AC tonnage from model number

How many tons is my AC unit?

The tonnage is printed on your unit, just never as the word "tons." It hides inside the model number on the data plate as a two or three digit code that counts thousands of BTU per hour. One ton of cooling equals 12,000 BTU per hour, so the code is always divisible by 6: a 36 means 36,000 BTU, which is 3 tons. A 48 is 4 tons. An 060 is 5 tons.

Walk out to the outdoor unit and find the data plate on the back or side panel, near where the copper refrigerant lines enter the cabinet. Copy the line labeled "Model No." or "M/N" into the decoder above and it does the rest. The catch, and the reason the decoder exists, is that model numbers are stuffed with other digits that look like size codes: SEER digits, voltage codes, product-family prefixes. Carrier starts many model numbers with 24 or 25, and neither of those is the tonnage. The decoder knows which positions each brand uses, so it grabs the right pair.

Trane, Carrier, Goodman, Lennox: where the size sits in each brand's model number

Every major US brand encodes the size the same way (thousands of BTU, divisible by 6) but puts it in a different spot. The table below shows the pattern for each brand family with a real model number, so you can read yours by hand even if the plate is half faded:

Two families deserve a flag. Armstrong Air and Ducane models read wrong so easily that some retailer listings get them wrong: the 136 in 4SCU14LE136P is a 1 (electrical code) followed by 36 (3 tons), not a 13.6-ton monster. And old Lennox units from the HS era used size codes that have nothing to do with BTU, so an HS29-411 is a 3-ton unit even though 41 is not divisible by anything useful. If the decoder refuses your old Lennox, that is why, and the printed BTU line on the data plate is your fallback.

AC tonnage chart: model number code to tons and BTU

Once you know which digits are the size code, the conversion is fixed across every brand. A common mistake is reading 042 as 42 tons or 4.2 tons. It is 42,000 BTU per hour, and dividing by 12,000 gives 3.5 tons:

Mini-splits run on the same math but skip the tons vocabulary: a 9,000 BTU head is three quarters of a ton, a 12,000 BTU head is 1 ton. If your model number shows a 09 or 12, you have a mini-split code, not a broken chart.

Model number vs serial number: which one has the size

The data plate carries both numbers side by side, and mixing them up is the most common reason a decode fails. The model number mixes letters and digits in a structured pattern and is shared by every unit of that configuration; that is where the size lives. The serial number is unique to your unit, usually runs longer and heavier on digits, and encodes the manufacture date instead. If you want to know how old the unit is, our HVAC serial number age tool decodes that side of the plate. Between the two numbers you can pull the size and the age off any unit in about a minute, which is most of what a contractor reads off the plate before quoting you.

How many BTU is my gas furnace?

Furnaces use the same trick with a different unit. The three-digit group in a furnace model number is the input in thousands of BTU per hour: 040, 060, 080, 100, or 120. A Goodman GMSS960803BN is an 80,000 BTU furnace. The number to watch is the one before it: the 96 is the AFUE efficiency code, and it matters because the model number states what the burner consumes, not what reaches your ducts. That 80,000 BTU input at 96 percent AFUE delivers about 76,800 BTU of heat. An older 80 percent furnace with the same 080 code only delivers 64,000. Two furnaces with identical size codes can differ by a full room's worth of heat, so keep the AFUE in view when you compare.

When the outdoor and indoor unit numbers don't match

People decode their condenser at 4 tons, then find a 42 on the indoor coil or a compressor part number that reads 3.5, and assume something was installed wrong. Usually nothing is. Manufacturers routinely pair a larger indoor coil with a smaller compressor to boost efficiency, and the outdoor unit's model number is the one that names the nominal system size. The number is also nominal rather than exact: a 3-ton system typically rates a few percent under 36,000 BTU once it is certified with its matched coil, which is normal and not a defect.

Where a mismatch does matter is on a replacement. The new indoor coil has to match the condenser's tonnage and refrigerant, and the pair should be certified together for the efficiency you are paying for. Ask the installer for the AHRI reference number of the matched pair; rebates often require it, and it is a two-minute check that catches a mismatched quote before it is on your patio.

Should your replacement be the same size?

Not automatically, and this is where the decoded number earns its keep. Most contractors quote like for like off the old data plate because it is fast. But the original unit was often sized by a rule of thumb, and homes change: new insulation, air sealing, or replacement windows can cut the load by half a ton or more, while an addition pushes it the other way. If the old unit cooled evenly and handled humidity, matching its size is reasonable. If it short cycled, left rooms clammy, or ran flat out on mild days, cloning it just buys the same problem with a fresh warranty.

Bigger is not the safe direction either. An oversized AC hits the thermostat setting fast and shuts off before it wrings the moisture out of the air, which is why oversized houses feel cold and damp at the same time, and the constant restarts wear the compressor. Before you sign, compare the tonnage you decoded against what the house actually needs with our AC tonnage calculator. If the two numbers disagree by a ton, that gap is worth a conversation with the contractor, and our HVAC replacement cost calculator shows what the swap should run at either size so you can price-check the quote too.