What the Air Conditioner Tonnage Calculator does
This calculator estimates the air conditioner size a room needs, in tons and BTU per hour, starting from the ENERGY STAR room air conditioner sizing chart. It applies that chart's published adjustments for shade, strong sun, extra occupants and kitchens, then a few clearly labelled rules of thumb for high ceilings, poor insulation and heat from appliances, and names the nearest standard unit sizes.
Every line of the heat-load breakdown says where it comes from - ENERGY STAR, physics, or an A2Z rule of thumb - so you can see which parts are documented guidance and which are estimates.
How to use it
- Enter the room's length, width and ceiling height in metres or feet.
- Pick the sun exposure: heavily shaded, normal, or very sunny (large west or south-facing windows in the northern hemisphere).
- Enter how many people normally use the room. ENERGY STAR adds 600 BTU/h for each person beyond two.
- Tick the kitchen box if the room is a kitchen or opens onto one; cooking adds a lot of heat.
- Choose the insulation level and add the watts of any appliances that run while you cool - computers, TVs, lamps.
- Read the result in tons and BTU/h, and compare with the cooling capacity printed on the units you are considering.
Reading the results
One ton of cooling is 12,000 BTU/h, about 3.52 kW. The ton label on a unit is a marketing size; the rated cooling capacity in BTU/h or kW on the data sheet is what to compare.
Nearest unit up is the smallest common size that covers the estimate. When the estimate is only just above a smaller size, the smaller unit may suit a well-shaded, well-insulated room.
Oversizing is not free. A unit that is too big cools the air quickly and switches off before it has removed much moisture, leaving the room cold and clammy.
Worked example: a sunny living room
A 6 x 5 m living room with a 3 m ceiling has 30 m², which is 322.9 sq ft. ENERGY STAR's chart gives 8,000 BTU/h for 300-350 sq ft.
It is very sunny (+10%, 800 BTU/h) and four people use it (two beyond the first two, +1,200 BTU/h): 10,000 BTU/h. The 3 m ceiling is 9.84 ft, so the A2Z volume rule scales that by 9.84 / 8, to 12,303 BTU/h. A television and lamps giving off 300 W add 300 x 3.412 = 1,024 BTU/h.
The total is about 13,327 BTU/h, or 1.11 tons. The nearest standard size up is a 1.5-ton unit (18,000 BTU/h); a 1-ton unit (12,000 BTU/h) would fall about 10% short on the hottest afternoons.
Formulas and scoring rules
- Base capacity
ENERGY STAR chart: 100-150 sq ft 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 BTU/hA boundary area takes the smaller figure.- ENERGY STAR adjustments
heavily shaded -10%; very sunny +10%; +600 BTU/h per occupant beyond two; kitchen +4,000 BTU/h- A2Z rules of thumb
ceiling above 8 ft: x (height / 8); poor insulation: x 1.15- Appliance heat
BTU/h = watts x 3.412- Tons and kW
tons = BTU/h / 12,000; kW = BTU/h / 3,412
Limitations: what the result does not prove
- The ENERGY STAR chart is for room air conditioners and ends at 2,500 sq ft. Whole homes and central systems need a proper load calculation such as ACCA Manual J; the calculator refuses to extrapolate beyond the chart.
- Climate is only captured through sun exposure. Very hot or humid regions may need more capacity than the chart suggests; local installers' guidance for your climate should take precedence.
- The ceiling and insulation adjustments are A2Z rules of thumb, not published standards. They are shown separately so you can judge them.
- It does not choose efficiency ratings, refrigerant, electrical supply or installation details, all of which matter as much as size.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- ENERGY STAR - room air conditioner sizing - checked 19 Sep 2026
- ENERGY STAR - Insulation R-values (why insulation changes cooling load)
Frequently asked questions
How many tons of AC do I need for a 12 x 12 ft room?
A 144 sq ft room falls in ENERGY STAR's 100-150 sq ft band, which is 5,000 BTU/h - under half a ton. In practice the smallest common split units are 0.75 or 1 ton, so a 0.75-ton unit is usually plenty unless the room is very sunny or packed with people and equipment.
How many BTU is one ton of air conditioning?
One ton of refrigeration is 12,000 BTU per hour - the rate of heat absorbed by melting one short ton of ice in 24 hours. So a 1.5-ton unit is rated around 18,000 BTU/h, or about 5.3 kW of cooling.
Is a bigger air conditioner better?
No. An oversized unit cools the air fast and switches off before it has run long enough to pull moisture out, so the room feels damp and the compressor cycles on and off, wasting energy and wearing the unit. The right size runs for longer, steadier cycles.
Why does a kitchen need 4,000 more BTU?
Cooking appliances release a great deal of heat, and ENERGY STAR's sizing guidance adds 4,000 BTU/h for a room air conditioner used in a kitchen. Tick the kitchen box for an open-plan kitchen-diner too, since the heat reaches the whole space.
Does ceiling height change the AC size?
It changes the volume of air and the wall area that gains heat. The ENERGY STAR chart assumes a standard ceiling of about 8 ft, so this calculator scales the load by height / 8 for taller rooms. That is a rule of thumb, labelled as such in the breakdown.
What is the difference between this and the room BTU calculator?
This one uses the ENERGY STAR area chart that room air conditioners are sold against. The Room BTU Calculator builds the load from walls, windows, insulation and temperatures using a heat-balance method, and also works for heating. Use both and compare if the choice is borderline.
Last reviewed by the A2Z.Tools team against the sources listed above.
