Air Conditioner Size Calculator: BTU and Tons by Room

Calculate required cooling capacity in BTU/hr and HVAC tons based on room square footage, ceiling height, sunlight, and occupants.

Air conditioner BTU by room size: how it is calculated

The tool reads the base capacity from an area → BTU/h chart (7,000 BTU/h for 300 sq ft, i.e. 27.9 m²), then adjusts it for sun exposure (±10%), climate, occupants beyond two (+600 BTU/h each), kitchen use (+4,000 BTU/h) and ceiling height. To convert to watts, divide by 3.412: 7,000 BTU/h is about 2.05 kW of cooling.

Step by step

  1. Area in square feet (1 m² = 10.7639 sq ft), then the base capacity: up to 150 sq ft 5,000 BTU/h, 250: 6,000, 300: 7,000, 350: 8,000, 400: 9,000, 450: 10,000, 550: 12,000, 700: 14,000, 1,000: 18,000, 1,400: 24,000, above that 25 BTU/h per sq ft.
  2. Sun: × 0.9 for a shaded room, × 1.1 for a very sunny one. Climate: × 1.15 hot, × 0.95 mild.
  3. +600 BTU/h per occupant beyond two; +4,000 BTU/h for a kitchen.
  4. Ceiling: × 1.1 above 8.5 ft (2.59 m), × 1.2 above 10 ft (3.05 m); in metres mode, the height entered is converted to feet for these thresholds.
  5. Rounded to 500 BTU/h; tonnage = BTU/h ÷ 12,000; the range shown runs from −5% to +15%.

Worked example

A 6 × 5 m room (30 m², 323 sq ft): base 8,000 BTU/h. Very sunny: 8,800; hot climate: 10,120; 4 occupants: +1,200, i.e. 11,320; 2.75 m (9 ft) ceiling: × 1.1 = 12,452, rounded to 12,500 BTU/h, i.e. 1.04 tons and about 3.66 kW (12,500 ÷ 3.412). A 5 × 4 m bedroom with no adjustment needs 6,000 BTU/h, about 1.76 kW.

Precautions and limits

  • The feet and metres buttons convert the values already entered, ceiling height included: 20 × 15 ft with an 8 ft ceiling becomes 6.096 × 4.572 m with a 2.438 m ceiling, and the capacity stays at 7,000 BTU/h.
  • The climate and ceiling factors are the tool's own reference figures. The bands above 300 sq ft could not be verified.
  • The tool ignores insulation, glazing and orientation: a heat-load study remains the reference for a fixed installation.

Sources and dates: ENERGY STAR room air conditioner sizing chart (EPA, based on an 8 ft ceiling), cited without a link because the page could not be displayed; −10% / +10%, +600 BTU per person and +4,000 BTU for a kitchen: Natural Resources Canada, guide to room air conditioners (page modified 10/06/2025); 1 W = 3.412 BTU/h (checked 14/10/2026)

Frequently Asked Questions

How many BTU do I need for a 20 m² room?

20 m² is 215 sq ft: the tool gives 6,000 BTU/h with no adjustment, about 1.76 kW of cooling. A very sunny room goes to 6,600 BTU/h, rounded to 6,500.

How do I convert BTU to watts?

1 W equals 3.412 BTU/h: divide BTU/h by 3.412 to get watts. A 9,000 BTU/h air conditioner therefore delivers about 2.64 kW of cooling.

Does a kitchen need more capacity?

Yes, the tool adds 4,000 BTU/h for a kitchen because of the heat given off by cooking. It also adds 600 BTU/h for each occupant beyond two.

What does air conditioner tonnage mean?

One ton of cooling equals 12,000 BTU/h, about 3.5 kW. A 12,500 BTU/h unit is therefore 1.04 tons; the unit is mainly used in the United States.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.