Model number decoder: tonnage and furnace size
The model number of an air conditioner, heat pump or gas furnace carries its size. On an outdoor unit it is a two- or three-digit code in thousands of BTU per hour: 36 or 036 means 36,000 BTU/h, which is 3 tons. Choose the brand and type the model number to read it.
Find the size from the model number
What do the size codes mean?
On an air conditioner or heat pump the code is the nominal cooling capacity in thousands of BTU per hour. One ton of cooling is 12,000 BTU/h, so the code divided by 12 is the tonnage. Some series print the code with a leading zero: 036 and 36 are the same size.
| Code | Nominal BTU/h | Tons | Variant codes in the same size class |
|---|---|---|---|
| 18 or 018 | 18,000 | 1.5 | 17, 19 |
| 24 or 024 | 24,000 | 2 | 25 |
| 30 or 030 | 30,000 | 2.5 | 31 |
| 36 or 036 | 36,000 | 3 | 37, 38 |
| 42 or 042 | 42,000 | 3.5 | 41, 43 |
| 48 or 048 | 48,000 | 4 | 47, 49 |
| 54 or 054 | 54,000 | 4.5 | |
| 60 or 060 | 60,000 | 5 | 59, 61 |
A variant code marks a slightly different version of the same size class. Not every brand uses every code.
On a gas furnace the code is the heating input in thousands of BTU per hour, always three digits: 040 is 40,000 BTU/h, 060 is 60,000, 080 is 80,000, 100 is 100,000 and 120 is 120,000. It is input, not output, and it is not tons.
Where is the size in the model number, by brand?
The position of the code differs from brand to brand, and often from series to series. Each page shows the layouts, worked examples and the document each layout comes from.
- Carrier, Bryant and Payne7 layouts: air conditioners, heat pumps and gas furnaces
- Trane and American Standard7 layouts: air conditioners, heat pumps and gas furnaces
- Goodman, Amana and Daikin12 layouts: air conditioners, heat pumps and gas furnaces
- Lennox2 layouts: air conditioners, heat pumps and gas furnaces
- Rheem and Ruud3 layouts: air conditioners, heat pumps and gas furnaces
- York, Coleman and Luxaire7 layouts: air conditioners, heat pumps and gas furnaces
How are the layouts checked?
A layout is taken from the manufacturer’s own document wherever one could be found. Every layout is then run against the two public registries of certified equipment: the US Department of Energy compliance certification database and the ENERGY STAR product list. Both give a certified capacity for each model number.
On October 5, 2026 the layouts read 1,686 of 1,688 certified outdoor units in the DOE database to the certified size, 629 of 639 heat pumps in the ENERGY STAR list, and 2,798 of 2,798 gas furnaces in the DOE database.
An outdoor unit counts as agreeing when the middle of its certified cooling capacity range is between 80% and 112% of the nominal size in the code. A furnace agrees when its certified input is between 90% and 106% of the code. Cold-climate heat pumps account for most of the misses: their certified cooling capacity sits well below the nominal size.
What the decoder does not read
- Ductless mini-splits, packaged units, evaporator coils, air handlers and commercial equipment. Their model numbers follow other layouts.
- Brands that are not in the list above.
- Older model numbers that match none of the layouts. The decoder then says so. It does not guess.
What to do with the size
The size of the unit you have is not proof of the size the house needs. Check it with the AC tonnage calculator or the furnace size calculator. The serial number on the same plate gives the age: serial number decoders by brand.
Questions and answers
How do I find the tonnage of my air conditioner?
Read the model number on the rating plate of the outdoor unit. It holds a two- or three-digit size code in thousands of BTU per hour. Divide the code by 12: 24 is 2 tons, 36 is 3 tons, 48 is 4 tons and 60 is 5 tons.
How many BTU is one ton of air conditioning?
One ton is 12,000 BTU per hour. A 3-ton air conditioner has a nominal cooling capacity of 36,000 BTU/h.
Is the tonnage printed on the unit?
Usually not as a word. The rating plate lists the model number, the electrical data and the refrigerant charge. The size is inside the model number.
Does a furnace model number show tonnage?
No. A gas furnace is sized by its heating input in BTU per hour, a three-digit code in the model number. Many furnace numbers also carry a blower-size code that is written as tons of cooling airflow. That code is not heating capacity.
Sources
- US Department of Energy compliance certification database: central air conditioners and heat pumps (certified cooling capacity for each outdoor model number): https://www.regulations.doe.gov/certification-data/CCMS-4-Air_Conditioners_and_Heat_Pumps_-_Central.html
- US Department of Energy compliance certification database: furnaces (certified input capacity for each model number): https://www.regulations.doe.gov/certification-data/CCMS-4-Furnaces.html
- ENERGY STAR certified air-source heat pumps (certified cooling capacity for each outdoor model number): https://data.energystar.gov/Active-Specifications/ENERGY-STAR-Certified-Air-Source-Heat-Pumps/w7cv-9xjt
The manufacturer documents behind each layout are listed on the brand pages.
Layouts last checked on October 5, 2026. Method and sources.