AC Size Calculator
Air conditioners are sold in three different languages: BTU per hour, tons and kilowatts. This AC size calculator converts between all three and rounds your load up to the next standard unit size.
Do not know your BTU load yet? Calculate it first with the BTU Calculator, which adjusts for insulation, sun and room type.
Enter valid values to see the results.
Formula
One ton of cooling equals 12,000 BTU/h — the heat needed to melt a ton of ice in 24 hours. Standard residential sizes: 6k, 7k, 9k, 12k, 14k, 18k, 24k, 30k, 36k BTU/h.
How to measure
- Calculate the room or house cooling load in BTU/h with the BTU Calculator.
- For multi-room split systems, add the loads of all rooms served by one outdoor unit.
- Enter the total here and round up to the next standard size.
Worked example
Practical tips
- Never round down: an undersized unit runs nonstop and still misses the setpoint on hot days.
- For inverter mini-splits, slight oversizing is fine — they modulate down instead of short-cycling.
- Check the SEER/EER efficiency rating alongside size; a higher-SEER unit pays for itself in hot climates.
- One ton per 40–50 m² (450–550 ft²) is a common rule of thumb for average homes.
Waste allowance
Round up to the next standard size — that is the only “allowance” needed. Do not add extra margin beyond one size step.
Standard AC sizes and coverage (average conditions)
| BTU/h | Tons / kW | Typical area |
|---|---|---|
| 7,000 | 0.6 t / 2.1 kW | 15–25 m² (160–270 ft²) |
| 9,000 | 0.75 t / 2.6 kW | 25–35 m² (270–375 ft²) |
| 12,000 | 1.0 t / 3.5 kW | 35–50 m² (375–540 ft²) |
| 18,000 | 1.5 t / 5.3 kW | 50–70 m² (540–750 ft²) |
| 24,000 | 2.0 t / 7.0 kW | 70–95 m² (750–1,020 ft²) |
Common mistakes
- 1 Confusing electrical power draw (kW input) with cooling capacity (kW cooling) — cooling kW is 2.5–3.5 times the electrical draw.
- 2 Sizing central AC by square footage alone without a Manual J load calculation.
- 3 Ignoring that portable units lose capacity through the exhaust hose — deduct about 30%.
- 4 Oversizing by two or more steps: humidity removal suffers and comfort drops.
Frequently asked questions
How many BTU is 1 ton of AC?
Exactly 12,000 BTU/h. A 1.5-ton unit is 18,000 BTU/h, a 2-ton unit is 24,000 BTU/h.
What size AC do I need for 30 m²?
About 9,000–12,000 BTU/h (0.75–1 ton) in average conditions. Use the BTU Calculator to adjust for sun and insulation first.
What is the difference between kW cooling and kW electric?
Cooling kW is heat removed; electric kW is power consumed. A 3.5 kW (12,000 BTU) unit typically draws only 1.0–1.3 kW of electricity thanks to the heat pump cycle.
Is it bad to oversize an AC?
Yes. An oversized unit cools too fast, shuts off before dehumidifying, and short-cycles — the room feels cold and sticky. One size step up from the calculation is the sensible maximum.
How many square feet does a 2-ton AC cool?
Roughly 900–1,100 ft² (85–100 m²) in average conditions with 8 ft ceilings, less in hot climates or poorly insulated homes.