A 12,000 BTU window unit is the honest 500 ft² answer for most rooms, 8,000 fights it all summer.
Is bigger BTU always better?
No. An oversized AC cools the air without pulling humidity (short cycling), and a window unit that short-cycles breeds the swamp-cooler smell and compressor wear. Buy the number the tool lands on, not the biggest box on the shelf.