BTU / AC Size Calculator
Enter the room and conditions. You get the BTU rating, the tonnage, and the honest answer on whether "just go one size up" is wrong for your case. This is the simplified Manual J method HVAC pros use, with every step shown.
How the math works (so you can check us)
Step 1. Baseline load = floor area × ceiling height ÷ 10 × 6.5 BTU per cubic foot per degree-hour basis — in practice: BTU ≈ area × 25 × height/8 with the height factor carrying the volume.
Step 2. Multiply by the sun, insulation, and climate factors above.
Step 3. Add 600 BTU per extra occupant and 4,000 BTU for a kitchen.
Step 4. Round UP to the nearest standard unit size: 5,000 / 6,000 / 8,000 / 10,000 / 12,000 / 15,000 / 18,000 / 24,000 BTU (0.5–2.0 tons for central).
BTU = area × 25 × (height/8) × sun × insulation × climate + occupants × 600 + kitchen 4,000
Method: simplified Manual J residential load calculation (ACCA Manual J, 8th ed.), consistent with ENERGY STAR room-AC sizing guidance and DOE cooling-load fundamentals. Factors are typical published values; a full Manual J accounts for window area, orientation per wall, duct location, and infiltration in detail.
Why "one size up" is usually the wrong advice
An AC doesn't just cool the air — it has to run long enough to pull moisture out of it. An oversized unit dumps cold air, satisfies the thermostat in minutes, and shuts off before any dehumidifying happens. The room goes cold-clammy, the compressor short-cycles itself to an early death, and your bill runs higher the whole time. If the math hands you 11,200 BTU, the right buy is the 12,000 — not the 15,000 "for safety." The safety margin is already inside the factors.
Sanity benchmarks (run through this exact engine)
- 150 sq ft bedroom, mild climate, average everything: 4,959 BTU calculated → buy the 5,000 BTU window unit. The "10,000 BTU for any bedroom" marketing is double oversize.
- 500 sq ft garage conversion, poor insulation, sunny, extreme climate: 23,400 BTU calculated → a 24,000 BTU (2 ton) mini-split. The honest way to make a garage livable in July.
- 350 sq ft living room, sunny, hot climate: 13,282 BTU calculated → 15,000 BTU. Note the engine jumps one size up from the raw number; the factors already carry the safety margin, so resist going further.
This method sizes a room or zone. Whole-home central systems stack up room loads the same way plus duct gains, so a full Manual J for a typical 3-bedroom house usually lands near 2.5–4 tons. If a contractor's quote is a ton or more above what the room-by-room math says, ask to see their load calculation worksheet. Any pro who did one will hand it over; any pro who didn't is guessing with your money.