BTU Calculator – What Size Air Conditioner Do I Need?

This BTU calculator tells you what size air conditioner a room needs: enter the floor area in square feet, pick sun exposure, occupants and whether it is a kitchen, and you instantly get the cooling capacity in BTU per hour (and kW) plus the next standard unit size. It follows the ENERGY STAR sizing chart.

Example: Room area 300 sq ft · Sun exposure: Average · People who regularly use the room 2 → Cooling capacity needed: 8,000 BTU/h ≈ 2.34 kW. Source: ENERGY STAR (U.S. EPA) – Room Air Conditioners: sizing chart (sq ft → BTU/h) and adjustments for shade, sun, occupants and kitchen. Updated: 30 September 2026.

Room area

sq ft

Length × width in feet; for an open floor plan use the whole area

Sun exposure

People who regularly use the room

The unit will cool a kitchen

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Result

Cooling capacity needed: 8,000 BTU/h ≈ 2.34 kW
Unit size to buy: at least 8,000 BTU/h (2.34 kW)
ENERGY STAR sizing chart: Area BTU/h kW
100–150 ft² 5,000 1.47
150–250 ft² 6,000 1.76
250–300 ft² 7,000 2.05
300–350 ft² 8,000 2.34
350–400 ft² 9,000 2.64
400–450 ft² 10,000 2.93
450–550 ft² 12,000 3.52
550–700 ft² 14,000 4.10
700–1,000 ft² 18,000 5.28
1,000–1,200 ft² 21,000 6.15
1,200–1,400 ft² 23,000 6.74
1,400–1,500 ft² 24,000 7.03
1,500–2,000 ft² 30,000 8.79
2,000–2,500 ft² 34,000 9.96
Working: 300 ft² → chart value 8,000 BTU/h; sun/shade + 0; people + 0; kitchen + 0 = 8,000 BTU/h ≈ 2.34 kW.

Area	BTU/h	kW
100–150 ft²	5,000	1.47
150–250 ft²	6,000	1.76
250–300 ft²	7,000	2.05
300–350 ft²	8,000	2.34
350–400 ft²	9,000	2.64
400–450 ft²	10,000	2.93
450–550 ft²	12,000	3.52
550–700 ft²	14,000	4.10
700–1,000 ft²	18,000	5.28
1,000–1,200 ft²	21,000	6.15
1,200–1,400 ft²	23,000	6.74
1,400–1,500 ft²	24,000	7.03
1,500–2,000 ft²	30,000	8.79
2,000–2,500 ft²	34,000	9.96

How it is calculated

How the BTU calculator works

The calculator uses the room air conditioner sizing chart published by ENERGY STAR (U.S. EPA). It assigns a cooling capacity to each range of floor area – for example 150 up to 250 sq ft → 6,000 BTU per hour. The same source then gives four adjustments:

heavily shaded room: reduce capacity by 10%,
very sunny room: increase capacity by 10%,
more than two people: add 600 BTU/h for each additional person,
kitchen: add 4,000 BTU/h.

Example: a sunny 12 × 22 ft living room is 264 sq ft, which falls in the 250–300 sq ft row with 7,000 BTU/h. Plus 10% for sun gives 7,700 BTU/h, and four people add 1,200 BTU/h – 8,900 BTU/h in total. A 9,000 BTU/h window unit is the right size.

BTU and kW

One BTU per hour equals 0.293 watts (NIST), so 12,000 BTU/h – often called “one ton” of cooling – is about 3.5 kW. The BTU rating describes the heat the unit removes, not the electricity it uses; the power draw is printed on the label in watts.

Why not just buy a bigger unit?

ENERGY STAR points out that an oversized air conditioner cools the room quickly but shuts off before it has removed the moisture, so the room feels cold and clammy. An undersized one runs constantly and cannot keep up on hot days. That is why the calculator suggests the smallest standard size that covers the load.

Limits

The chart is a rule of thumb for single rooms with typical ceiling heights. Rooms under an uninsulated roof, with large glass areas or with heat-producing equipment need more. For a whole house or a ducted or multi-zone mini-split system, an HVAC contractor calculates the load room by room. To estimate what the unit costs to run, use the electricity cost calculator; the dew point calculator shows when walls get condensation, and the HP to kW converter handles other power units.

Frequently asked questions

How many BTU do I need per square foot?

Roughly 20 to 40 BTU/h per square foot, falling as rooms get larger: 300 sq ft needs 8,000 BTU/h, 500 sq ft 12,000 BTU/h and 1,000 sq ft 21,000 BTU/h according to the ENERGY STAR chart, before adjustments.

What size air conditioner do I need for a 12×12 room?

A 12 × 12 ft room is 144 sq ft, which is in the 100–150 sq ft row: 5,000 BTU/h. If it gets a lot of sun, add 10% and choose a 6,000 BTU/h unit.

How many BTU is a ton of air conditioning?

One ton of cooling is 12,000 BTU per hour, about 3.5 kW. A 2-ton system delivers 24,000 BTU/h.

What is the formula for BTU sizing?

Take the chart value for the floor area, then subtract 10% for heavy shade or add 10% for strong sun, add 600 BTU/h per person beyond two and 4,000 BTU/h for a kitchen – the adjustments recommended by ENERGY STAR.

Does ceiling height matter?

The chart assumes normal ceiling heights and works from floor area. With high or vaulted ceilings, go one size up or have the load calculated.

Sources and legal basis

As of: 30 September 2026