What size air conditioner do I need? How to use the BTU calculator
- 1Measure the room's length and width, or enter its floor area.
- 2Enter the ceiling height if it is higher than 8 ft (2.4 m).
- 3Choose your summer climate — very hot for places with 40–50 °C summers such as Pakistan, North India and the Gulf.
- 4Choose how sunny the room is, whether it is on the top floor, how many people normally use it and whether it is a kitchen.
- 5Choose the kind of unit you are buying, then read the cooling capacity and the size to look for in BTU/h, kW or tons. Compare it with the cooling capacity on the unit's label or data sheet.
How to calculate BTU for a room (formula)
- 01Floor area → capacity from the ENERGY STAR chart (e.g. 150–250 ft² → 6,000 BTU/h)
- 02Hot summers: × 1.3 · Very hot: floor area in ft² × 95 BTU/h instead of the chart
- 03× your ceiling height ÷ 8 ft (very hot: ÷ 10 ft), only if the ceiling is higher (our adjustment)
- 04× 1.1 if very sunny, × 0.9 if heavily shaded · × 1.25 on a top floor under a sun-baked roof
- 05+ 600 BTU/h for each regular occupant beyond two
- 06+ 4,000 BTU/h if the room is a kitchen
- 07Size to look for = the next size on sale at or above the result · 1 kW ≈ 3,412 BTU/h · 1 ton = 12,000 BTU/h
ENERGY STAR, the US government's energy-efficiency programme, publishes a simple chart that matches a room's floor area to the air conditioner capacity it needs — for example 5,000 BTU/h for 100–150 ft² and 12,000 BTU/h for 450–550 ft². The chart assumes an 8 ft ceiling, two people and average sun, in a US climate.
It then adjusts: 10 % less for a heavily shaded room, 10 % more for a very sunny one, 600 BTU/h for each extra person and 4,000 BTU/h for a kitchen. We apply the sun adjustment to the chart figure and then add the people and kitchen figures, because that heat doesn't depend on the sun.
Hotter places need more. For summers of about 35–40 °C we add 30 %: AIRAH's calculator gives the same insulated room 3.0 kW in Perth against 2.3 kW in Sydney. Where summers reach 40–50 °C, as in Pakistan, North India and the Gulf, homes with concrete roofs and walls are sized by a different rule: AC makers' charts rise in a straight line with the floor area, about 95 BTU/h per ft² (1 ton for every 126 ft²). A single percentage on the ENERGY STAR chart can't match them, because that chart stays at 6,000 BTU/h from 150 to 250 ft². A room right under a roof in full sun gets another 25 %.
For ceilings above 8 ft (10 ft in the very-hot setting) we scale the figure in proportion to the height (10 ft → × 1.25), because the room holds more air and has more wall to gain heat through. ENERGY STAR only says to consider a bigger unit for higher ceilings, so this part is our estimate. The size to look for is the next size on sale: the ENERGY STAR steps for window units, 0.75–4 ton for split units sold by the ton, and 2.0–9.5 kW for split units sold by the kW.
BTU calculation example
A 15 ft × 12 ft room is 180 ft², which the ENERGY STAR chart puts at 6,000 BTU/h.
With average sun, 2 people and no cooking, nothing is added, so the room needs 6,000 BTU/h: look for a 6,000 BTU/h unit. If the room were very sunny it would need 10 % more.
Tips before you buy
- Measure only the space the unit has to cool — but if a wide opening joins another room, include that room too, or the unit will struggle.
- Close curtains or blinds on sunny windows during the day. Sun through glass is often the biggest source of heat in a room.
- Compare the unit's rated cooling capacity (BTU/h or kW of cooling), not the electrical power it uses — they are different numbers.
- Install a window unit level so it drains properly, and seal the gaps around it with the insulation supplied, as ENERGY STAR advises.
- A loft room under the roof, or one with big windows that get the afternoon sun, is usually "very sunny" even if the chart room would not be.
- In 45 °C heat, look for a unit rated for high outdoor temperatures (often labelled T3), and keep its outdoor part in the shade with room for air to flow.
Frequently asked questions
What size AC do I need in Pakistan or India?
Where summers reach 40–50 °C, AC makers' charts give about 1 ton for up to 120–130 ft², 1.5 ton up to about 190 ft² and 2 ton up to about 250 ft² (Voltas, Haier India). Choose "Very hot" and "Split AC sold in tons" to get the same result, and tick "Top floor" for a room under the roof.
How many square feet does a 1.5-ton AC cool?
In a very hot climate, about 130–190 ft² (12–18 m²) with a 10 ft ceiling — less on a top floor under a sun-baked roof. In a mild US climate the ENERGY STAR chart gives an 18,000 BTU/h (1.5 ton) unit 700–1,000 ft².
How many BTU do I need per square foot?
There is no single figure. In the ENERGY STAR chart it ranges from about 50 BTU/h per ft² for a 100 ft² room (5,000 BTU/h) down to about 14 for 2,500 ft² (34,000 BTU/h). The often-quoted "20 BTU per square foot" roughly fits mid-sized rooms of about 500–1,000 ft², which is why this calculator uses the chart instead.
What size air conditioner do I need for a 12 × 12 room?
A 12 × 12 ft room is 144 ft², which the ENERGY STAR chart puts at 5,000 BTU/h (about 1.5 kW). Add 10 % if it is very sunny, 600 BTU/h for each regular occupant beyond two, and 4,000 BTU/h if it is a kitchen.
How many BTU is a ton of air conditioning?
12,000 BTU per hour, about 3.5 kW. The name comes from the cooling effect of melting one ton of ice over 24 hours.
How do I convert kW to BTU/h?
Multiply kW by about 3,412. A 2.5 kW split unit is about 8,500 BTU/h, and a 3.5 kW unit about 12,000 BTU/h. To go from BTU/h to kW, divide by 3,412.
Is a bigger air conditioner better?
No. ENERGY STAR warns that an oversized unit costs more to buy, wastes energy and removes less humidity, so the room can feel cold and clammy. Choose a size close to what the room needs.
Can I use this for central air or a whole house?
No — it is for one room. Whole-house systems should be sized with a heat-load calculation such as ACCA Manual J, which accounts for insulation, windows, air leakage and your local climate. Ask an HVAC contractor for one.
Also searched as: what size air conditioner do I needAC size calculatorair conditioner BTU calculatorair con size calculatorroom AC size calculatorhow many BTU do I need
Sources
- ENERGY STAR – Room air conditioners: sizing chart and adjustments (sun, people, kitchen, 8 ft ceiling)
- CHOICE – What size air conditioner do I need? (AIRAH cooling calculator: 30 m² room, Perth 3.0 kW, Sydney 2.3 kW; 2.9 kW with no roof insulation)
- Voltas – AC room size chart (≤ 90 ft² 0.75 ton, 91–130 ft² 1 ton, 131–190 ft² 1.5 ton, 191–250 ft² 2 ton; 10 ft ceiling, 2 people)
- Haier India – How to calculate AC tonnage for your room (up to 120 ft² 1 ton, 120–180 ft² 1.5 ton)
- Daikin Australia – split system range (2.0, 2.5, 3.5, 5.0, 6.0, 7.1 kW …)
- ACCA – Manual J, the ANSI-recognised standard for residential heating and cooling load calculations
- NIST SP 811, Appendix B.8 – 1 BTU(IT)/h = 0.2930711 W; 1 ton of refrigeration = 3,516.853 W
Formulas last reviewed: September 30, 2026 ·How we check our formulas