Construction
BTU Calculator
Enter your room dimensions and conditions to estimate the air-conditioner size in BTU per hour, the equivalent cooling tonnage, and the standard window-unit class that fits.
Quick answer: A room needs roughly 20 BTU of cooling per square foot — about 5,000 BTU for a 12×12 bedroom and 12,000 BTU (1 ton) for 550–600 sq ft — adjusted up for high ceilings, poor insulation, strong sun, kitchens, and extra occupants.
- Room area180 sq ft
- Cooling tons0.29
- Window-unit size5,000 BTU
Rule-of-thumb sizing per DOE guidance (~20 BTU per square foot). For whole-house systems, a contractor’s Manual J load calculation is the accurate method — an oversized unit short-cycles and dehumidifies poorly.
How it works
1. Start at 20 BTU per square foot
The standard rule of thumb sizes cooling at about 20 BTU/hr per square foot of floor area, assuming 8-foot ceilings and average conditions. The calculator scales for taller ceilings, since it's really the air volume being cooled.
2. Adjust for conditions
Poor insulation and strong sun add load (up to ~25% together); good insulation and shade reduce it. Kitchens add roughly 4,000 BTU for appliance heat, and each person beyond two adds about 600 BTU.
3. Match to real equipment
Window units come in standard classes (5,000, 6,000, 8,000 … 24,000 BTU) — pick the smallest that meets your number. Oversizing cools fast but short-cycles and leaves the room humid; central systems are sized in tons (12,000 BTU each).
Frequently asked questions
How many BTU do I need per square foot?
About 20 BTU/hr per square foot for cooling under average conditions. A 300 sq ft room needs ~6,000 BTU; 550–600 sq ft needs ~12,000 BTU (one ton). Add capacity for kitchens, sun-baked rooms, and high ceilings.
What happens if my AC is too big?
It short-cycles — cooling the air quickly and shutting off before it dehumidifies, leaving the room cold but clammy, with more wear and higher bills. Slightly undersized and running steadily usually feels better than oversized.
Is this a substitute for a Manual J calculation?
No — it's rule-of-thumb sizing that works well for single rooms and window units. For central air or a heat pump, have a contractor run a Manual J load calculation, which accounts for windows, orientation, ductwork, and local climate.