Room Air Conditioner Sizing Calculator
Choosing an air conditioner without calculating the real cooling load first is one of the most common—and expensive—mistakes homeowners make. An undersized unit runs at full capacity without reaching the desired temperature, fails early, and spikes electricity bills. An oversized unit short-cycles constantly, fails to dehumidify the air, and is equally inefficient. Getting it right means using the industry-standard cooling load formula. This calculator uses the same method applied by HVAC professionals in Argentina and across Latin America: room volume × 50 (the base thermal coefficient in kcal/h·m³), plus 150 frigorías per habitual occupant, plus electrical appliances (watts × 0.86), all corrected for window orientation and insulation quality. The result is your room's actual cooling load in frigorías and BTU/h, plus the nearest commercial split unit size. 1 frigoría (kcal/h) = 3.968 BTU/h. This is the unit used on most Latin American split AC packaging.
When to use this calculator
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Bedroom 4 × 3.5 m, north-facing, average insulation, middle floor
- Volume load: 4 × 3.5 × 2.6 × 50 = 1,820 frigorías
- People: 2 × 150 = 300 frigorías
- Appliances: 100 W × 0.86 = 86 frigorías
- Subtotal: 1,820 + 300 + 86 = 2,206 frigorías
- North orientation factor: 2,206 × 1.15 = 2,537 frigorías
- Average insulation: × 1.00 (no change)
- Not top floor: × 1.00
- Total cooling load: 2,537 frigorías ≈ 10,063 BTU/h
- Recommended split: 3,200 frigorías (nearest commercial size above)
How it works
1 min readRoom air conditioning sizing follows the industry-standard cooling load method widely used in Latin America and based on fundamental HVAC principles.
How It's Calculated
Cooling Load (frigorías) = (L × W × H × 50) + (people × 150) + (watts × 0.86)
× orientation_factor × insulation_factor × floor_factor
Orientation factors (window direction):
South → ×1.00 (least direct solar gain in southern hemisphere)
East → ×1.10 (morning sun, lower ambient temperature)
North → ×1.15 (strong midday solar gain)
West → ×1.20 (worst case: afternoon sun at peak temperatures)
Insulation factors:
Good → ×0.90 (modern build, double glazing, insulated roof)
Average → ×1.00 (standard construction)
Poor → ×1.20 (old building, single glazing, no insulation)
Top floor factor:
Yes → ×1.18 (exposed roof gains heat all day)
No → ×1.00
Unit conversion: 1 frigoria (kcal/h) = 3.968 BTU/hTypical Commercial Split Sizes
| Frigorías | BTU/h | Typical room |
|---|---|---|
| 2,600 | 10,300 | Small bedroom 10–14 m² |
| 3,200 | 12,700 | Standard bedroom 14–18 m² |
| 3,500 | 13,900 | Large bedroom / study |
| 4,500 | 17,800 | Living room 18–28 m² |
| 5,500 | 21,800 | Large living / dining |
| 7,500 | 29,800 | Open loft / large space |
| 9,000 | 35,700 | Large open-plan area |
Common Sizing Mistakes
Final Notes
This method is used by HVAC installers throughout Latin America. For highly critical installations (large commercial spaces, data centers, cleanrooms), a full Manual J or ASHRAE heat balance analysis is recommended.
Frequently asked questions
How many BTU do I need per square meter?
There is no single correct BTU/m² value — it depends on ceiling height, sun exposure, insulation, and occupants. The full industry formula is: (length × width × height × 50) + (people × 150) + (appliance watts × 0.86), then adjust for orientation and insulation. As a rough guide: a well-insulated bedroom might land around 150–180 frigorías/m² (600–715 BTU/m²), while a poorly insulated room facing west could exceed 220 frig/m² (875 BTU/m²).
Why does a west-facing room need more cooling capacity?
West-facing windows receive afternoon sun in summer, which coincides with the hottest time of day. A 2 m² unshaded west-facing window can add 400–600 frigorías to the cooling load. In the southern hemisphere (Argentina, Chile, etc.), north-facing windows receive strong midday sun — the equivalent of south-facing in the northern hemisphere. Installing blackout curtains or adjustable blinds can reduce solar gain by 70–85%, lowering your required AC capacity.
What is a frigoria and how does it convert to BTU?
A frigoria is a unit of cooling capacity equal to 1 kcal/h. It's the standard unit used on Latin American split AC product specifications. The conversion is: 1 frigoria = 3.968 BTU/h (essentially 4 BTU/h for quick estimates). A 3,500-frigoria split is roughly a 12,000-BTU/h (1-ton) unit. When comparing products across markets, check whether ratings are in BTU/h, frigorías, or kW — 1 kW = 860 frigorías = 3,412 BTU/h.
Why multiply appliance watts by 0.86?
1 watt = 0.86 kcal/h (the conversion factor between SI electrical units and the thermal calorie). Every electrical device running in a room converts almost all its power consumption to heat released into the space. A 150 W computer adds 129 frigorías of heat load. In home offices with multiple computers, this can account for 20–30% of total cooling load — easily the difference between one split size and the next.
How much does being on the top floor add to cooling requirements?
Typically 15–20%. An exposed roof slab without insulation receives direct solar radiation all day and can have a heat flux of 40–80 W/m². This calculator applies +18% for top-floor rooms. If you have roof insulation (spray polyurethane foam, EPS panels), select 'good' insulation to partially offset this, rather than leaving insulation as 'average' — the two factors can compound.
What is the difference between Inverter and on/off AC technology?
On/off ACs run at 100% capacity when the compressor is on, reach the set temperature, then shut off. Each restart draws a high current spike. Inverter units modulate compressor speed between ~30% and 100%: once the room is cool, they drop to minimum power and maintain temperature continuously. Documented energy savings are 30–50% in sustained use. Inverter units also dehumidify better because they run longer at lower power, giving the evaporator more contact time with humid air.
Is it better to get one large unit or two medium units for an open-plan space?
For open spaces over 35 m², two medium units are usually better: more uniform air distribution (no hot corners), the ability to run just one unit on mild days for efficiency, and redundancy if one fails. Upfront cost is often similar or only slightly higher than a single large unit, but the operational flexibility pays back over several seasons.
How accurate is this calculator?
This calculator uses the standard industry method applied by HVAC installers throughout Latin America, producing estimates accurate to ±10–15% for typical residential rooms. Main sources of variation: actual window area and glazing type, wall material thermal mass, local climate extremes, and duct losses (if ducted). For precision sizing of large commercial systems, a full Manual J or ASHRAE heat balance analysis is recommended.
Can I use a split AC for heating in winter?
Yes. All modern splits sold in most markets operate as reversible heat pumps — they cool in summer and heat in winter by reversing the refrigeration cycle. They are highly efficient: a unit with COP 3.5 delivers 3.5 kW of heat for every 1 kW of electricity consumed, making it about three times more efficient than a resistance electric heater. Most standard splits work efficiently down to about −5°C to −10°C outdoor temperature. For colder climates, look for 'cold climate' or 'Arctic' heat pump models rated to −15°C or lower.
How do I account for a room with unusually high ceilings?
The formula automatically handles this through the height (H) variable. Just enter the actual ceiling height in meters. A 20 m² room at 4 m ceiling has 80 m³ of air to cool versus 48 m³ at 2.4 m — a 67% difference in volume. For rooms with sloped or vaulted ceilings, use the average height (add minimum and maximum heights, divide by 2). Installing the indoor unit no higher than 2.5 m from the floor ensures optimal air distribution even in tall rooms.
What maintenance does an AC split need to stay efficient?
Annual professional service is recommended (ideally before summer: September–October in the southern hemisphere). Filters should be cleaned by the user every 15–30 days of use with warm water — a dirty filter increases energy consumption 10–20% and reduces cooling capacity. The annual service includes evaporator and condenser cleaning, refrigerant pressure check, condensate drain clearing, and electrical connection inspection. Neglecting this can lead to refrigerant leaks (unit cools poorly but consumes the same), mold growth on the evaporator, and premature compressor failure.