How Many BTU Do I Need for My Split AC? Calculator by m²
This calculator determines the correct BTU (British Thermal Unit) capacity needed for a split air conditioner based on your room's square footage and local climate. BTU is the standard unit measuring how much heat an AC unit can remove per hour. The core formula is: BTU = Room Area (m²) × Climate Factor (BTU/m²). Climate factors range from 600 BTU/m² in mild zones to 850+ BTU/m² in hot/humid regions. Getting this right is critical: an undersized unit runs non-stop and never cools properly, while an oversized unit short-cycles, leaving the air humid and uncomfortable.
To size a split AC, multiply your room area in m² by the climate factor: 600 BTU/m² in temperate zones, 700 BTU/m² in warm zones, 850 BTU/m² in very hot zones. A 20 m² room in a warm climate needs 14,000 BTU (≈3,528 frigorías). 1 BTU/h = 0.252 frigorías (kcal/h). Always round up to the next commercial unit size.
When to use this calculator
- Choosing the right split AC unit before purchase for a 15 m² bedroom in a hot climate
- Sizing a mini-split for a converted garage of 40 m² with poor insulation and direct sun exposure
- Replacing an old window unit with a ductless split in a 25 m² living room to reduce electricity bills
- Calculating cooling needs for a home office in a temperate climate where summer peaks occur
- Verifying that a landlord-installed 9,000 BTU split is actually sufficient for a 22 m² apartment room in a warm climate
Worked Example
- Room: 20 m², warm climate (factor = 700 BTU/m²)
- BTU = 20 × 700 = 14,000 BTU/h
- Frigorías = 14,000 × 0.252 = 3,528 kcal/h
How it works
3 min readHow It's Calculated
The fundamental formula used by HVAC engineers and endorsed by ENERGY STAR guidelines is:
BTU/h = Room Area (m²) × Climate Factor (BTU/m²)
Frigorías (kcal/h) = BTU/h × 0.252
Example:
Room = 20 m², Warm climate (Factor = 700)
BTU/h = 20 × 700 = 14,000 BTU/h
Frigorías = 14,000 × 0.252 = 3,528 kcal/h
→ Select: 14,000–18,000 BTU splitThe climate factor captures the combined effect of outdoor temperature, solar gain, humidity, and typical insulation levels for each zone. The DOE's Manual J methodology uses similar load calculations; this formula is a practical approximation widely used for residential split AC sizing.
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BTU Chart by Room Size and Climate
| Room Size (m²) | Temperate (600) | Warm (700) | Very Hot (850) | Recommended Split |
|---|---|---|---|---|
| 10 | 6,000 BTU | 7,000 BTU | 8,500 BTU | 9,000 BTU / 0.75 ton |
| 15 | 9,000 BTU | 10,500 BTU | 12,750 BTU | 12,000 BTU / 1 ton |
| 20 | 12,000 BTU | 14,000 BTU | 17,000 BTU | 12,000–18,000 BTU / 1–1.5 ton |
| 25 | 15,000 BTU | 17,500 BTU | 21,250 BTU | 18,000 BTU / 1.5 ton |
| 30 | 18,000 BTU | 21,000 BTU | 25,500 BTU | 24,000 BTU / 2 ton |
| 40 | 24,000 BTU | 28,000 BTU | 34,000 BTU | 30,000–36,000 BTU / 2.5–3 ton |
| 50 | 30,000 BTU | 35,000 BTU | 42,500 BTU | 36,000–48,000 BTU / 3–4 ton |
> Always round up to the next available commercial size — never down.
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Climate Factor by Zone
| Climate Type | Typical Regions | BTU per m² |
|---|---|---|
| Temperate | Northern Europe, Pacific Northwest, NZ | 500–600 |
| Warm | Mediterranean, Southeast US, Northern Argentina | 600–700 |
| Hot | South Florida, Gulf Coast, Buenos Aires summers | 700–800 |
| Very Hot & Humid | Tropical zones, Corrientes AR, Puerto Rico | 800–850 |
| Hot & Dry | Phoenix AZ, Las Vegas NV, desert zones | 850–950 |
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BTU to Commercial Split AC Sizes
| BTU Range | Frigorías | Commercial Label | Nominal Tons |
|---|---|---|---|
| 7,000–9,000 | 1,764–2,268 | 9,000 BTU | 0.75 ton |
| 9,001–12,000 | 2,268–3,024 | 12,000 BTU | 1 ton |
| 12,001–14,000 | 3,024–3,528 | 14,000 BTU | 1 ton+ |
| 14,001–18,000 | 3,528–4,536 | 18,000 BTU | 1.5 ton |
| 18,001–24,000 | 4,536–6,048 | 24,000 BTU | 2 ton |
| 24,001–30,000 | 6,048–7,560 | 30,000 BTU | 2.5 ton |
| 30,001–36,000 | 7,560–9,072 | 36,000 BTU | 3 ton |
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Common Mistakes
1. Ignoring ceiling height. The base formula assumes a standard 2.5 m (8 ft) ceiling. Rooms taller than 3 m need a volume correction: multiply BTU result by (actual ceiling height ÷ 2.5).
2. Not accounting for sun exposure. South- and west-facing rooms with large glass windows can receive 400–600 W/m² of solar gain. Add 10–15% to the BTU result for heavy sun exposure rooms.
3. Choosing the smallest unit to save money. An undersized split runs at 100% capacity continuously, wears out faster, and never reaches the setpoint temperature.
4. Ignoring internal heat sources. Each person in a room adds ~250–400 BTU/h. A kitchen adds 4,000+ BTU/h from cooking. Offices with multiple computers can add 1,000–3,000 BTU/h of load.
5. Oversizing the unit. An oversized unit short-cycles — it reaches the setpoint quickly and shuts off before removing enough moisture. This leaves the room cold but clammy.
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Related Calculators
Frequently asked questions
How many BTU do I need for a 20 m² (215 sq ft) room?
In a warm climate: 20 m² × 700 BTU/m² = 14,000 BTU/h (3,528 frigorías). In a temperate climate: 20 × 600 = 12,000 BTU/h. The closest commercial split sizes are 12,000 BTU (1 ton) or 18,000 BTU (1.5 ton) — choose based on whether your climate factor result is above or below the midpoint.
How many BTU per square meter do I need?
600 BTU/m² for temperate climates (Pacific Northwest, northern Europe), 700 BTU/m² for warm climates (Mediterranean, southern US, Argentina), and 850 BTU/m² for very hot/humid zones (tropical regions, Gulf Coast, desert). These factors align with the DOE Manual J approach and ENERGY STAR guidance.
What is the difference between BTU and frigorías (kcal/h)?
Both measure cooling power in different unit systems. 1 BTU/h = 0.252 kcal/h, so 1 frigoría = 1 kcal/h ≈ 3.968 BTU/h. In Latin America and parts of Europe, split AC units are rated in frigorías. A 12,000 BTU unit equals roughly 3,024 frigorías.
Should I choose a fixed-speed or inverter split AC?
For most residential applications, an inverter (variable-speed) compressor is strongly preferred. Inverter units modulate output between ~30–100% of rated capacity, saving 25–40% in electricity vs. fixed-speed units per ENERGY STAR data. They also maintain more stable temperatures and have longer compressor life.
How do I convert BTU to tons of refrigeration?
1 ton of refrigeration = 12,000 BTU/h. So a 24,000 BTU split = 2 tons; an 18,000 BTU split = 1.5 tons. This ton-based naming is standard in the US HVAC industry and on most equipment spec sheets.
Does the BTU formula change for a kitchen?
Yes. Kitchens have major internal heat loads from cooking appliances. Add 4,000–6,000 BTU/h for a residential kitchen with a gas or electric range. So a 15 m² kitchen in a warm climate: (15 × 700) + 5,000 = 10,500 + 5,000 = 15,500 BTU/h → select an 18,000 BTU unit instead of the 12,000 BTU you'd use for a bedroom of the same size.
What happens if I install a split AC that is too large (oversized)?
An oversized unit short-cycles — it reaches the thermostat setpoint very quickly and shuts off before removing enough moisture from the air. This leaves the room feeling cold but clammy. The frequent on-off cycles also stress the compressor, reducing lifespan. ENERGY STAR specifically warns against oversizing; the goal is to size within 15% of the calculated load.
Can I use one large split AC to cool multiple rooms?
A single-zone split AC cools only the room where the indoor unit is installed. Adjacent rooms rely on air circulation through open doors, which is inefficient and uneven. For multi-room cooling, a multi-zone (multi-split) system connects 2–5 indoor units to one outdoor compressor. Each indoor zone should be sized independently.
What is SEER2 and why does it matter?
SEER2 (Seasonal Energy Efficiency Ratio 2) is the updated DOE efficiency metric. It measures total cooling output (BTU) divided by total electrical energy used (Wh) over a cooling season. The US federal minimum for split systems ≤45,000 BTU is SEER2 13.4. Higher SEER2 = lower electricity bills; top units reach SEER2 20–26.