Air Conditioner BTU Calculator
Estimate the cooling capacity for one room from its area, sun exposure, people and kitchen use, and see how three published rules of thumb compare
Room Details
How BTUs Are Calculated
BTU (British Thermal Unit) is a traditional unit of heat. To properly cool a room, your AC must remove heat faster than it is generated or absorbed by the space.
The formula: 20 BTU per square foot of floor (about 215 BTU per m²), the rule the US Department of Energy gives. ENERGY STAR then adjusts it: −10% for a heavily shaded room, +10% for a very sunny one, +600 BTU for each person beyond two who regularly uses the room, and +4,000 BTU for a kitchen.
Required Cooling Capacity
Rounded down to the nearest thousand BTU, as ENERGY STAR advises, and never below 5,000 BTU, the smallest size in its example.
- Bosch (Spain), 90–100 frigorías per m²: 7,134–7,926 BTU, up to 30% more in strong sun.
- Brazilian retail rule, 600 BTU per m² (800 in sun): 11,985–15,979 BTU.
Rules differ by country and climate; the US figure is the lowest. In a hot climate, check with an installer.
How to use this tool
- Choose square meters or square feet and enter the floor area of the room.
- Pick the sun exposure: heavily shaded, average or very sunny.
- Enter how many people regularly use the room, and say whether the unit will cool a kitchen.
- Read the cooling load, the ENERGY STAR size guide and how the same room comes out under two other published rules.
Worked example
A 20 m² (215 sq ft) bedroom with average sun and two people needs 215.3 × 20 = 4,306 BTU. ENERGY STAR rounds down to the nearest thousand, which is below 5,000, so the guide shows 5,000 BTU. Make it very sunny and the load becomes 4,736; add two more people and it rises by 1,200. Used as a kitchen, the same room adds 4,000 BTU. For comparison, the Bosch rule gives 7,143–7,937 BTU and the Brazilian retail rule 12,000–16,000 BTU for the same 20 m².
Why this tool exists and where its numbers come from
A room air conditioner that is too small struggles on hot days, and one that is too large cools without removing humidity. The tool uses the US Department of Energy rule of 20 BTU per square foot and the ENERGY STAR adjustments (±10% for shade or sun, +600 BTU per person beyond two, +4,000 BTU for a kitchen, rounding down to the nearest thousand). For comparison it shows two other published rules: Bosch in Spain (90–100 frigorías per m², up to 30% more for strong sun) and a Brazilian retail rule (600 BTU per m², 800 in sun). None of them replaces a professional load calculation.
Sources
- US Department of Energy: Room Air Conditioners
- ENERGY STAR: choosing the right-sized window air conditioner (shade/sun ±10%, extra occupants)
- Bosch Home Comfort (Spain): how many frigorías per m² (90–100 frig/m² for bedrooms)
- Webarcondicionado (Brazil): how to calculate the BTUs of an air conditioner (600 BTU/m² rule)
For the full method, see the Methodology section just below.
Methodology
The estimate starts from the US Department of Energy rule of 20 BTU per square foot of living space (about 215 BTU per m²) and applies the ENERGY STAR adjustments for shade, sun, extra people and kitchens. It is not a professional heat-load calculation such as ACCA Manual J, which also accounts for insulation, windows and climate.
Eight rooms under the US rule
Values the calculator returns with 20 BTU per square foot and the ENERGY STAR adjustments.
| m² | Sun | People | Kitchen | Load (BTU) | Size guide (BTU) |
|---|---|---|---|---|---|
| 10 | Shaded | 2 | No | 1,938 | 5,000 |
| 15 | Average | 2 | No | 3,229 | 5,000 |
| 20 | Average | 2 | No | 4,306 | 5,000 |
| 20 | Sunny | 2 | No | 4,736 | 5,000 |
| 20 | Average | 4 | No | 5,506 | 5,000 |
| 12 | Average | 2 | Yes | 6,583 | 6,000 |
| 30 | Sunny | 3 | No | 7,704 | 7,000 |
| 45 | Average | 4 | No | 10,888 | 10,000 |
Why the same room can need 5,000 or 12,000 BTU: the US rule assumes typical American housing, while hotter climates use bigger factors. The comparison box shows both, so you can see the gap before you buy.
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Frequently asked questions
What happens if I buy an AC with too few BTUs? ▼
An undersized unit will run constantly, driving up your electricity bill while failing to effectively cool the room on hot days. It will also wear out the compressor much faster.
Can I buy a much larger AC just to be safe? ▼
No, an oversized AC is just as bad. It will cool the room so quickly that it shuts off before it has a chance to dehumidify the air. This leaves the room feeling cold, clammy and uncomfortably humid.
Why is the size guide sometimes below the calculated load? ▼
ENERGY STAR advises rounding down to the nearest thousand BTU after the adjustments, and an oversized unit cools too fast without removing humidity. A load of 5,506 BTU therefore maps to 5,000. The guide never goes below 5,000 BTU, the smallest size in ENERGY STAR’s example.
Can you walk through a full example, and how do BTU and kW relate? ▼
A 20 m² (215 sq ft) room with average sun and two people needs 215.3 × 20 = 4,306 BTU per hour, so the guide shows 5,000 BTU. One kilowatt of cooling equals about 3,412 BTU per hour, so 5,000 BTU/h is about 1.5 kW, which helps when a label is in kW.
Why do other sites give much bigger numbers? ▼
Rules of thumb differ by country. The US Department of Energy uses 20 BTU per square foot (about 215 per m²). Bosch in Spain uses 90–100 frigorías per m² (about 357–397 BTU), and a common Brazilian retail rule uses 600 BTU per m², 800 in sun. The comparison box shows all three for your room. In a hot climate or a top-floor room, an installer’s load calculation is worth it.