AC Size Calculator: Room Area to BTU and Tons
Enter the room area, how sunny it is, how many people use it and whether it is a kitchen. See the room air conditioner size from the ENERGY STAR sizing chart, in BTU per hour, tons and kilowatts.
A rule-of-thumb size. Very hot climates, top floors, large windows and poor insulation may need more. A professional heat-load calculation (such as ACCA Manual J) is the proper method.
- Cooling capacity
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- Size in tons of refrigeration
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- Cooling capacity
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- Chart size before adjustments
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- Room area
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Assumptions
- The base size comes from the ENERGY STAR room air conditioner chart (June 2003) for the room area.
- Shade or sun changes the chart size by 10%; then 600 BTU/h is added per person above two, and 4,000 BTU/h for a kitchen.
- One ton of refrigeration is 12,000 BTU/h (NIST SP 811).
- Climate, ceiling height, insulation, windows and appliances are not modelled.
Estimate Checked 28 October 2026 ENERGY STAR: Qualified Room Air Conditioners, sizing chart and special considerations (June 2003) All sources
How it is calculated
- Base size = ENERGY STAR chart value for the room area (for example 150–250 sq ft → 6,000 BTU/h)
- Size = base × 0.9 if heavily shaded, × 1.1 if very sunny
- + 600 BTU/h for each regular occupant above two, + 4,000 BTU/h for a kitchen
- Tons = BTU/h ÷ 12,000; kW = BTU/h × 0.2930711 ÷ 1,000
Worked example
A 20 m² bedroom (about 215 sq ft) used by two people.
| You enter | |
|---|---|
| Room floor area | 20 |
| Area unit | Square metres (m²) |
| Sun on the room | Normal |
| People who regularly use the room | 2 |
| Is it a kitchen? | No |
| You get | |
| Cooling capacity | 6,000 BTU/h |
| Size in tons of refrigeration | 0.5 |
| Cooling capacity | 1.76 kW |
| Chart size before adjustments | 6,000 BTU/h |
| Room area | 215.3 ft² |
A room of 215 sq ft starts at 6,000 BTU/h on the chart. With your adjustments, look for about 6,000 BTU/h, which is 0.5 tons or 1.76 kW of cooling. This is ENERGY STAR's chart for US homes; in very hot climates, top floors or rooms with large windows, size up and get a heat-load calculation.
Questions
What is a "ton" of air conditioning?
A ton of refrigeration is 12,000 BTU per hour of cooling, about 3.52 kW. A "1.5 ton" split AC therefore cools about 18,000 BTU/h.
Why not just buy a bigger AC?
ENERGY STAR notes that an oversized unit cools the room quickly but leaves humidity behind, so the room feels cold and clammy, and it wastes energy.
Sources and checks
- ENERGY STAR: Qualified Room Air Conditioners, sizing chart and special considerations (June 2003)
- NIST SP 811 (2008), Appendix B: ton of refrigeration (12,000 Btu/h), Btu per hour and square foot factors
Last checked: 28 October 2026. This tool passes 7 automatic tests before every release. How we check our tools
Your numbers never leave your device: the calculation runs in your browser and nothing you type is sent or stored.
This is an estimate from a published rule-of-thumb chart, not a heat-load calculation. A unit that is too big cools quickly but leaves the room damp, and one that is too small runs all the time. Results are estimates for planning and learning. They are not a design check and do not replace a qualified engineer or your local building/electrical code.