AC Size Calculator (BTU & Tons)
Find what size air conditioner a room needs in BTU, tons and kW. Advanced adds insulation, top floor, windows, a kitchen, heat from gadgets, the kind of AC, heating and running cost settings.
Hot is like Indian summers; very hot is like Gulf summers.
Good: insulated walls and roof, double glazing. Poor: thin walls, no roof insulation, single glass, draughts.
Windows: one normal window (about 3 × 5 ft or 1.4 m²) is already counted; each extra one adds heat. Other heat: add the watts of things that run while you cool the room, like a TV, computer or lights.
Find it on your bill as the unit rate.
Cost uses the AC size picked below. More detail on appliance costs: electricity cost calculator.
How your AC size is worked out
BTU = area × BTU per sq ft for your climate, adjusted for the room. 12,000 BTU = 1 ton.
- 1
Floor area
12 ft × 15 ft= 180 sq ft
- 2
Climate and height
180 × 45 BTU× 10 ÷ 8 ft height= 10,125 BTU/h
- 3
Room extras
no extras= 10,125 BTU/h
- 4
AC size
10,125 ÷ 12,000= 0.84 ton→ 1 ton AC
Read more: the formula, a worked example, the Energy Star chart, sizes and running cost
This is a quick rule of thumb used by AC sellers and online sizing charts, not a full heat-load survey (such as Manual J). It starts with the floor area and a typical number of BTU per square foot for your climate, then adjusts for the room. All the figures here are typical, so treat the result as a guide and ask an installer to check big or unusual rooms.
The formula
area (sq ft) = length × width (1 m² = 10.764 sq ft)
BTU/h = area × BTU per sq ft for your climate × ceiling height ÷ 8 ft
× sun (shaded 0.9, some 1, lots 1.1) × insulation (good 0.8, average 1, poor 1.25) × top floor 1.15
+ 1,000 × sun for each window after the first
+ 600 for each person after two + 4,000 for a kitchen
+ watts of other heat × 3.412
then + safety margin %
tons = BTU/h ÷ 12,000; kW = BTU/h ÷ 3,412
AC to buy = the smallest standard size at or above the BTU/h
running watts = AC BTU ÷ (COP × 3.412) or ÷ EER
cost = watts ÷ 1000 × hours × on-time × price per kWh
Worked example
A room 12 ft × 15 ft with a 10 ft ceiling in a hot climate, some sun, two people, average insulation, not on the top floor: 12 × 15 = 180 sq ft. 180 × 45 BTU = 8,100 BTU/h. The ceiling is 10 ft, not 8 ft, so 8,100 × 10 ÷ 8 = 10,125 BTU/h. There are no extras to add. 10,125 ÷ 12,000 = 0.84 ton, or 10,125 ÷ 3,412 = 2.97 kW. The smallest standard split AC that covers it is 1 ton (12,000 BTU). Running it at a COP of 3.5 draws 12,000 ÷ (3.5 × 3.412) = 1,005 W. For 8 hours a day with the compressor on 60% of the time, that is 1.005 × 8 × 0.6 = 4.82 kWh a day. At $0.15 per kWh that costs $0.72 a day, $21.70 a month of 30 days and $130.22 for a 6-month season.
BTU per square foot by climate (typical)
| Climate | Hottest days | BTU per sq ft | 1 ton covers (8 ft ceiling) |
|---|---|---|---|
| Cool summers | highs below 27 °C / 80 °F | 18 | 667 sq ft (62 m²) |
| Mild | highs about 27–32 °C / 80–90 °F | 20 | 600 sq ft (56 m²) |
| Warm | highs about 32–37 °C / 90–99 °F | 30 | 400 sq ft (37 m²) |
| Hot | highs about 38–44 °C / 100–111 °F | 45 | 267 sq ft (25 m²) |
| Very hot | highs 45 °C / 113 °F and above | 60 | 200 sq ft (19 m²) |
The per-square-foot figures are typical rules of thumb, not a standard. Hotter places need more because the gap between outside and inside is bigger and ACs give less cooling when it is very hot outside. A top floor, west-facing glass or poor insulation needs more again.
Standard AC sizes
| Tons | BTU/h | kW of cooling |
|---|---|---|
| 0.75 ton | 9,000 | 2.64 |
| 1 ton | 12,000 | 3.52 |
| 1.5 ton | 18,000 | 5.28 |
| 2 ton | 24,000 | 7.03 |
| 2.5 ton | 30,000 | 8.79 |
| 3 ton | 36,000 | 10.55 |
Split ACs are usually sold in tons (0.75, 1, 1.5, 2, 2.5, 3 ton). Window and portable ACs are often sold by BTU (5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 24,000). Central systems come in half-ton steps from 1.5 to 5 tons. The cooling capacity on the label is what counts, not the size of the box.
US Energy Star room AC chart
The US Energy Star programme publishes a simple chart for room ACs, made for typical US weather. Use it as a second opinion: it is lower than our hot and very hot settings. Adjustments: heavily shaded −10%, very sunny +10%, +600 BTU for each person after two, +4,000 BTU for a kitchen.
| Floor area | BTU/h |
|---|---|
| 100–150 sq ft | 5,000 |
| 150–250 sq ft | 6,000 |
| 250–300 sq ft | 7,000 |
| 300–350 sq ft | 8,000 |
| 350–400 sq ft | 9,000 |
| 400–450 sq ft | 10,000 |
| 450–550 sq ft | 12,000 |
| 550–700 sq ft | 14,000 |
| 700–1,000 sq ft | 18,000 |
| 1,000–1,200 sq ft | 21,000 |
| 1,200–1,400 sq ft | 23,000 |
| 1,400–1,500 sq ft | 24,000 |
| 1,500–2,000 sq ft | 30,000 |
| 2,000–2,500 sq ft | 34,000 |
Source: energystar.gov room air conditioners, checked on 11 October 2026 (chart rows and the shade, sun, people and kitchen adjustments).
Heating
Switch to Heating in Advanced to size a heater or a heat-pump AC. It uses typical figures of 25 BTU per sq ft for mild winters, 35 for cool, 45 for cold and 55 for very cold winters, with the same height, insulation, top floor and window adjustments. Sun, people and a kitchen are left out because they help warm the room. Divide BTU/h by 3,412 for the heater's kW: a 2 kW electric heater gives about 6,824 BTU/h. A heat-pump AC gives less heat when it is very cold outside, so check its heating capacity at low temperatures.
Efficiency labels
- COP, EER in W/W and ISEER (India): cooling watts out for each watt in. Multiply by 3.412 to get BTU/Wh.
- EER and SEER (US): BTU of cooling for each watt-hour. SEER is a season average, so real use often matches it better than EER.
- India star rating: this page uses a typical ISEER for each star (1 star 3.4, 2 star 3.75, 3 star 4.2, 4 star 4.7, 5 star 5.2). The bands change every few years, so the ISEER printed on your label is better. Source: BEE India, not yet checked.
- Compressor on-time: once the room is cool, the compressor slows down or switches off. 60% is a typical guess; an inverter AC in a well-sized room may be lower.
Tips
- Don't oversize: a too-big AC switches on and off often and leaves the room damp.
- Shade sunny windows and insulate the roof: it can cut the size you need.
- Open-plan spaces count as one room: add up the whole floor area.
- For a whole house, many rooms or unusual buildings, ask an installer for a proper load calculation.
Common questions
How many BTU do I need for my room?
Multiply the floor area in square feet by a figure for your climate: about 20 BTU per sq ft in mild weather and about 45 in hot summers. Then adjust for ceiling height, sun, people and a kitchen. A 12 × 15 ft room with a 10 ft ceiling in a hot climate needs 180 × 45 × 10 ÷ 8 = 10,125 BTU/h, so a 1 ton AC.
How many square feet does a 1 ton AC cool?
One ton is 12,000 BTU/h. With an 8 ft ceiling, that covers about 600 sq ft at 20 BTU per sq ft in mild weather, about 267 sq ft at 45 in hot summers and about 200 sq ft at 60 in very hot summers. A top floor, strong sun or poor insulation means less.
What does a ton mean for an air conditioner?
A ton is a measure of cooling power, not weight. One ton is 12,000 BTU per hour, or about 3.52 kW of cooling. So a 1.5 ton AC gives 18,000 BTU/h and a 2 ton AC gives 24,000 BTU/h. Window and portable ACs are often sold by BTU instead.
Is a bigger AC better?
Not usually. An AC that is too big cools the room fast and then switches off, so it removes less moisture and the room can feel cold and damp. It also costs more to buy. Pick the smallest standard size that covers your need. An inverter AC copes better with a range of loads.
How much does a 1 ton AC cost to run?
A 1 ton AC with an efficiency (COP or ISEER) of 3.5 draws about 1,005 W at full power. Run 8 hours a day with the compressor on 60% of the time, it uses about 4.82 kWh a day. Multiply by your price per kWh: at 0.15 that is about 0.72 a day.
What size heater do I need for a room?
Heating needs about 25 BTU per sq ft where winters are mild and about 55 where they are very cold, at an 8 ft ceiling. A 180 sq ft top-floor room in a cold winter needs 180 × 45 × 1.15 = 9,315 BTU/h, which is about 2.73 kW of heat.
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