BTU AC Calculator

Sizing BTU/h for air conditioning

The rule installers fall back on is 600 BTU/h per m² + 600 per extra person (beyond the 2nd) + 600 per active electronic appliance in the room. Take a 25 m² room with 3 people and 2 electronics: 25·600 + 1·600 + 2·600 = 15,000 + 600 + 1,200 = 16,800 BTU/h, which you round up to an 18,000 BTU/h unit. Tack on +20% for high ceilings (> 2.8 m) and another +20% for west-facing sun exposure. Inverter compressors vary their speed and run more efficiently than fixed-speed ones. Check for the Procel/INMETRO seal, where class A marks the most efficient units.

Applications

Use it to size a split system (residential, efficient, quiet), a window unit (simple, cheap), a portable model (last resort), a multi-split (one outdoor unit feeding several rooms), or a VRF (Variable Refrigerant Flow, the kind used in commercial buildings and offices). The same BTU rule of thumb carries across every one of these formats. What changes between them is the installation cost and the noise.

FAQ

Should I oversize for safety? No. An oversized unit short-cycles on and off, never gets the humidity down, and burns extra energy doing it. Match the calculation, or go just a notch above it.

Does insolation really matter that much? It does. West-facing windows catch the harshest afternoon sun, so if the room is badly exposed to it, bump the capacity by around 20%.

Inverter vs fixed-speed? An inverter keeps the compressor running at a varying speed and can cut energy use 30–40% next to an on/off model. You pay more up front, but with regular use it earns that back in 1–3 years.