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BTU Calculator

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

sq ft
feet
Recommended AC Size
8,000 BTU
0.67 tons
Base BTU
6,800
Adjusted BTU
7,650
Tonnage
0.67 tons
AdjustmentBTU
Base (340 sq ft × 20 BTU/sqft)6,800
Ceiling height (9 ft)+850
Total (adjusted)7,650

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 NeededAC Size
100-2005,000-6,000Window unit
200-3506,000-8,000Window unit
350-5008,000-12,000Large window/portable
500-70012,000-14,000Mini split
700-1,00014,000-18,000Mini split
1,000-1,50018,000-24,0001.5-2 ton central
1,500-2,50024,000-36,0002-3 ton central
2,500+36,000-60,0003-5 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

InsulationFactorTypical Building
Poor0.55Old home, no insulation, single-pane windows
Normal0.40Average home, some insulation
Good0.28Well-insulated walls, double-pane windows
Excellent0.20New construction, spray foam, triple-pane

Temperature Change Examples

LocationWinter LowTargetTemp Change
Minneapolis-15°F70°F85°F
Boston-5°F70°F75°F
Denver5°F70°F65°F
Atlanta25°F70°F45°F
Miami45°F70°F25°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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Frequently Asked Questions

What is a BTU?

A British Thermal Unit (BTU) is the amount of heat needed to raise one pound of water by 1°F. In HVAC, BTU/hr measures the cooling or heating capacity of an air conditioner, furnace, or heat pump.

How many BTU do I need per square foot?

A general rule is 20 BTU per square foot for cooling. However, this varies with ceiling height, insulation, sun exposure, climate, and occupancy. The AC calculator accounts for all these factors.

What does AC tonnage mean?

One ton of cooling equals 12,000 BTU/hr. A 2-ton AC unit provides 24,000 BTU/hr. Residential ACs typically range from 1 to 5 tons. The term comes from the cooling effect of melting one ton of ice per day.

How do I calculate BTU for heating?

Use the General calculator. Enter room dimensions, insulation quality, and the desired temperature change (difference between coldest outdoor temp and target indoor temp). The calculator estimates BTU needed to maintain that temperature.

What is desired temperature change?

It is the difference between the outdoor temperature and your target indoor temperature. For example, if Boston winter drops to -5°F and you want 70°F indoors, the desired change is 75°F. For Atlanta winter (25°F to 70°F), use 45°F.

Is it bad to oversize an AC?

Yes. An oversized AC cools too quickly, shutting off before properly dehumidifying the air. This causes clammy air, uneven temperatures, and higher energy bills from frequent cycling. Size correctly or slightly under.