AC unit size calculator
The size of a central air conditioner is measured in tons — one ton equals 12,000 BTU/h of cooling. Getting the tonnage right matters: an undersized unit never keeps up on the hottest days, while an oversized one short-cycles, cools unevenly and leaves the home humid.
This calculator sizes a whole-home, cooling-only central AC from your floor area and climate zone, with adjustments for ceiling height, sun, insulation, occupants and a kitchen. It returns the load in tons, BTU/h and kW, the nearest standard central size, and a check for a contractor's quote — all in your browser. For a room-by-room breakdown or heater sizing, use the BTU calculator instead.
Tons of AC per square foot
The quickest rule of thumb is around one ton of cooling per 400 to 600 square feet, tighter in hot climates and looser in mild ones. Because that spread is almost entirely about climate, this calculator starts from a climate-adjusted cooling rate in BTU per square foot rather than a single flat number.
The table below shows the editable default rate for each climate band and the floor area a single ton covers at that rate. Change your climate zone above and the base rate — and the recommended size — moves with it.
The cooling base rate rises with the climate, so a ton of air conditioning covers fewer square feet in a hot zone than a cold one. These are the editable defaults behind the calculator.
| Climate band | IECC zones | Cooling BTU/ft² | Approx. ft² per ton |
|---|---|---|---|
| Hot | 1, 2 | 23 | ≈ 522 |
| Warm | 3 | 21 | ≈ 571 |
| Moderate | 4 | 20 | ≈ 600 |
| Cool | 5 | 19 | ≈ 632 |
| Cold | 6, 7, 8 | 18 | ≈ 667 |
How the tonnage is calculated
The cooling load is built additively so every factor is a visible part of the total: a climate-adjusted base-area load, plus a ceiling-volume delta above 8 ft, plus sun and insulation percentages on the base, plus flat adders for extra occupants and a kitchen. Dividing the load by 12,000 gives tons.
Every figure here is built from a handful of transparent formulas — no black box.
Cooling load
cooling BTU/h = rate(zone) × area(ft²) + ceiling + sun + insulation + occupants + kitchen- The rate is the climate band's BTU/ft²: 20 at an 8 ft ceiling in a moderate zone, higher in hot zones.
- Ceiling adds base × (height ÷ 8 − 1) above 8 ft; standard 8 ft ceilings add nothing.
- Sun is base × ±10%; insulation is base × (−10% good … +15% poor).
- Occupants add 600 BTU/h each beyond two; a kitchen adds 4,000 BTU/h.
Tons and capacity
tons = BTU/h ÷ 12,000 · kW = BTU/h ÷ 3,412- One ton of cooling is 12,000 BTU/h; one kilowatt is 3,412 BTU/h.
- A 36,000 BTU/h load is 3.0 tons or about 10.5 kW.
Nearest standard size
unit = smallest central rung ≥ load (1, 1.5, 2 … 5 tons)- Central AC comes in half-ton steps; the tool snaps the load up to the next real size, never below it.
- It then flags how far above the load that size sits — a guard tighter than ACCA Manual S.
What the live diagram shows
The tool draws two figures for your home: a stacked contribution bar that splits the cooling load into its factors, and a tonnage ladder that snaps the load up to a real central size. Here they are for a 2,000 ft² whole home in a moderate climate.
- base area
- 40,000 BTU/h
- ceiling
- +5,000 BTU/h
- sun
- 0 BTU/h
- insulation
- −4,000 BTU/h
- occupants
- 0 BTU/h
- kitchen
- 0 BTU/h
Tons, BTU/h and kW
Air conditioners are rated three ways for the same capacity: tons, BTU per hour and kilowatts. One ton is 12,000 BTU/h, and one kilowatt is 3,412 BTU/h, so a 36,000 BTU/h load is 3.0 tons or about 10.5 kW. Use the reference table below to convert between them for any standard central size.
Every standard central-AC size in tons, with its cooling capacity in BTU/h and kW. Divide any cooling load by 12,000 to get tons, then round up to the next size here.
| Tons | BTU/h | kW |
|---|---|---|
| 1.0 | 12,000 | 3.52 |
| 1.5 | 18,000 | 5.28 |
| 2.0 | 24,000 | 7.03 |
| 2.5 | 30,000 | 8.79 |
| 3.0 | 36,000 | 10.55 |
| 3.5 | 42,000 | 12.31 |
| 4.0 | 48,000 | 14.07 |
| 5.0 | 60,000 | 17.58 |
Choosing a standard size
Central AC is sold in fixed capacities — 1, 1.5, 2, 2.5, 3, 3.5, 4 and 5 tons — so the calculator snaps your load up to the nearest available size, never below it, because undersizing is the worse failure on a design day. It then flags how far above the load that size sits, using a guard tighter than ACCA Manual S so you don't drift into a unit that is far too big.
What changes the size
Six things move the cooling load beyond raw floor area. Change any one in the calculator and the matching segment of the contribution bar and the tonnage update immediately, so you can see exactly what is driving the size.
| Factor | Effect on the load |
|---|---|
| Base area | The climate rate × floor area — 20 BTU/h per ft² at an 8 ft ceiling in a moderate zone is the starting point. |
| Climate zone | Sets the cooling BTU/ft²: 23 in hot zones down to 18 in cold ones. This is the lever a plain BTU calculator leaves flat. |
| Ceiling height | Above 8 ft, adds base × (height ÷ 8 − 1) for the extra volume to cool; standard 8 ft ceilings add nothing. |
| Sun exposure | A sunny, unshaded home adds 10% to the base; a well-shaded one subtracts 10%. |
| Insulation | Good insulation and tight windows subtract 10%; poor or older insulation adds 15%. |
| Occupants | Each person beyond two adds a flat 600 BTU/h of body heat. |
| Kitchen | A kitchen adds a flat 4,000 BTU/h for appliance and cooking heat. |
Oversizing and contractor quotes
Bigger is not better with air conditioning. An oversized unit reaches the thermostat setpoint quickly and shuts off before it has run long enough to pull humidity out of the air, so the house feels cold and clammy, the compressor wears from frequent starts, and efficiency drops. Contractors sometimes round up 'to be safe', which is how a home that needs 3 tons ends up with 4.
Enter a quoted tonnage in the quote-check panel and the tool compares it with your estimated load, flagging a quote that runs well above it. Treat a half-ton of headroom as normal and a full ton or more as a question worth asking.
When to get a Manual-J
This calculator is a rule-of-thumb estimate for a starting point, not a professional load calculation. A quote that lands within a half-ton of the number here is a reassuring sign; one that is a full ton or more above it is worth questioning.
A full ACCA Manual-J accounts for window area, glazing and orientation, wall and roof assemblies, measured air leakage, duct gains and losses, and your local design temperatures. Use this estimate to sanity-check a quote or narrow a size, then have an HVAC professional run a Manual-J and Manual-S selection before you buy equipment.
Worked examples
Two estimates at the default constants — the same 2,000 ft² home in a moderate and a hot climate. Your own numbers will differ once you set the zone, insulation and factors to your space.
Two estimates at the default constants. Only the climate zone changes between them, to show how much it moves the size.
Whole home — moderate climate
Cooling- Given
- 2,000 ft² total, 9 ft ceilings
- Zone 4 · moderate (20 BTU/ft²), neutral sun, 2 occupants, good insulation
- Formula
20 × 2,000 + ceiling + insulation- Steps
- Base: 20 × 2,000 = 40,000 BTU/h
- Ceiling: 40,000 × (9 ÷ 8 − 1) = +5,000
- Insulation (good −10%): 40,000 × −0.10 = −4,000
- Total: 40,000 + 5,000 − 4,000 = 41,000 BTU/h
- Result
- 41,000 BTU/h ≈ 3.4 tons ≈ 12.0 kW → snaps up to a 3.5-ton (42,000 BTU/h) central unit
Same home — hot climate
Cooling- Given
- 2,000 ft² total, 9 ft ceilings
- Zone 2 · hot (23 BTU/ft²), neutral sun, 2 occupants, good insulation
- Formula
23 × 2,000 + ceiling + insulation- Steps
- Base: 23 × 2,000 = 46,000 BTU/h
- Ceiling: 46,000 × (9 ÷ 8 − 1) = +5,750
- Insulation (good −10%): 46,000 × −0.10 = −4,600
- Total: 46,000 + 5,750 − 4,600 = 47,150 BTU/h
- Result
- 47,150 BTU/h ≈ 3.9 tons ≈ 13.8 kW → snaps up to a 4.0-ton (48,000 BTU/h) central unit — one size up for the same house
How to size a central AC with this calculator
- Enter your area and unitsPick metric or imperial, then enter your home's total conditioned floor area — the single biggest driver of the size.
- Set your climate zoneChoose your IECC climate zone. Hotter zones raise the cooling BTU/ft²; this is the lever a plain BTU calculator leaves flat.
- Adjust ceiling, sun, insulation and occupantsTune ceiling height, sun exposure, insulation quality, occupants and whether there is a kitchen, and watch the contribution bar show what drives the load.
- Read the tonnage and check a quoteRead the recommended size in tons, BTU/h and kW with its nearest standard central unit, and enter a contractor's quoted tonnage to check it for oversizing.
Frequently asked questions
What size air conditioner do I need?
As a starting point, size central AC at roughly one ton per 400–600 square feet, depending on your climate — closer to 400 ft² per ton in a hot zone and 600 in a mild one. Enter your home's area and climate zone above and the calculator returns the load in tons, BTU/h and kW and snaps it up to the nearest standard central size. It is guidance, not a Manual-J calculation.
How many tons of AC per square foot?
About one ton (12,000 BTU/h) per 500 square feet is a common rule of thumb at 20 BTU/h per ft², but the real figure depends on climate, ceiling height, sun, insulation and occupancy. This tool starts from a climate-adjusted BTU/ft² — 23 in hot zones down to 18 in cold ones — and adjusts for the rest, rather than using a single flat number.
Is a bigger air conditioner better?
No. An oversized AC short-cycles: it cools the air fast, shuts off before it removes much humidity, then restarts, leaving the home cold and clammy while wearing the compressor and wasting energy. Right-sizing — the smallest unit that still covers the peak load — is the goal, which is why the calculator snaps up only to the next standard size and warns when a quote runs well above your load.
Is this a Manual-J load calculation?
No. This is a rule-of-thumb estimate to give you a starting point and sanity-check a contractor's quote. A full ACCA Manual-J accounts for window area and orientation, air leakage, duct losses and local design temperatures. Verify with an HVAC professional before purchasing equipment.
Does it work in metric?
Yes. Switch units at the top and enter your area in square metres; the tool reads kW first, with BTU/h and tonnage alongside. Tonnage is shown in both systems, since air conditioners are sold by the ton worldwide.