How can I tell what tonnage my AC unit is?
You tell the tonnage from the model number, not by measuring or weighing anything. Walk out to the outdoor unit (the metal cabinet with the fan on top that sits next to the house), find the model number printed on the rating plate, and look inside it for a two- or three-digit block that is a multiple of 6: 018, 024, 030, 036, 042, 048, or 060. Manufacturers embed the nominal cooling capacity in that spot, expressed in thousands of BTU per hour. Divide that number by 12 and you have the tonnage.
For example, in a model number like CA13NA036, the 036 is the capacity: 36,000 BTU per hour, which is 3 tons (36,000 divided by 12,000). A number ending in 048 is a 4-ton unit, 060 is a 5-ton, and 024 is a 2-ton. This model-number method is the reliable one because it does not depend on the age of the system or on any label wearing off. It reads the same way across nearly every brand of central AC and heat pump. If you also want to confirm whether that outdoor box is an air conditioner or a heat pump, our guide on how to know if you have a heat pump covers the nameplate tells for that.
Where do I find the tonnage on my AC unit?
The tonnage lives in the model number on the rating plate (data plate), a metal or foil sticker on the side of the outdoor condenser unit, usually near the top where the electrical and refrigerant lines enter. It lists the model number, the serial number, the voltage, and the refrigerant type. The tonnage itself is almost never printed as the word "tons"; you read it out of the model number as described above. Wipe the plate clean if it is dusty, and take a phone photo so you can read the small print in good light.
If the outdoor plate is faded or unreachable, check the indoor equipment too. The evaporator coil and the air handler or furnace it sits on carry their own rating plates, and the coil's model number encodes a matching capacity. On a ductless mini split, the capacity is often printed right on the indoor head's label in plain BTU (9,000, 12,000, 18,000, 24,000), so you may not even need to decode anything. One caution: the serial number on the plate tells you the manufacture date and the unit's age, not its size, so do not confuse the two.
How do I read the tonnage from the model number?
Match the capacity number to this short table, because the math is the same every time: 018 = 1.5 tons (18,000 BTU), 024 = 2 tons (24,000 BTU), 030 = 2.5 tons (30,000 BTU), 036 = 3 tons (36,000 BTU), 042 = 3.5 tons (42,000 BTU), 048 = 4 tons (48,000 BTU), and 060 = 5 tons (60,000 BTU). A rare 012 is a 1-ton unit, common only on small mini splits. Every step is a half ton, or 6,000 BTU, which is why the capacity number is always a multiple of 6.
The reason it works is the definition of a ton of cooling: one ton removes 12,000 BTU of heat per hour, a figure that dates back to the cooling power of a ton of melting ice. So the model number is not telling you a weight; a 3-ton AC does not weigh three tons. It is telling you the unit moves 36,000 BTU per hour out of your house. If you want the deeper breakdown of what any single size covers, we have dedicated pages for the 3-ton AC unit, the 4-ton AC unit, and the 5-ton AC unit, each with the BTU, the typical cost, and the breaker and airflow it needs.
How do I tell a 1.5-ton apart from a 3 or 4-ton unit?
You tell them apart by that same capacity number, and the gaps are wide enough that there is no guesswork. A 1.5-ton unit reads 018 (18,000 BTU); a 2-ton reads 024. To separate a 3-ton from a 4-ton, look for 036 versus 048: a 3-ton moves 36,000 BTU and a 4-ton moves 48,000 BTU, a full ton apart. If you are trying to decide between 1 ton and 1.5 ton, that is 012 versus 018, and on central systems 1-ton units barely exist, so a small residential condenser is almost always the 1.5-ton 018.
The physical size of the cabinet is a weak clue and not a reliable one, because two brands can package the same tonnage in different footprints, and a higher-efficiency unit of the same tonnage is often physically larger. Always trust the model number over how big the box looks. If the number is smudged, the count of refrigerant lines, the fan size, or the breaker amperage can hint at capacity, but none of those pins down the exact tonnage the way the model number does.
What if the model number does not show an obvious tonnage?
A few brands break from the multiple-of-6 pattern, so if nothing in the model number looks like 018 through 060, do not force it. The safest move is to type the full model number into the manufacturer's website or call their support line with it, and they will tell you the exact nominal capacity. An HVAC contractor can also read it in seconds, and it is worth asking during any service visit so you have it on file. Photograph both the outdoor plate and the indoor coil plate before you call, since the capacity is sometimes clearer on one than the other.
Avoid the shortcuts that seem clever but mislead. The serial number does not encode tonnage, only the build date. The compressor's amperage (RLA or FLA) roughly tracks size but is not a clean tons number. And you cannot judge tonnage by how cold the air feels or how loud the unit is. When in doubt, get the reading confirmed rather than guessing, especially before you spend money on a repair part or a replacement quote that depends on the size being right.
Should I match my old tonnage when I replace the AC?
Not automatically. Knowing your current tonnage is useful for ordering the right repair part and for sanity-checking a quote, but copying the old size by default is a common and expensive mistake. The previous unit may have been oversized to begin with, and homes change: new windows, added insulation, air sealing, or an addition all shift the load. An oversized AC short cycles, cools the air fast without pulling out humidity, and leaves the house cold and clammy, while an undersized one runs nonstop in a heat wave and never catches up.
So use your existing tonnage as a reference point, then confirm the right size for the house today with a proper load calculation. Run the numbers with our BTU calculator before you take any quote, and if a contractor's proposed size is far off from both your old unit and the calculator, ask them to show their Manual J load calculation. For what a new system actually runs by size and brand, see our guide to central air conditioner prices.
Frequently asked questions
Is a 3-ton AC too big for 1500 square feet?
It can be, but square footage alone does not decide it. The right tonnage depends on your climate, insulation, ceiling height, window area, and how sealed the house is, so two 1500 sq ft homes can need different sizes. An oversized AC short cycles and leaves the air humid, so do not size by a rule of thumb. Run your actual numbers through the BTU calculator at /calculators/btu-calculator, and if a quote looks oversized, ask the contractor for their Manual J load calculation.
How do I check if my AC is 1 ton or 1.5 ton?
Read the capacity number in the model number on the outdoor unit's data plate: 012 is a 1-ton unit (12,000 BTU) and 018 is a 1.5-ton unit (18,000 BTU). On central air conditioners, true 1-ton units are rare, so a small residential condenser is almost always the 1.5-ton 018. One-ton and smaller capacities show up mostly on ductless mini splits, where the BTU rating is often printed plainly on the indoor head.
How do I know if an AC unit is 3 ton or 4 ton?
Look for 036 versus 048 in the model number. A 3-ton unit reads 036 and moves 36,000 BTU per hour; a 4-ton reads 048 and moves 48,000 BTU per hour. That is a full ton, or 12,000 BTU, apart, so the numbers are easy to distinguish. Do not judge it by the size of the cabinet, since efficiency and brand change the footprint for the same tonnage.
Can I find my AC's tonnage from the serial number?
No. The serial number encodes the manufacture date and is used to figure out the unit's age, not its capacity. Tonnage is read from the model number instead, specifically the two- or three-digit multiple of 6 (018 through 060) that gives the cooling capacity in thousands of BTU per hour. If you need the age of the unit, that is what the serial number is for.
What size AC do I need for my house?
Do not assume it matches your old unit's tonnage, which may have been wrong to begin with. Proper sizing comes from a load calculation that accounts for your climate, square footage, insulation, and windows. Estimate it with the BTU calculator at /calculators/btu-calculator, then have a contractor confirm with a Manual J calculation. Getting the size right matters more than picking a brand, because both oversizing and undersizing hurt comfort and efficiency.