Find what size air conditioner you need — in tons and BTU/hr — from floor area, climate, insulation and sun exposure. Use Quick Estimate for a fast answer, or Detailed Load for an engineering calculation with walls, glass, people, lighting and ventilation.
Enter your space details and click Calculate
| Room Size | Moderate | Hot Climate |
|---|---|---|
| 100–150 sq ft | 0.75 Ton | 1.0 Ton |
| 150–250 sq ft | 1.0 Ton | 1.5 Ton |
| 250–400 sq ft | 2.0 Ton | 2.5 Ton |
| 400–600 sq ft | 2.5 Ton | 3.5 Ton |
| 600–900 sq ft | 4.0 Ton | 5.0 Ton |
"Tonnage" is just cooling capacity: 1 ton = 12,000 BTU/hr = 3.517 kW — the cooling effect of melting one ton of ice in 24 hours. Sizing an AC means finding the space's cooling load in BTU/hr, then dividing by 12,000 and rounding up to the next standard unit size.
The base intensity depends on what you're cooling: about 22 BTU/hr per sq ft for a whole dwelling (the familiar ASHRAE-style figure), but 55–60 for a single exterior room, 35 for an open-plan office floor and 85 for a commercial kitchen. The calculator then applies climate, insulation, sun and ceiling-height multipliers, and adds occupants and appliance heat.
Detailed mode gives you the split between sensible and latent load and the resulting SHR — the numbers that actually determine your supply air temperature and airflow.
A 150 sq ft (14 m²) bedroom with one exposed wall and a window, 2.7 m ceiling, hot Gulf climate, average insulation, 2 occupants:
Cross-check with Detailed Load mode (wall 10 m² block, 2 m² west window, 46°C outdoor, 2 people, 8 L/s fresh air, 10% safety): the component method gives 3,029 W ≈ 0.86 tons — the same answer from a completely different route. That's the sanity check every rule of thumb should survive.
You'll see "20 BTU per sq ft" quoted everywhere — but that figure is for a whole dwelling, which averages exterior rooms together with interior rooms, corridors and bathrooms that have little or no envelope. A single exterior bedroom is the worst slice of that average: it carries its own external wall, its own window and its own fresh-air load, all divided by a small floor area.
That's why this calculator uses ~55–60 BTU/ft² for single rooms and ~22 BTU/ft² for a whole apartment. Both are correct — they're answering different questions. Pick the space type that matches what you're actually cooling.
| Room Size (sq ft) | Room Size (m²) | Moderate Climate | Hot Climate |
|---|---|---|---|
| 100–150 | 9–14 | 0.75 Ton | 1.0 Ton |
| 150–250 | 14–23 | 1.0 Ton | 1.5 Ton |
| 250–400 | 23–37 | 2.0 Ton | 2.5 Ton |
| 400–600 | 37–56 | 2.5 Ton | 3.5 Ton |
| 600–900 | 56–84 | 4.0 Ton | 5.0 Ton |
| Home Size (sq ft) | Home Size (m²) | Moderate Climate | Hot Climate |
|---|---|---|---|
| 600–800 | 56–74 | 1.5 Ton | 2.0 Ton |
| 800–1000 | 74–93 | 2.0 Ton | 2.5 Ton |
| 1000–1400 | 93–130 | 2.5 Ton | 3.5 Ton |
| 1400–1800 | 130–167 | 3.0 Ton | 4.0 Ton |
| 1800–2400 | 167–223 | 4.0 Ton | 6.0 Ton |
Average insulation, normal sun exposure, standard 2.7 m ceiling. Moderate ≈ 32°C design, Hot ≈ 46°C design. Every value in both tables is generated by this calculator — enter the same inputs and you'll get the same answer.
Disclaimer: This calculator gives preliminary estimates. Quick mode uses rule-of-thumb intensities; detailed mode uses a simplified ETD/CLTD approach with typical U-values and solar gain factors. For construction documents, perform a full ACCA Manual J or ASHRAE load calculation with project-specific data.