To size an air conditioner for a room, measure the room's area in square feet, then match it to a cooling capacity in British Thermal Units per hour (BTU/hr). A room of 100 to 150 square feet needs about 5,000 BTU; a room of 300 to 350 square feet needs 8,000 BTU; a room of 450 to 550 square feet needs 12,000 BTU. These figures come from the ENERGY STAR room air conditioner sizing chart, calculated for an 8-foot ceiling. This AC BTU calculator multiplies the room's length by width to find the area, applies the ENERGY STAR chart, adjusts for sun, ceiling height, occupants, and kitchen use, then converts the result to tons, where 12,000 BTU per hour equals 1 ton.
This calculator estimates cooling capacity from the ENERGY STAR sizing chart. A precise cooling load uses a professional Manual J calculation, which also accounts for insulation, windows, and local climate. Load-calculation procedures such as Manual J sit alongside ASHRAE HVAC guidance. The ENERGY STAR room chart does not replace either method.
How to calculate AC size for a room
To calculate the air conditioner size for a room, follow four steps. First, multiply the room's length by its width to find the area in square feet. Second, match the area to the ENERGY STAR bracket that contains it. Third, adjust the base figure for sun, ceiling height, occupants, and kitchen use. Fourth, divide the final BTU by 12,000 to find the size in tons. A 12-foot by 12-foot room is 144 square feet and needs 5,000 BTU per hour, or 0.42 tons.
Calculate air conditioner size for room work starts with tape-measure values, not a guess at square footage. Calculate room size for AC by recording length and width in the same unit (feet). BTU for room size then comes from the ENERGY STAR bracket that contains that area, not from a single multiplier.
Measure length and width
Measure the interior length and the interior width of the room in feet. Multiply those two numbers to get area in square feet. A 12 ft by 12 ft room is 144 sq ft. A 20 ft by 20 ft room is 400 sq ft. A 20 ft by 25 ft room is 500 sq ft. Irregular rooms should be split into rectangles, with each rectangle multiplied and then added, before the ENERGY STAR chart is used.
Match the ENERGY STAR bracket
AC size by room size uses the ENERGY STAR room air conditioner chart for an 8-foot ceiling. An area on a bracket boundary uses the higher bracket: 300 sq ft uses 8,000 BTU, and 400 sq ft uses 10,000 BTU. Rooms under 100 sq ft use a 5,000 BTU floor. Rooms over 2,500 sq ft need a professional HVAC consult rather than this chart.
Apply sun, ceiling, occupants, and kitchen
The ENERGY STAR chart applies four adjustments on top of the base figure. Reduce the capacity by 10% for a heavily shaded room. Increase it by 10% for a very sunny room. Add 600 BTU for each occupant beyond two who regularly uses the room. Add 4,000 BTU if the unit cools a kitchen. Rooms with ceilings above 8 feet need proportionally more capacity, scaled by ceiling height divided by 8.
Convert the result to tons
Divide the final BTU per hour by 12,000 to get AC tonnage by room size. 5,000 BTU per hour is 0.42 tons. 12,000 BTU per hour is 1.0 ton. 24,000 BTU per hour is 2 tons. Window air conditioners are usually sold in BTU. Central air conditioner systems are usually sold in tons. Mini-split heads are sold in BTU and should be rounded up to the next listed size.
Room size to BTU chart
The table below lists the cooling capacity the ENERGY STAR chart recommends for each room size, measured for an 8-foot ceiling. AC size by room size starts at 5,000 BTU for 100 to 150 square feet and reaches 12,000 BTU for 450 to 550 square feet. Rooms under 100 square feet use the 5,000 BTU floor. Rooms over 2,500 square feet need a professional consult.
| Area to be cooled (sq ft) | Capacity needed (BTU/hr) |
|---|---|
| 100-150 | 5,000 |
| 150-250 | 6,000 |
| 250-300 | 7,000 |
| 300-350 | 8,000 |
| 350-400 | 9,000 |
| 400-450 | 10,000 |
| 450-550 | 12,000 |
| 550-700 | 14,000 |
| 700-1,000 | 18,000 |
| 1,000-1,200 | 21,000 |
| 1,200-1,400 | 23,000 |
| 1,400-1,500 | 24,000 |
| 1,500-2,000 | 30,000 |
| 2,000-2,500 | 34,000 |
What size AC for 500 sq ft is the 450–550 bracket: 12,000 BTU per hour (1 ton). What size air conditioner a 400 sq ft room needs is the 400–450 bracket: 10,000 BTU per hour. Do not use a flat 20 BTU per square foot in place of this table for small rooms; that shortcut undersizes a 12×12 room.
Read a boundary value
Each row is lower-inclusive and upper-exclusive at the shared boundary. 150 sq ft uses the 150–250 row (6,000 BTU), not the 100–150 row. 250 sq ft uses 7,000 BTU. 350 sq ft uses 9,000 BTU. 550 sq ft uses 14,000 BTU. Enter the measured area above to avoid picking the lower row by mistake.
When the chart stops
The ENERGY STAR room air conditioner chart covers 100 to 2,500 square feet. Whole-house central air conditioner sizing is a cooling load problem for insulation, windows, orientation, and climate. Calculate aircon size for room tools on this page stay on the room chart. A space larger than 2,500 sq ft should use Manual J rather than another row of this table.
Adjustments that change the BTU
The ENERGY STAR chart applies four adjustments on top of the base figure. Reduce the capacity by 10% for a heavily shaded room. Increase it by 10% for a very sunny room. Add 600 BTU for each occupant beyond two who regularly uses the room. Add 4,000 BTU if the unit cools a kitchen. Rooms with ceilings above 8 feet need proportionally more capacity, scaled by the ceiling height divided by 8.
Apply the adjustments after the bracket, not before. Find the base BTU from area, multiply by ceiling height ÷ 8, multiply by the sun factor (0.9, 1.0, or 1.1), then add occupant and kitchen BTU. The recommended figure is rounded up to the nearest 500 BTU per hour.
Sun and shade
A very sunny 12×12 room starts at 5,000 BTU and then uses × 1.1, which equals 5,500 BTU and rounds to a 6,000 BTU unit. A heavily shaded room of the same size uses × 0.9, which equals 4,500 BTU. Keep Normal (× 1.0) when the room is neither a west-facing glass box nor a fully shaded interior.
Occupants and kitchen heat
Two occupants are already built into the ENERGY STAR base. Each extra person who regularly uses the room adds 600 BTU. Four occupants in a 300 sq ft room: 8,000 + 1,200 = 9,200 BTU. A kitchen adds 4,000 BTU because cooking equipment raises the cooling load. Kitchen 300 sq ft, two occupants, normal sun: 8,000 + 4,000 = 12,000 BTU.
Ceiling height above 8 feet
The ENERGY STAR figures assume an 8-foot ceiling. A 10-foot ceiling uses the factor 10 ÷ 8 = 1.25. A 144 sq ft room at 10 feet: 5,000 × 1.25 = 6,250 BTU, which rounds up to 6,500 BTU recommended. Because 6,250 exceeds 6,000, the next window unit is 8,000 BTU. Vaulted rooms should use the average height, not the peak alone.
How many BTUs for a 12×12 room
A 12-foot by 12-foot room measures 144 square feet, which falls in the 100-to-150-square-foot bracket and needs 5,000 BTU per hour. A very sunny 12×12 room needs about 5,500 BTU, which rounds up to a 6,000 BTU unit. The smallest mini-split head sold is 6,000 BTU, so a mini-split for a 12×12 room is 6,000 BTU.
How many BTU for a 12x12 room is the most common sizing question on this page because 12×12 is a standard bedroom. 5,000 BTU per hour equals 0.42 tons. A flat 20 BTU per square foot would have suggested 2,880 BTU, which is below every window unit sold and below the ENERGY STAR floor.
Window unit versus mini-split
Nearest window unit sizes on this tool are 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, and 18,000 BTU. A normal 12×12 room maps to 5,000 BTU. Mini split size for room heads starts at 6,000 BTU, then 9,000, 12,000, 18,000, 24,000, and 36,000 BTU. Round the load up; do not install a smaller head than the load.
A 20×20 room compared
A 20-foot by 20-foot room is 400 square feet and needs about 9,000 to 10,000 BTU per hour, based on the ENERGY STAR chart. The 400 sq ft boundary uses 10,000 BTU. That is double the 12×12 result and still well below 1 ton. Use the same four-step method for every rectangle rather than scaling the 12×12 answer by eye.
Convert BTU to tons
To convert BTU per hour to tons of cooling, divide by 12,000, because one ton of cooling equals 12,000 BTU per hour. For example, a 24,000 BTU air conditioner is a 2-ton unit (24,000 ÷ 12,000). Window air conditioners are usually sold in BTU, and central systems are usually sold in tons.
AC tonnage by room size is the same cooling capacity as BTU, written in a different unit. 12,000 BTU = 1 ton. 18,000 BTU = 1.5 tons. 36,000 BTU = 3 tons. Convert BTU to tons whenever a central air conditioner quote lists tons and the room chart listed BTU.
Worked conversions
5,000 ÷ 12,000 = 0.42 tons (12×12 room). 8,000 ÷ 12,000 = 0.67 tons (300–350 sq ft). 10,000 ÷ 12,000 = 0.83 tons (400–450 sq ft). 12,000 ÷ 12,000 = 1.00 ton (450–550 sq ft, including 500 sq ft). 30,000 ÷ 12,000 = 2.50 tons (1,500–2,000 sq ft).
Which label stores use
A window air conditioner carton prints BTU/hr. A mini-split indoor head prints BTU/hr. A central air conditioner outdoor unit prints tons. Calculate aircon size for room results in both labels so the same cooling capacity can be matched to the product you are buying.
BTU vs tons vs SEER
A BTU (British Thermal Unit) measures cooling capacity per hour; a higher BTU rating cools a larger room. A ton measures the same capacity in a different unit, where 1 ton equals 12,000 BTU per hour. SEER (Seasonal Energy Efficiency Ratio) and EER (Energy Efficiency Ratio) measure efficiency, not capacity; a higher SEER uses less electricity for the same cooling. Size the unit by BTU or tons, then compare models by SEER or EER.
What size air conditioner do I need for a room is a capacity question (BTU or tons). How much electricity the unit uses is an efficiency question (SEER or EER). Mixing the two labels is the usual shopping error: a 12,000 BTU unit with a high SEER still cools 1 ton of load, not more.
Capacity versus efficiency
Cooling capacity is the heat the unit can remove, stated in BTU per hour or tons. Efficiency is how much cooling you get per watt. EER is a snapshot at a set outdoor temperature. SEER is a seasonal average. Raise SEER to cut running cost. Raise BTU only when the room load requires it.
Pick capacity first
Match ENERGY STAR BTU for room size first. Then compare window air conditioner, mini-split, or central air conditioner models that meet that capacity and list a SEER or EER that fits the electric bill target. An efficient oversized unit still short-cycles. An inefficient correctly sized unit still cools the load.
Why bigger is not better
An oversized air conditioner does not cool a room better. It reaches the set temperature quickly and shuts off before it removes humidity, which leaves the room cold and damp. The frequent starting and stopping, called short-cycling, wastes electricity and wears the compressor. Size the unit to the room's load rather than above it.
Cooling load is the heat that must be removed to hold the setpoint. Extra BTU above that load shortens run time. Moisture removal needs a longer run. That is why a correctly sized 5,000 BTU window unit in a 12×12 room dehumidifies better than a 12,000 BTU unit in the same room.
Short-cycling
Short-cycling is the on/off pattern of an oversized compressor. Each start draws a surge of current. Repeated starts raise wear and can trip a breaker on a shared bedroom circuit. If two standard sizes sit close to the result, take the size that meets the load. Do not jump a full extra ton for margin.
When to stop using the room chart
Open floor plans, large west glass, poor insulation, or a whole dwelling need a Manual J cooling load, which also accounts for insulation, windows, and local climate. ASHRAE publishes related HVAC guidance used by that professional process. The ENERGY STAR chart remains the right tool for a single room window air conditioner or mini-split head.
AC BTU calculator summary
What size air conditioner for a room starts with square footage, then an ENERGY STAR BTU bracket, then sun, ceiling, occupant, and kitchen adjustments. BTU states cooling capacity per hour. One ton equals 12,000 BTU per hour. Size by BTU or tons, then compare SEER or EER for efficiency.
Keep the worked anchors: a 12×12 room (144 sq ft) needs 5,000 BTU per hour (0.42 tons); 300–350 sq ft needs 8,000 BTU; 450–550 sq ft, including 500 sq ft, needs 12,000 BTU (1 ton). Round the load up to the next window unit or mini-split head. Use Manual J, not this chart, for a whole-house central air conditioner cooling load.