ClutchCalcs

Home

AC Sizing Calculator

Buying a window AC for a single room or sizing a mini-split? You don't need a Manual J load calculation for individual room sizing — the rule of thumb is 25 BTU per square foot at 8-ft ceilings, adjusted up for sun exposure, more occupants, kitchens, or taller ceilings. This calculator gives you a quick BTU estimate that lands within 10-15% of what you'd buy. Don't oversize: a 12,000 BTU AC in a 250 sq ft bedroom short-cycles, fails to dehumidify, and leaves the room cold-and-clammy. Right-sizing matters more than people think.

Enter the room area.

The 25-BTU rule and adjustments

Base AC sizing for a single room: 25 BTU per square foot at standard 8-ft ceilings. Adjust:

  • Ceiling height: scale linearly with height ÷ 8. 10-ft ceiling = multiply by 1.25.
  • Sun exposure: shaded room ×0.9; very sunny (south or west-facing with big windows) ×1.1.
  • Occupants beyond 2: add 600 BTU per additional person (body heat + breath).
  • Kitchen: add 4,000 BTU for cooking heat load.
  • Electronics-heavy: home office with 2+ computers running adds 1,000-2,000 BTU.

Worked example: 200 sq ft bedroom, 8-ft ceiling, average sun, 2 occupants. Base = 200 × 25 = 5,000 BTU. Round up to nearest stocked size: 5,000-6,000 BTU window unit. A 1.5-ton mini-split (18,000 BTU) would be wildly oversized.

Common room sizes

  • 100-200 sq ft (small bedroom, office): 5,000-6,000 BTU
  • 200-300 sq ft (medium bedroom, living room): 7,000-8,000 BTU
  • 300-400 sq ft (large living room, family room): 9,000-10,000 BTU
  • 400-550 sq ft (open kitchen+dining or family room): 12,000 BTU (1 ton)
  • 550-700 sq ft (great room, finished basement): 14,000-15,000 BTU
  • 700-1,000 sq ft: 18,000 BTU (1.5 tons)
  • 1,000-1,400 sq ft: 24,000 BTU (2 tons)

For whole-house central AC, multiply total conditioned square footage by 20-25 BTU/sq ft (depending on climate and envelope) to get total cooling demand, then divide by 12,000 to get tons. But always commission a real Manual J for whole-house systems — the room-by-room math is significantly more accurate than the rule of thumb.

How to use this calculator

  1. Room area in square feet.
  2. Ceiling height in feet (default 8).
  3. Sun exposure: shaded, average, or very sunny.
  4. People in room: regular occupancy count.
  5. Kitchen?: add cooking heat load if the room contains the stove.
  6. Output: BTU recommendation rounded to nearest 500, plus cooling capacity in tons.

Common scenarios

250 sq ft master bedroom, 9-ft ceiling, average sun, 2 people. 250 × 25 × 1.125 = 7,031 BTU. Round up to 8,000 BTU window AC or 9,000 BTU mini-split.

450 sq ft open kitchen-living, 10-ft ceiling, sunny west exposure, 4 people, includes kitchen. 450 × 25 × 1.25 × 1.1 + 1,200 (extra people) + 4,000 (kitchen) = 21,250 BTU. Round to 22,000 or 24,000 (2-ton) mini-split. Window units don't go this big — mini-split is the right answer.

120 sq ft home office, 8-ft ceiling, 1 person, electronics. 120 × 25 = 3,000 BTU base. Bump 1,500 BTU for electronics heat = 4,500 BTU. 5,000 BTU window unit is the smallest stocked size and just right.

FAQ

Why is oversizing bad? +
An oversized AC cools the room fast (10 minutes), shuts off, and the compressor never runs long enough to pull humidity out of the air. Result: cold but clammy room. Within 20 minutes the temp drifts back up and the cycle repeats. Energy waste plus discomfort. A properly-sized AC runs for 30+ minute cycles, removing humidity and providing even cooling.
Window unit vs mini-split vs central? +
Window unit: cheapest ($150-400), single-room, noisy, blocks the window. Mini-split: $1,500-3,000 installed per zone, quiet, efficient, no window obstruction, doubles as heat pump. Central: $5,000-15,000+ for whole-house, ducted distribution, hidden equipment. For 1-2 rooms: window or mini-split. For 3+ rooms: central AC or multi-zone mini-split.
What's a ton of cooling? +
1 ton = 12,000 BTU/hr of heat removal capacity. Historical origin: heat needed to melt 1 ton of ice in 24 hours. Residential central AC ranges 1.5 to 5 tons (18,000-60,000 BTU/hr). Mini-splits come in 9K, 12K, 18K, 24K, 36K BTU sizes.
Does insulation level affect AC sizing? +
Yes — dramatically for whole-house calculations, modestly for single-room. An uninsulated room loses cooling 3-4x faster than a well-insulated one. The 25 BTU/sq ft rule assumes typical wall and ceiling insulation; for a finished basement or insulated room, drop to 18-20 BTU/sq ft.
Should I get a heat pump instead of just AC? +
For new construction or replacing aging equipment: yes. Modern heat pumps (especially mini-splits) cool as efficiently as dedicated AC, AND heat efficiently down to 0°F or lower. In moderate climates, a heat pump replaces both furnace and AC. In cold climates (zone 6+), pair with backup heat strips or gas furnace for sub-zero days.
What's SEER and SEER2? +
Seasonal Energy Efficiency Ratio — BTU of cooling per watt-hour of electricity. Higher = more efficient. SEER 13 was the old federal minimum; SEER 14-15 is current minimum. Premium units run SEER 18-22+. SEER2 is the updated 2023 testing standard that's slightly more realistic than SEER — SEER2 numbers run about 5% lower than the equivalent SEER number on the same equipment.
How loud are window AC units? +
50-65 dB at the unit, similar to a quiet refrigerator (50) up to a normal conversation (60). Mini-splits are quieter — the noisy compressor is outside; the indoor head is 25-35 dB (whisper quiet). For light-sleeper bedrooms, mini-splits beat window units significantly.
Inverter vs non-inverter AC — worth the price? +
Inverter compressors modulate output to match load — they run at partial capacity most of the time instead of cycling on/off. Result: tighter temp control, lower humidity, 30-40% less electricity, longer compressor life. The premium ($200-500 more) pays back in 2-4 years on a daily-use unit. Worth it.