What Size Air Conditioner Do I Need? BTU Sizing by Room Size

What size air conditioner do I need? Start with about 20 BTU per square foot of room area. A 500 sq ft room needs roughly 10,000 BTU. A 1,000 sq ft space needs about 20,000 BTU, which is 1.67 tons. Measure the room, multiply the square footage by 20, then adjust for ceiling height, sun, insulation and how the room gets used. Run the numbers with the BTU Calculator.

That is the short answer. The longer answer matters more, because the most common sizing mistake is not buying too small. It is buying too big.

How many BTU do I need per square foot?

About 20 BTU per square foot for cooling. That is the working rule of thumb, and it is where almost every sizing conversation starts. Multiply your room area by 20 and you have a baseline number in BTU per hour.

A BTU is a unit of heat. A cooling BTU rating tells you how much heat the unit can pull out of a space every hour. More square footage means more heat to move, so the number scales with area.

The rule works because it bakes in a set of assumptions: an 8 foot ceiling, average insulation, a normal amount of sun, a couple of people in the room. Change those assumptions and the number moves. We cover the adjustments further down.

If you want the reasoning behind the multiplier in more depth, read our breakdown of how many BTU per square foot you actually need.

What size AC do I need for my room size?

Here is the baseline table at 20 BTU per square foot. Find your room area and read across. Tons are shown because that is how larger equipment is sold.

• 150 sq ft, 3,000 BTU (0.25 ton)
• 250 sq ft, 5,000 BTU (0.42 ton)
• 300 sq ft, 6,000 BTU (0.50 ton)
• 400 sq ft, 8,000 BTU (0.67 ton)
• 500 sq ft, 10,000 BTU (0.83 ton)
• 700 sq ft, 14,000 BTU (1.17 ton)
• 1,000 sq ft, 20,000 BTU (1.67 ton)
• 1,200 sq ft, 24,000 BTU (2.00 ton)
• 1,500 sq ft, 30,000 BTU (2.50 ton)
• 2,000 sq ft, 40,000 BTU (3.33 ton)

Two of these come up constantly. If you are cooling a primary bedroom or a small apartment, see how many BTU you need for 500 square feet. If you are sizing for a whole small home, see what size air conditioner works for 1,000 square feet.

Before you use the table, get the area right. Length times width for a rectangle. For an L shape, split it into rectangles and add them up. Open plan spaces that share air with a hallway or stairwell count toward the load even if you were not planning to cool them. The Square Footage Calculator handles the odd shapes.

How many square feet does a 1 ton AC cover?

About 600 square feet at 20 BTU per square foot. One ton equals 12,000 BTU per hour, and 12,000 divided by 20 gives you 600. That is the conversion behind every tonnage chart you will see.

Tons are a holdover from the ice trade. One ton of cooling was the heat needed to melt a ton of ice in a day. It stuck. Window and portable units get labeled in BTU. Mini splits and central systems get labeled in tons. Same measurement, different packaging.

• 1 ton, 12,000 BTU, about 600 sq ft
• 1.5 ton, 18,000 BTU, about 900 sq ft
• 2 ton, 24,000 BTU, about 1,200 sq ft
• 3 ton, 36,000 BTU, about 1,800 sq ft
• 5 ton, 60,000 BTU, about 3,000 sq ft

To convert either direction: BTU divided by 12,000 gives tons, tons times 12,000 gives BTU. More on the math in how many BTU is a 1 ton AC.

What adjustments change the BTU number?

The 20 BTU rule assumes an average room. Yours probably is not. Apply these adjustments to the base number, and they stack. A sunny kitchen with high ceilings can land far above where the table put it.

• Ceiling over 8 ft, add 1,250 BTU per extra foot
• Very sunny room, add 1,000 BTU
• Heavily shaded room, subtract 1,000 BTU
• Very poor insulation, add 2,000 BTU
• Very good insulation, subtract 2,000 BTU
• Kitchen, add 4,000 BTU
• Each person over 2 regularly in the room, add 600 BTU

Here is how it plays out. Take a 500 sq ft room with a 10 foot ceiling, lots of afternoon sun and poor insulation. Base is 10,000 BTU. The two extra feet of ceiling add 2,500. Sun adds 1,000. Poor insulation adds 2,000. Total: 15,500 BTU, or about 1.29 tons.

That is a 55 percent jump over the table value. Same floor area. This is exactly why a square footage chart alone is not a sizing method. The BTU Calculator applies these adjustments for you so you are not stacking them by hand.

What happens if my AC is too big?

It short cycles, and the room ends up cold and clammy. This is the single most useful thing to understand about AC sizing, and it is the reason bigger is not better.

An air conditioner does two jobs. It drops the temperature, and it pulls moisture out of the air. The second job only happens while the unit is actually running. Air has to keep passing over the cold coil long enough for water to condense out and drain away.

An oversized unit hits the thermostat setpoint fast. Maybe ten minutes. Then it shuts off. It never runs long enough to do the dehumidifying half of the job. You get a room that reads 72 on the thermostat and still feels damp and sticky. People respond by turning the temperature down further, which makes it colder and clammier.

Short cycling costs you elsewhere too. Starting a compressor is the hardest thing it does. An oversized unit starts and stops all day, which is more wear on the compressor and worse efficiency than a right-sized unit running longer, steadier cycles. Rooms also cool unevenly, because the air never circulates long enough to mix.

Undersized has the opposite failure. The unit runs constantly, never reaches setpoint on the hottest days, and you paid for equipment that cannot do the job. Both directions are bad. The target is right-sized, not generous. We go deeper in what happens if your AC is too big.

So when a contractor suggests rounding up a full ton "just to be safe," ask why. Sometimes there is a real reason. Often it is a hedge that hands you a humidity problem.

Window unit, portable, mini split or central: does sizing change?

The BTU math is the same. What changes is how much slop the system tolerates and how much a mistake costs you.

Window and portable units

These cool one room. The rule of thumb works well here, because the load is contained and the failure mode is cheap to fix. Portables are the exception worth flagging: they vent hot air through a hose and pull room air to do it, so real world performance often trails the label. Sizing at or slightly above your adjusted number is reasonable for a portable.

Mini splits

One outdoor unit, one or more indoor heads. Sizing is per zone, so you calculate each room and add them for the outdoor unit. Mini splits handle part loads better than most equipment, but they are not immune to oversizing. Garages are their own animal because of insulation and door openings, covered in what size mini split for a garage.

Central air

This is where the rule of thumb stops being enough. A central system serves the whole house through ducts, and duct losses alone can move the number meaningfully. Use the table to sanity-check a quote. Do not use it to pick the equipment.

When do I need a Manual J load calculation?

Any time you are buying a whole-home system. Manual J is the actual load calculation, done by an HVAC professional, and it is what proper sizing means. The 20 BTU rule is a rough estimate that gets you in the neighborhood. Manual J gets you the address.

A Manual J accounts for things the rule of thumb ignores entirely: your climate zone, the number and type and orientation of your windows, how well the house is air sealed, duct location and losses, wall and attic construction, internal heat from appliances and occupants. Two identical 1,500 sq ft houses in different climates with different window placement can need meaningfully different equipment.

Rule of thumb is right for: picking a window unit, choosing a portable, sizing a single mini split head, checking whether a quote is in a sane range. If a contractor quotes 5 tons for a 1,200 sq ft house, the table tells you to ask questions.

Rule of thumb is wrong for: specifying a central system, replacing a furnace and coil, anything you are permitting. A contractor who sizes your whole house off square footage alone is guessing. Ask for the Manual J.

What about heating BTU?

Heating uses a different multiplier: roughly 30 to 40 BTU per square foot, with about 35 as a reasonable midpoint. A 1,000 sq ft space needs about 35,000 BTU, in a range of 30,000 to 40,000.

The range is wide because climate drives heating more than it drives cooling. A mild coastal winter and a hard continental one are not the same problem. That is a big part of why the heating rule of thumb is even rougher than the cooling one, and why Manual J matters more as you go north.

Bottom line

Multiply your square footage by 20 for a cooling baseline. Adjust for ceiling height, sun, insulation, kitchen use and extra people. Convert to tons by dividing by 12,000 if you are shopping equipment rated that way. Then remember the part most people skip: an oversized AC leaves the room cold and damp and wears itself out doing it. Right-sized beats big.

Use these numbers to pick a window or portable unit, or to sanity-check a quote before you sign it. For a whole-home system, get a Manual J. Start with your room measurements and the BTU Calculator.