AC Tonnage Calculator
Air conditioner capacity is expressed in two equivalent units: BTU per hour (BTU/h) and tons of refrigeration. One ton equals exactly 12,000 BTU/h, a convention derived from the heat of melting one ton of ice per day. Residential systems typically range from 1.5 to 5 tons (18,000 to 60,000 BTU/h); commercial systems are much larger. This calculator converts in both directions: enter BTU/h to get tons, or enter tons to get BTU/h. It also estimates required tonnage from your room's square footage using the 20 BTU per square foot rule of thumb.
Tonnage conversion formula
Tons = BTU/h ÷ 12,000
Estimated BTU/h = Area (sq ft) × 20
Estimated tons = Area (sq ft) × 20 ÷ 12,000
The conversion factor of 12,000 BTU/h per ton is exact and standardized. The 20 BTU per square foot estimate assumes 8-foot ceilings, average insulation, and a temperate US climate. Adjust upward for hot climates (use 25-28 BTU/sq ft) or downward for well-insulated spaces in cool climates (15-18 BTU/sq ft).
Common residential AC sizes
- 1.5 tons (18,000 BTU/h): suitable for rooms up to about 700 sq ft in temperate climates.
- 2 tons (24,000 BTU/h): small homes or large rooms, approximately 900 to 1,100 sq ft.
- 2.5 tons (30,000 BTU/h): mid-size homes, approximately 1,100 to 1,400 sq ft.
- 3 tons (36,000 BTU/h): larger homes or hot climates, approximately 1,400 to 1,800 sq ft.
- 4 tons (48,000 BTU/h): large homes, approximately 1,800 to 2,400 sq ft.
- 5 tons (60,000 BTU/h): very large homes, approximately 2,400 to 3,000 sq ft.
AC tonnage: frequently asked questions
What does one ton of air conditioning mean?
One ton of refrigeration equals 12,000 BTU per hour. The term originates from the amount of heat required to melt one ton (2,000 lb) of ice in 24 hours, which is approximately 288,000 BTU, or 12,000 BTU/h. Modern AC equipment is still rated in tons.
How many tons do I need for a 1,500 square foot home?
A rough estimate for a 1,500 sq ft home in a temperate US climate is 2 to 2.5 tons (24,000 to 30,000 BTU/h), based on the 20 BTU per square foot guideline. Hot climates, vaulted ceilings, or poor insulation push this higher. A Manual J calculation gives the precise answer.
Can I look up my current AC tonnage from the model number?
Yes. Most residential AC units encode tonnage in the model number. Look for a two-digit number divisible by 6 or 12. For example, '24' indicates 24,000 BTU/h (2 tons), '36' indicates 3 tons, '48' indicates 4 tons. Check your unit's data plate or the manufacturer's spec sheet to confirm.
Is a larger ton rating always better?
No. An oversized AC unit cools the space too quickly and shuts off before removing sufficient humidity, leading to a clammy feeling despite the correct temperature. Short-cycling also increases wear on compressor components and reduces efficiency. Proper sizing matches the actual heat load.
What is the SEER rating and how does it relate to tonnage?
SEER (Seasonal Energy Efficiency Ratio) measures cooling output per unit of energy consumed, expressed as BTU per watt-hour. It is independent of tonnage; you can have a 2-ton unit with SEER 14 or SEER 20. Higher SEER means lower operating cost for the same cooling capacity.
Official sources
- U.S. DOE Energy Saver: Air Conditioning.
- ASHRAE: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.