What size AC unit do I need?

You need the size that matches your home’s calculated cooling load, which a Manual J calculation finds. Central air conditioners for homes come in nominal sizes from 1.5 to 5 tons, and one ton is 12,000 BTU/h. The US Department of Energy (DOE) says the common estimate of one ton per 400 to 500 square feet fails to account for local climate and house efficiency.

DOE says your contractor should use Manual J to calculate the proper size of HVAC equipment. The Air Conditioning Contractors of America (ACCA) publishes Manual J. Read what a Manual J load calculation is before you ask for quotes.

Is there a quick estimate by square footage?

Yes, but it is rough. DOE cites a common estimate of one ton of cooling per 400 to 500 square feet. DOE says that estimate fails to take into account local climate and the efficiency of a house. It says one ton can condition 800 to 1,000 square feet of a very efficient house. For a table by floor area, use the AC tonnage calculator.

What sizes do central air conditioners come in?

Central air conditioners for homes come in nominal sizes from 1.5 to 5 tons, and one ton is 12,000 BTU/h. That is the range DOE’s Building America guide lists, and the three product lines we read match it. Payne’s PA13NR line has nominal sizes from 018 through 060. Lennox calls its ML14KC1 a 1.5 to 5 ton air conditioner. Goodman lists its GSX14 at 18,000 to 60,000 BTU/h.

DOE’s Building America guide says most manufacturers sell nominal sizes in half-ton steps. These three lines list 1.5, 2, 2.5, 3, 3.5, 4 and 5 tons, with no 4.5 ton model.

Size (tons)Capacity (BTU/h)Model number digits
1.518,000018
224,000024
2.530,000030
336,000036
3.542,000042
448,000048
560,000060

The digits are the three-digit codes used in Goodman and Lennox model numbers. DOE’s Building America guide lists the two-digit form, such as 24 for 2 tons. Some models carry a code one step away for the same size: Lennox lists 041, 047 and 059, and Goodman lists 019, 025, 031, 037 and 043.

What goes wrong if the air conditioner is too big?

DOE says “oversized cooling systems can cost too much, provide inadequate comfort, and lead to mold and poor indoor air quality.” ENERGY STAR says a system that is too large may not keep your home comfortable. It cites frequent on/off cycling, which causes humidity control problems and inefficient operation.

ENERGY STAR also says oversized units tend to have shorter lives. It says short-cycling inflicts excessive wear and tear on the compressor.

DOE’s Building America guide says an oversized system can shut off before it has had time to remove moisture from the air. That can cause high humidity problems in the home. The DOE load calculation guideline says an oversized system has a higher initial cost and a higher cost of operation.

How do I read my air conditioner’s size from its model number?

Look for the capacity digits in the model number: 036, for example, means 36,000 BTU/h, or 3 tons. Find the model number on the rating plate of the outdoor unit, not the serial number. DOE’s Building America guide says the capacity is often indicated in the model number.

The guide’s example is model SSX160241, a 2 ton (24,000 BTU/h) air conditioner. Lennox’s ML14KC1 sheet lists the digits as the nominal cooling capacity, with 018 for 1.5 tons.

The position of the digits differs by brand. Goodman’s GSX14 nomenclature reads G S X 14 036 1 AA, with 036 as the capacity. Lennox’s ML14KC1 puts the capacity after a dash, as in ML14KC1-036. Check the maker’s nomenclature before you trust a guess.

The model number size decoder reads sizes for supported brands. To find the plate first, see where to find the rating plate.

When is my old air conditioner’s size a bad guide?

It is a bad guide if the home has changed or the old unit was not sized by a load calculation. Replacing a unit with one of equal capacity can carry over an over-sizing or under-sizing problem, DOE says. DOE says installers frequently specify the same capacity or larger for an existing home. Without a new load calculation, they may be assuming the old system was correctly sized. They may also be assuming the home has not changed through additions, occupancy changes or window replacements.

ENERGY STAR says increased insulation, air sealing, high-performance windows and improved duct systems can dramatically reduce cooling loads. It says smaller air conditioners can usually be installed. It adds that it is common for contractors to install oversized units because they cool more quickly.

Rheem says you can give a dealer the model and serial numbers of your old units, found on the rating plate, as a starting point. It adds that you will still require a load calculation.

How is the right air conditioner size worked out?

A contractor finds the load with Manual J, then selects equipment with Manual S. ENERGY STAR says to make sure your contractor verifies the proper size with Manual J. DOE says the loads are used to right-size the equipment with Manual S.

ACCA says Manual S provides size limits. A size factor is the equipment capacity divided by the load. The limits below are from the 2023 edition, as printed in ACCA’s 2025 proposed Addendum c. For the standard sizing condition, the total cooling size factor must be at least 0.90. It may be at most 1.20 for single-speed units with a cooling load of 24,000 BTU/h or less, 1.15 above that and 1.25 for two-speed units. Variable-capacity cooling-only units may go up to 1.30. A separate table covers the dry sizing condition, where the sensible share of the cooling load is 0.95 or more.

DOE’s 2012 guideline says Manual S recommends that cooling equipment capacity not exceed the total load by more than 15%. Sizes come in steps. DOE’s example is a 19,000 BTU/h cooling load with units sold only at 17,500 and 23,400 BTU/h. The larger unit is more than 15% oversized, but DOE says to select it so the peak load is met.

Questions and answers

How many tons of AC do I need for 2,000 square feet?

DOE cites one ton per 400 to 500 square feet, which gives 4 to 5 tons for 2,000 square feet. DOE says one ton can condition 800 to 1,000 square feet of a very efficient house, which gives 2 to 2.5 tons. Only a Manual J load calculation gives your number.

Is a bigger air conditioner better?

No. DOE says oversized cooling systems can cost too much, provide inadequate comfort, and lead to mold and poor indoor air quality. ENERGY STAR says a system that is too large may not keep your home comfortable because of frequent on/off cycling, which causes humidity control problems and inefficient operation.

Can I replace my air conditioner with the same size?

Only after checking that nothing has changed. DOE says replacing a system with one of equal capacity can perpetuate over-sizing or under-sizing, and installers may be assuming the old system was correctly sized and the home has not changed. Rheem says you will still require a load calculation.

How do I find my air conditioner’s size from the model number?

Find the model number on the outdoor unit’s rating plate, not the serial number. DOE’s Building America guide says the capacity is often indicated in the model number. Goodman and Lennox use three digits, such as 036 for 3 tons. Positions differ by brand, so check the maker’s nomenclature.

Sources

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The facts in this guide rest on 65 passages in the sources above. Each passage was found in its document on October 5, 2026. Method and sources.