How to Tell the Tonnage of My AC Unit (Model Number Guide)

To tell the tonnage of your AC unit, read the model number on the data plate of the outdoor condenser. Almost every brand encodes cooling capacity in that number as a two- or three-digit figure that represents thousands of BTUs per hour, and one ton of cooling equals 12,000 BTU/hr. So you find the capacity figure, then divide by 12 (or divide the full BTU number by 12,000) to get tonnage. For example, a model number containing 24, 030, or 036 means 24,000, 30,000, or 36,000 BTU, which works out to 2, 2.5, and 3 tons. Residential units come in standard sizes of 1.5, 2, 2.5, 3, 3.5, 4, and 5 tons, so your answer should land on one of those. If the plate is missing or unreadable, look up the model number on the manufacturer’s site or check the owner’s manual. Below we walk through the model-number method step by step and answer the most common sizing questions.

Where do I find the tonnage on my AC unit?

The fastest place to look is the manufacturer’s data plate on the outdoor condensing unit. It lists the model number, and the model number carries the capacity code you need. Most plates do not print the word “tonnage” directly, so reading the model number is the reliable route.

If you cannot read the plate, the owner’s manual usually states the capacity, and you can also enter the model number into the manufacturer’s website or a parts lookup to pull the specs. Knowing the tonnage matters because it tells you whether the unit is correctly matched to your home. A unit that is too large or too small for the space runs inefficiently and drives up energy bills.

So to find tonnage: read the model number on the condenser plate first, fall back to the manual, and use an online lookup if both fail.

How do you tell the tonnage of an AC unit by the model number?

This is the method HVAC techs actually use, and it works across most brands. Find the model number on the outdoor unit’s data plate, then look for a two- or three-digit number that represents the cooling capacity in thousands of BTU per hour. It almost always appears as one of these standard values: 18, 24, 30, 36, 42, 48, 54, or 60 (sometimes written as 018, 024, 030, and so on).

That number is the BTU capacity in thousands. To convert to tons, divide it by 12, because one ton of cooling equals 12,000 BTU/hr. So 24 means 24,000 BTU, which is 2 tons. 036 means 36,000 BTU, which is 3 tons. 060 means 60,000 BTU, which is 5 tons.

Brands like Goodman, Carrier, Trane, Lennox, and Rheem all embed the capacity this way, though the exact position of the digits in the string varies. When in doubt, the capacity number will round to one of the standard sizes (1.5 to 5 tons), so if your math gives an odd answer, you probably grabbed the wrong digits. Cross-check against the manufacturer’s nomenclature guide for your specific model.

How many tons is a standard AC unit?

Most central air conditioning units range from 1.5 to 5 tons, in half-ton steps. The right size depends on the square footage of the home, its insulation, ceiling height, window load, and your climate.

As a rough guide, a 1.5-ton unit suits a small home of roughly 600 to 900 square feet, while a 5-ton unit serves around 2,000 square feet or more. Each additional ton covers a few hundred more square feet, but this is only a starting point. Proper sizing uses a Manual J load calculation, not square footage alone, because two homes of the same size can have very different cooling needs.

Tonnage, BTU, and the 12,000 rule

The whole system rests on one fact: one ton of air conditioning equals 12,000 British thermal units (BTU) per hour. The term is a holdover from the days of cooling with ice, but it is now a fixed unit of cooling capacity.

That single conversion lets you move between any of the three numbers. If you know BTU, divide by 12,000 for tons. If you know tons, multiply by 12,000 for BTU. And because the model number encodes BTU in thousands, the same rule decodes the model number. There is no separate trick for tonnage versus BTU; it is all the same 12,000 conversion.

AC unit tonnage chart

This chart ties model-number codes, BTU, tons, and rough coverage together so you can read your unit at a glance:

• 18 / 018 = 18,000 BTU = 1.5 tons (roughly 600–900 sq ft)
• 24 / 024 = 24,000 BTU = 2 tons (roughly 900–1,200 sq ft)
• 30 / 030 = 30,000 BTU = 2.5 tons (roughly 1,200–1,500 sq ft)
• 36 / 036 = 36,000 BTU = 3 tons (roughly 1,500–1,800 sq ft)
• 42 / 042 = 42,000 BTU = 3.5 tons (roughly 1,800–2,100 sq ft)
• 48 / 048 = 48,000 BTU = 4 tons (roughly 2,100–2,400 sq ft)
• 60 / 060 = 60,000 BTU = 5 tons (roughly 2,400–3,000 sq ft)

Coverage figures are estimates. A common rule of thumb is about 20 to 25 BTU per square foot, so for a 1,440 sq ft space you would need roughly 30,000 to 36,000 BTU, or 2.5 to 3 tons. Climate, sun exposure, insulation, and occupancy all shift the real number, so use a chart to sanity-check, not to size from scratch.

How to tell tonnage of a Carrier AC unit

Carrier uses the same approach as everyone else: the capacity is encoded in the model number on the condenser data plate. Find the two- or three-digit capacity figure (such as 24, 036, or 048), then divide by 12 to get tons.

So a Carrier model showing 036 is a 3-ton unit (36,000 BTU), and one showing 060 is a 5-ton unit. Ignore the idea of measuring the cabinet with a tape measure; physical size does not reliably indicate capacity. The model number is the accurate source. If you are unsure which digits represent capacity, look up Carrier’s model nomenclature for your series, or enter the full model number on Carrier’s site to confirm what size unit you have.

How to read an AC model number

An AC model number packs in several details: the product family, efficiency, refrigerant type, and the capacity code you care about for tonnage. Manufacturers do not all use an identical format, but the capacity digits are nearly always one of the standard values (018 through 060).

Do not confuse the model number with the serial number. The serial number is what encodes the manufacturing date (often the year and week or month), not the capacity. For tonnage, you want the model number and its capacity code. When the format is unclear, the manufacturer’s nomenclature guide or owner’s manual is the definitive reference.

What size AC unit do I need?

Sizing a replacement is a different question from reading your current tonnage, and bigger is not better. An oversized unit cools the air fast but shuts off before it removes humidity, leaving the home clammy and short-cycling the equipment. An undersized unit runs nonstop and still cannot keep up on hot days.

The square footage of your home is the starting input, along with climate, insulation, ceiling height, window area and orientation, and how many people live there. A rough estimate uses about 20 to 25 BTU per square foot, then divides by 12,000 for tons. For a 2,000 sq ft home that lands somewhere around 3 to 3.5 tons, but the only accurate method is a Manual J load calculation by a professional, which accounts for all those variables. Use the BTUs of your AC and the 12,000 conversion to translate between any of these numbers.

How to determine AC tonnage from a Goodman model number

Goodman is one of the clearest examples of the method. On the outdoor unit, find the two-digit capacity number in the model string, which will be one of 18, 24, 30, 36, 42, 48, 54, or 60. That is the capacity in thousands of BTU.

Divide that number by 12 to get tonnage. For a model number such as GSX130241, the “24” means 24,000 BTU, which is 2 tons. A “36” means 36,000 BTU, or 3 tons. (Note: multiplying the capacity digits by 12,000 gives total BTU only if you read them as a single ton value; the reliable rule is capacity-in-thousands divided by 12 equals tons.) When in doubt, cross-check against Goodman’s published nomenclature.

Air conditioner tonnage calculation: how to find air conditioner size

Rule of thumb for AC tonnage calculation

A quick rule of thumb for air conditioning tonnage calculation is to multiply the square footage of the space by 20 to 25 BTU, then divide by 12,000 to get tons. For a 1,500 sq ft area that is roughly 30,000 to 37,500 BTU, or about 2.5 to 3 tons.

This is only a ballpark. Ceiling height, sun exposure, insulation quality, local climate, and the number of occupants all change the real load. Treat the rule of thumb as a sanity check and rely on a professional load calculation before buying.

4 ton AC unit

A 4-ton air conditioner delivers 48,000 BTU per hour and typically appears in the model number as 48 or 048. It is sized for larger homes in the neighborhood of 2,000 to 2,400 square feet, depending on climate and construction.

If your model plate reads 48, you have a 4-ton unit. As always, confirm that the home’s load actually calls for 4 tons before replacing like-for-like, because the original install may have been oversized or undersized.

Frequently Asked Questions

Can I tell tonnage by measuring the cabinet? Not reliably. Two units of similar physical size can have different capacities. The model-number capacity code is the accurate method.

What if my model number capacity digit is something like 25 or 35? Capacity codes round to standard values (18, 24, 30, 36, 42, 48, 60). An odd reading usually means you grabbed the wrong digits; recheck against the brand’s nomenclature.

Does the serial number tell tonnage? No. The serial number generally encodes the manufacture date. Capacity lives in the model number.

Next Steps

To tell your AC’s tonnage, read the capacity code in the model number on the outdoor unit and divide by 12. The data plate is the source, and the 12,000 BTU per ton rule is the only math you need:

Tonnage = (BTU/hr) / 12,000

Once you know your tonnage, you can confirm whether the unit is properly matched to your home, and use the same conversion when shopping for a replacement AC unit for your home.