BTU Calculator

Calculate BTU needed for air conditioning and heating. Estimate AC size based on room size, insulation, climate, and more.

AC BTU Worksheet

Room size

sq ft

Ceiling height

Number of people

Room type

Insulation

Sun exposure

Climate

Recommended AC size 8,000 BTU

0.67 tons

Base (340 sq ft × 20 BTU/sqft)6,800

Ceiling height (9 ft)+850

Total (adjusted)7,650

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AC BTU Calculator

The AC BTU calculator estimates the cooling capacity needed for a room or house. It starts with a base calculation of 20 BTU per square foot and adjusts for ceiling height, occupancy, room type, insulation, sun exposure, and climate zone.

BTU by Room Size (Quick Reference)

Room Size (sq ft)	BTU Needed	AC Size
100-200	5,000-6,000	Window unit
200-350	6,000-8,000	Window unit
350-500	8,000-12,000	Large window/portable
500-700	12,000-14,000	Mini split
700-1,000	14,000-18,000	Mini split
1,000-1,500	18,000-24,000	1.5-2 ton central
1,500-2,500	24,000-36,000	2-3 ton central
2,500+	36,000-60,000	3-5 ton central

General Heating/Cooling BTU

The general calculator uses room volume (width × length × ceiling height), insulation quality, and desired temperature change to estimate BTU requirements for both heating and cooling. This works for furnaces, heat pumps, and space heaters.

Insulation Factors

Insulation	Factor	Typical Building
Poor	0.55	Old home, no insulation, single-pane windows
Normal	0.40	Average home, some insulation
Good	0.28	Well-insulated walls, double-pane windows
Excellent	0.20	New construction, spray foam, triple-pane

Temperature Change Examples

Location	Winter Low	Target	Temp Change
Minneapolis	-15°F	70°F	85°F
Boston	-5°F	70°F	75°F
Denver	5°F	70°F	65°F
Atlanta	25°F	70°F	45°F
Miami	45°F	70°F	25°F

Tips for HVAC Sizing

Don't oversize: An oversized unit cycles on/off too frequently, wasting energy and causing humidity issues
Consider humidity: In humid climates, slightly undersizing helps the AC run longer and dehumidify better
Factor in ductwork: Add 20-25% for duct losses in unconditioned spaces (attics, crawlspaces)
Get a Manual J: For whole-house HVAC, a professional Manual J calculation is the gold standard
SEER rating matters: Higher SEER (Seasonal Energy Efficiency Ratio) means lower operating costs. Current minimum is SEER 14-15.

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On conv.awajis.com

BTU to Joules Convert heating energy to SI units.

Frequently Asked Questions

How many BTU do I need per square foot?

The general rule is 20 BTU per square foot for cooling, so a 500 sq ft room needs about 10,000 BTU per hour. The real figure shifts with ceiling height, insulation, sun exposure, climate and how many people use the room. Add roughly 1,000 BTU for each large sunny window facing south or west.

How many BTU is a ton of air conditioning?

One ton of cooling equals 12,000 BTU per hour, so a 2-ton AC unit delivers 24,000 BTU per hour. Residential air conditioners typically range from 1 to 5 tons. The term comes from the cooling effect of melting one ton of ice over a full day. To convert BTU to tons, divide the BTU figure by 12,000.

Is it bad to oversize an AC unit?

Yes. An oversized air conditioner cools the room too quickly and shuts off before it removes enough humidity. The result is clammy air, uneven temperatures and higher energy bills from frequent on-off cycling. Size the unit to the calculated load, or slightly under it, rather than buying the biggest unit the budget allows.

How do high ceilings change BTU requirements?

Each foot of ceiling height above 8 feet adds roughly 10 to 15% to the BTU needed, because there is more air volume to heat or cool. A room with 12-foot ceilings needs about 40 to 60% more capacity than the same floor area with standard 8-foot ceilings. Most per-square-foot rules assume 8-foot ceilings.