Find the model number
The nameplate is usually on a side panel. A number such as 36 in the model code often means 36,000 BTU, or 3 tons. Compare it with the load the calculator gives.
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AC size and BTUAC size calculator: BTU and tons
How big an air conditioner do you need? Size a room unit in BTU from the ENERGY STAR chart, or a central system in tons for the whole home, and see where common unit sizes fall against your load.
Reviewed 8 October 2026
Written and maintained byMantu Kumar
Evidence review8 October 2026
Calculation rulesRead the methodology
Quality standardEditorial policy
Key facts at a glance
- 1 ton = 12,000 BTU per hour of cooling.
- A 300–350 sq ft room needs about 8,000 BTU on the ENERGY STAR chart.
- Add 10% for a very sunny room; take off 10% for heavy shade.
- Add 600 BTU per person over two and 4,000 BTU for a kitchen.
- Many homes need about 500–700 sq ft per ton.
- Oversized units short-cycle; ask for a Manual J before buying.
Sources: ENERGY STAR, ACCA and the Department of Energy, listed at the end of this page.
Air conditioner size calculator
No email · nothing sentAt the outdoor unit
Read the size off the equipment you already have
The outdoor condenser of a central system shows its size on the nameplate. The photograph shows a typical unit on a pad beside the house.
The three detail views below enlarge parts of the same photograph. Use them with the checks underneath.
Check the electrical and refrigerant lines
The disconnect box and the copper line set carry power and refrigerant to the indoor coil. A larger new unit may need different wiring or lines.
Keep the airflow clear
Shrubs and walls too close to the coil reduce capacity. Leave the clearance the manufacturer specifies, usually 1 to 2 feet around the sides.
Before you buy a new air conditioner, confirm these
- Load calculation
- A Manual J for central systems, or the ENERGY STAR chart for a room unit
- Existing size
- The tonnage on the old unit, and whether rooms were too hot or too clammy
- Ductwork
- Whether the ducts can carry the airflow the new size needs
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On this page
The ENERGY STAR room sizing chart
For a single room, ENERGY STAR publishes a simple chart that matches floor area to cooling capacity in BTU per hour. It is the starting point for window, portable and wall units, and the calculator uses it directly.
| Room area (sq ft) | Cooling capacity (BTU per hour) |
|---|---|
| 100 to 150 | 5,000 |
| 150 to 250 | 6,000 |
| 250 to 300 | 7,000 |
| 300 to 350 | 8,000 |
| 350 to 400 | 9,000 |
| 400 to 450 | 10,000 |
| 450 to 550 | 12,000 |
| 550 to 700 | 14,000 |
| 700 to 1,000 | 18,000 |
| 1,000 to 1,200 | 21,000 |
| 1,200 to 1,400 | 23,000 |
| 1,400 to 1,500 | 24,000 |
| 1,500 to 2,000 | 30,000 |
| 2,000 to 2,500 | 34,000 |
Adjusting for sun, people and the kitchen
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two people: add 600 BTU for each additional person.
- Kitchen: add 4,000 BTU.
High ceilings and a top-floor room under a hot roof are not in the ENERGY STAR chart. The calculator adds them as separate allowances so you can see their effect and leave them out if you prefer.
Whole-home sizing in tons
Central air conditioners and heat pumps are sold in tons; one ton removes 12,000 BTU of heat per hour. Residential systems run from about 1.5 to 5 tons. Whole-home mode multiplies the floor area by a cooling load per square foot for your climate, then adjusts for insulation, sun, ceiling height and occupants. Many homes land between 500 and 700 square feet per ton; hot climates and older, leaky homes need more capacity, efficient homes less.
Why a bigger unit is not better
An oversized air conditioner cools the air quickly and shuts off before it has run long enough to pull moisture out. The result is a cold, clammy room, more on-off cycles, more wear and uneven temperatures between rooms. Pick the nearest size at or just above the load; the green band on the result chart shows the sensible range.
Ask for a Manual J before you buy
A rule of thumb is a planning check. Contractors size central systems with the ACCA Manual J load calculation, which looks at walls, windows, insulation, orientation and air leakage room by room, and Manual S to choose equipment. A new system sized by Manual J is often smaller than the one it replaces. Price the system you need with the HVAC replacement cost calculator.
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Questions
How many BTU do I need per square foot?
For a single room, the ENERGY STAR chart works out at roughly 20 BTU per square foot for mid-sized rooms, for example 8,000 BTU for 300 to 350 sq ft. Sun, people and a kitchen then adjust it.
What size AC do I need for a 2,000 sq ft house?
In a mixed climate with average insulation, this calculator gives about 3 to 3.5 tons. Hot climates can need 4 tons or more, and well-insulated homes less. A Manual J load calculation gives the real figure.
How many square feet does a ton of AC cool?
Commonly about 500 to 700 square feet per ton, depending on climate, insulation, windows and ceiling height.
How many BTU is one ton of air conditioning?
One ton equals 12,000 BTU per hour. A 3-ton system is 36,000 BTU per hour.
Is it bad to oversize an air conditioner?
Yes. An oversized unit short-cycles, removes less humidity, leaves rooms clammy and wears out sooner. Choose the nearest size at or just above the calculated load.
What size window AC do I need for a bedroom?
A 150 to 250 sq ft bedroom needs about 6,000 BTU on the ENERGY STAR chart; a 300 to 350 sq ft room about 8,000 BTU. Add 10% for a very sunny room.
What is a Manual J calculation?
The ACCA standard method for working out a home’s heating and cooling load room by room. Ask a contractor for one before buying a central system.
Related
- HVAC replacement cost calculator — price the system you need
- Repair or replace calculator — test whether the old unit is worth fixing
- Solar cost calculator — offset cooling costs
- Window replacement calculator — cut heat gain through glass
How to use this result
This is a planning estimate, not an ACCA Manual J load calculation. Windows, insulation, air leakage and duct condition change the real load. Use it to check a quote, not to replace one.
Evidence trail
Reviewed 8 October 2026Sources used to check this tool
Room sizing follows the ENERGY STAR chart and adjustments; whole-home sizing is a documented rule of thumb, with Manual J named as the professional method.
Room AC sizingENERGY STAR room air conditioners
Load calculationACCA Manual J
Central airEnergy Saver: central air conditioning
Room unitsEnergy Saver: room air conditioners
These organizations do not endorse ReplaceCost. The written policy, program rules, contractor scope and applicable law control the real result. See the full methodology for the source hierarchy and update process.
Air conditioner sizing
Match the capacity to the load, not the old unit
Too small never catches up; too big leaves the house cold and damp.
BTU by room size
The ENERGY STAR chart sets the starting capacity.
Tons for the whole home
One ton is 12,000 BTU per hour.
Oversizing
Short cycles remove less humidity.
Manual J
The professional room-by-room load method.
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