AC Size Calculator

Use this AC size calculator to size whole-house central air in tons from conditioned square footage, IECC climate zone, ceiling height, insulation and sun exposure.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size central air, multiply the conditioned square footage by 20 to 30 BTU per square foot for your IECC climate zone. Adjust for ceiling height, insulation, sun and internal gains, then divide by 12,000 and round to the nearest half ton. A 2,000 sq ft house in zone 4 with average insulation and 8 ft ceilings comes out at 48,000 BTU per hour, which is 4 tons.

Your measurements

A rectangular volume: length times width times depth.

Conditioned floor areasq ftEvery floor the ducts serve, including a conditioned basement. Leave out an unconditioned attic, garage or crawlspace even if a duct passes through it.Climate zoneThe 2021 IECC map runs zone 1 in south Florida to zone 8 in interior Alaska. Cooling load per square foot falls as you go north, from about 30 down to 20.Average ceiling heightftThe load scales by height divided by 8 ft, because the system cools volume. Average the whole house if only the great room is vaulted.Insulation and windowsGlass drives the cooling load more than wall insulation does. Clear double-pane glass on a west wall gains roughly twice what low-SHGC glass gains.Sun exposureJudge it from the west and south walls in late afternoon, which is when the house peaks. Morning shade on the east side does not count for much.People living in the houseManual J counts 230 BTU per hour of sensible heat and 200 of latent heat per person. This adds 430 BTU for each person above four.Extra heat sourcesOnly add what the square-foot factor does not already cover. A normal kitchen in a 2,000 sq ft house is inside the factor; a commercial range or a server rack is not.

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Results

Cooling capacity needed: 4 tons; For 2,000 sq ft in zone 4, rounded to the nearest half ton because that is how equipment is sold
Cooling load: 48,000 BTU per hour; 24 BTU per sq ft plus 0 BTU of internal gain
Standard equipment size: 4 tons; 48,000 BTU per hour nominal. Split systems are sold at 1.5, 2, 2.5, 3, 3.5, 4 and 5 tons, and most lines have no 4.5 ton step
BTU per square foot: 24 BTU per sq ft; 24 for zone 4, 0% for the envelope, 0% for sun and 0% for ceiling height
Sizing check: Within the rule of thumb; 500 sq ft of floor per ton. Manual J on a tight, well shaded house often lands at 700 sq ft to 1,000 sq ft per ton, so treat this as the upper bound
Before you buy: Manual J, then Manual S; Planning estimate only. IRC M1401.3 and IECC R403.7 require the equipment to be picked by Manual S from a Manual J load, and a licensed mechanical contractor pulls the permit

AC Size formula

BTU per sq ft = Zone factor (20 to 30) x envelope factor x sun factor x Ceiling height / 8
Cooling load = Area (sq ft) x BTU per sq ft + 430 x (people - 4) + heat source adder
Tons = Cooling load / 12,000, rounded to the nearest 0.5
Equipment size = nearest of 1.5, 2, 2.5, 3, 3.5, 4 or 5 tons

Worked example

A 1,400 sq ft house on the Gulf Coast in IECC zone 2, with 8 ft ceilings, a tight shell with low-SHGC glass, west-facing and unshaded, five people and an open kitchen. The zone 2 factor is 28 BTU per square foot. Good insulation takes it to 28 x 0.85 = 23.8, and the sun factor takes it to 23.8 x 1.1 = 26.18 BTU per square foot.

The shell load is 1,400 x 26.18 = 36,652 BTU per hour. The fifth person adds 430 and the kitchen adds 1,200, for 38,282 BTU per hour. Divide by 12,000 and you get 3.19 tons, which rounds to 3 tons of standard equipment at 36,000 BTU per hour. That works out to 467 square feet per ton, aggressive even for zone 2, and a Manual J on a house with low-SHGC glass would likely come back at 2.5 tons.

Quick reference

Tons of central air by house area and climate zone (average insulation and sun, 8 ft ceilings, four people)
House area	Zone 2, 28 BTU per sq ft	Zone 4, 24 BTU per sq ft	Zone 6, 21 BTU per sq ft
1,000 sq ft	2.5 tons	2 tons	2 tons
1,200 sq ft	3 tons	2.5 tons	2 tons
1,500 sq ft	3.5 tons	3 tons	2.5 tons
1,800 sq ft	4 tons	3.5 tons	3 tons
2,000 sq ft	4.5 tons	4 tons	3.5 tons
2,500 sq ft	6 tons, two systems	5 tons	4.5 tons
3,000 sq ft	7 tons, two systems	6 tons, two systems	5.5 tons, two systems

How to size central air in tons

Take the conditioned floor area, multiply by a climate factor, and divide by 12,000. The factor runs about 30 BTU per square foot in zones 1 and 2, 24 in zone 4, and 20 to 21 in the northern zones. It falls going north because the gap between 75 degrees indoors and the design day outdoors shrinks. Scale for ceiling height, since the system cools volume, and adjust 15 percent either way for the shell. Then round to the nearest half ton, because that is the step equipment is built in.

Round to the nearest half rather than always up. A 3.19 ton result belongs on a 3 ton condenser, not a 3.5. Manual S permits a single-speed system to cover 90 to 115 percent of the load, so a unit slightly under the calculated number is normal and correct. Check the answer against square feet per ton as well: this method gives 400 to 600, while a Manual J on a house with a tight shell and good glass often returns 700 to 1,000.

Why oversizing is the classic mistake

An air conditioner does two jobs. It drops the air temperature, which happens fast, and it condenses water out of the air on the cold coil, which happens slowly and only once the coil is fully wet. A unit one size too big hits the setpoint in 5 to 8 minutes and shuts off. The coil never gets wet enough to drain, so the moisture it did collect evaporates back into the house on the next blower cycle. The result is a room at 72 degrees that still feels clammy, which is the complaint that drives people to set the thermostat even lower.

Short cycles cost money in other ways. A compressor draws locked-rotor current on every start, so a system that starts six times an hour instead of twice wears the contactor, the start components and the compressor. Room-to-room temperatures get worse too, because the blower stops before the air in the far bedroom has turned over. If the calculated load lands just above a size, get a Manual J before you order. On a code-built house it almost always comes back smaller than the square-foot rule.

What this calculator leaves out

A square-foot rule cannot see the things that actually set a cooling load. It misses how many square feet of west glass there are and the solar heat gain coefficient on that glass. It also misses ducts running through a 130 degree attic, how leaky the shell is, and how the load splits between floors. A Manual J covers all of that room by room. It also separates sensible from latent load, which is the number that decides whether a house in Houston stays dry. Not included here: the duct design, the return path, the line set and pad, the disconnect and circuit. Nor the condensate drain and secondary pan, the thermostat, permits, or any piping change the move from R-410A to R-454B forces.

Code limits that apply

IRC M1401.3checked
Equipment selected by ACCA Manual S from an ACCA Manual J load calculation
If you miss it: A permit filed without the load sheet stalls, and the oversized system that gets installed never controls humidity
IECC R403.7
Heating and cooling equipment sized to Manual S using Manual J design loads
If you miss it: The energy inspection is held until the contractor produces the load calculation for the house as built
ACCA Manual S
Total cooling capacity 90 to 115 percent of the design load in a humid climate
If you miss it: Capacity above 115 percent leaves the coil dry and the house near 60 percent relative humidity at 72 degrees
IRC M1601.1
Duct systems sized by ACCA Manual D or another approved method
If you miss it: Old ducts on a bigger system run high static pressure, cut airflow and ice the evaporator coil
IRC M1602.2
Return air taken from inside the conditioned space, not an attic, garage or crawlspace
If you miss it: A return panned into a closet or garage pulls in dust, fumes and humid attic air the system then has to treat
IRC M1305.1.1
30 in by 30 in of level working space at the service side of the air handler
If you miss it: An air handler boxed into a closet fails the final and the wall has to be opened back up

Provision	Limit	If you miss it
IRC M1401.3checked	Equipment selected by ACCA Manual S from an ACCA Manual J load calculation	A permit filed without the load sheet stalls, and the oversized system that gets installed never controls humidity
IECC R403.7	Heating and cooling equipment sized to Manual S using Manual J design loads	The energy inspection is held until the contractor produces the load calculation for the house as built
ACCA Manual S	Total cooling capacity 90 to 115 percent of the design load in a humid climate	Capacity above 115 percent leaves the coil dry and the house near 60 percent relative humidity at 72 degrees
IRC M1601.1	Duct systems sized by ACCA Manual D or another approved method	Old ducts on a bigger system run high static pressure, cut airflow and ice the evaporator coil
IRC M1602.2	Return air taken from inside the conditioned space, not an attic, garage or crawlspace	A return panned into a closet or garage pulls in dust, fumes and humid attic air the system then has to treat
IRC M1305.1.1	30 in by 30 in of level working space at the service side of the air handler	An air handler boxed into a closet fails the final and the wall has to be opened back up

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Matching the old unit's tonnage. Systems installed before 2000 were routinely a full ton over, so the replacement inherits the mistake.
Adding a half ton for an addition that may never be built. That capacity short cycles for the 15 years the system lives.
Leaving the ducts alone. A 4 ton system wants about 1,600 CFM, and a single 14 in return trunk cannot carry it.
Sizing a two-story house as one zone. The upper floor can be 60 percent of the load while the thermostat sits downstairs.
Pairing a new condenser with the old indoor coil. The AHRI certified match sets the rated capacity and the compressor warranty.
Chasing humidity with more tonnage in the Southeast. Long runtime is what dries a house out, so the smaller unit wins.

Key facts

One ton of cooling is 12,000 BTU per hour, and residential split systems are sold in 1.5, 2, 2.5, 3, 3.5, 4 and 5 ton sizes, with no 4.5 ton step in most lines.
ACCA Manual S allows a single-speed air conditioner's total capacity to run from 90 to 115 percent of the Manual J total cooling load in a humid climate.
ACCA Manual J counts 230 BTU per hour of sensible heat and 200 BTU per hour of latent heat for each person in the house.
IRC M1401.3 and IECC R403.7 both require heating and cooling equipment to be selected by Manual S from an ACCA Manual J load calculation.
An oversized air conditioner satisfies the thermostat in 5 to 8 minutes, short of the 10 to 15 minutes of runtime the coil needs to pull real moisture out of the air.

Frequently asked questions

What size AC unit do I need for a 2,000 sq ft house?

About 4 tons by the square-foot rule in a mixed zone 4 climate, which is 48,000 BTU per hour. In zone 2 the same house comes out near 4.5 tons and in zone 6 near 3.5. A Manual J on a code-built house of that size often returns 2.5 to 3 tons instead.

How many square feet does a 3 ton AC cool?

Roughly 1,285 square feet in zone 2 at 28 BTU per square foot, and about 1,715 square feet in zone 6 at 21. Those are rule-of-thumb numbers. A tight house with low-SHGC glass can run 2,000 square feet or more on 3 tons.

Why does my new air conditioner leave the house humid?

Almost always because it is a size too big. It reaches the setpoint before the evaporator coil is wet enough to drain, so it removes very little moisture. A variable speed system, or a correctly sized single-speed unit, runs longer cycles and pulls the humidity down.

Do I need a Manual J to replace my AC?

IRC M1401.3 and IECC R403.7 both require it, and most jurisdictions ask for the load sheet with the mechanical permit. A contractor runs one in about an hour with software, typically $150 to $400 on its own and included with most bids.

Sources and references

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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.