How to Size AC Capacity for a Room: Cooling Load, Not Floor Area

"What size AC does this room need?" is one of the most common air-conditioning questions. The quick answers online are usually a rule per square metre. That rule works for a small bedroom, but it often misses for offices, classrooms and meeting rooms.

This article is for design consultants, procurement officers and building managers in Indonesia who need to set AC capacity on a defensible basis. The short answer: AC capacity is set by the cooling load, the heat entering and generated in the room, not by floor area alone.

The cooling load is the amount of heat an AC must remove to hold the room at its design temperature and humidity, expressed in BTU/h or kW. It comes from 7 main sources: walls, roof, glass, people, lighting, equipment and outdoor air.

What do PK, BTU/h and kW mean on an AC?

PK is a trade label, while BTU/h and kW are the actual units of cooling capacity. A 1 PK split AC in the Indonesian market is usually about 9,000 BTU/h. Technical specifications should use BTU/h or kW, because units with the same PK label can have different capacities across manufacturers.

Split AC trade label	Typical cooling capacity in the market
½ PK	about 5,000 BTU/h
¾ PK	about 7,000 BTU/h
1 PK	about 9,000 BTU/h
1½ PK	about 12,000 BTU/h
2 PK	about 18,000 BTU/h

These are common ranges, not a standard. Always check the cooling capacity in the brochure or on the unit label.

Conversions used in the calculation:

• 1 kW = 3,412 BTU/h
• 1 TR (ton of refrigeration) = 12,000 BTU/h = 3.517 kW

Why do per-square-metre rules often miss?

Per-square-metre rules ignore the heat sources that differ most between rooms: occupancy, glass area, sun direction and equipment. Two 48 m² rooms can need very different capacities.

For example, a 48 m² office with 6 people and a small north-facing window has a fairly light load. A 48 m² meeting room with 20 people, a projector and a west-facing glass wall has a much larger one. If both get AC sized by the same area rule, the meeting room will feel hot when it is full.

Which components make up the cooling load?

The cooling load is made of external and internal loads, plus the outdoor air load. Each component is calculated separately and then added up.

Component	Heat source	What affects it
Wall and roof conduction	Solar heat passing through the building fabric	Material, insulation, colour, orientation
Glass	Direct solar radiation and conduction	Glass area, orientation, glass type, shading
People	Body heat (sensible and latent)	Number of people and activity
Lighting	Heat from lamp power	Lamp type and power per m²
Equipment	Computers, printers, projectors, servers	Power and hours of use
Outdoor air	Ventilation and infiltration	Fresh air volume, outdoor temperature and humidity

Latent load comes from moisture, such as people's breath and humid outdoor air. In a tropical climate like Indonesia's, the latent load from outdoor air is often large and should not be ignored.

Worked example: internal load of a meeting room

The internal load of a 48 m² meeting room with 20 people reaches about 13,900 BTU/h, or 4.1 kW, from people, lighting and equipment alone. The calculation below shows where that figure comes from. Assumed values are marked so you can replace them with your own room data.

Assume a 48 m² meeting room with 20 people:

1. People: ASHRAE tables for moderately active office work give about 75 W sensible and 55 W latent heat per person. For 20 people: 20 × 130 W = 2,600 W.
2. Lighting (assumed 10 W/m²): 48 × 10 W = 480 W.
3. Equipment (assumed projector and laptops): 1,000 W.
4. Total internal load: 2,600 + 480 + 1,000 = 4,080 W, or about 4.1 kW.
5. In BTU/h: 4,080 × 3.412 ≈ 13,900 BTU/h.

The internal load alone is already more than one 1½ PK unit. Heat from walls, glass and outdoor air is not yet included, and in a west-facing glazed room those components can be large. That is why a room-by-room calculation is needed before choosing units.

What are the steps to size AC correctly?

AC sizing follows six steps, from room data to unit selection. The order:

1. Collect room data: plan, ceiling height, wall and roof materials, glass area and orientation.
2. Set design conditions: indoor temperature and humidity, and outdoor conditions for the location.
3. Record internal loads: number of people, lighting power, equipment power and hours of use.
4. Set the fresh air requirement for the room's use.
5. Calculate each load component and add them up for the peak load hour.
6. Choose a unit whose cooling capacity matches the peak load, then check its actual capacity at site conditions from the manufacturer's data.

In buildings, the calculation is done per room or per zone. The result decides whether split, cassette or VRV/VRF suits best. For larger buildings, see our VRV vs chiller comparison.

Which standards apply in Indonesia?

The main reference for building air conditioning in Indonesia is SNI 6390:2020 on energy conservation in air-conditioning systems for buildings. Load calculation methods usually follow ASHRAE.

For state buildings, Minister of Public Works Regulation 22/PRT/M/2018 requires air conditioning that considers energy conservation and suits the building's function. The capacity from the load calculation is what goes into the AC technical specification for procurement.

Conclusion

Room AC capacity is set by the cooling load in BTU/h or kW, not floor area alone. A 1 PK split AC is usually about 9,000 BTU/h, so the right capacity can only be chosen once the load is calculated. The 48 m² meeting room with 20 people in the example above already has an internal load of about 13,900 BTU/h before walls, glass and outdoor air are counted.

SOKANTARA is an HVAC, VRV/VRF and MEP contractor with its head office in Medan, serving projects across Indonesia. We calculate the load room by room, model the system in BIM MEP, then deliver HVAC installation through to commissioning. If you need a capacity calculation for a building, contact our team.