Quick answer: Cooling load is the total heat a room gains — through walls, roof and glass, plus people, lights and equipment, plus fresh air. Add it all up, then convert to tonnage at 1 TR = 3.517 kW = 12,000 BTU/h. A quick Indian rule of thumb is ~1 TR per 100–150 sq ft, but a glass-heavy top-floor room needs far more. Get the room-accurate figure with the heat load calculator.
Why the "rule of thumb" gets AC sizing wrong
“One ton per 100 square feet” is a starting point, not an answer. Two rooms of the same size can differ by 50% in load: a north-facing internal room versus a west-facing top-floor room with large windows. Oversize the unit and it short-cycles — it cools the air fast, switches off before it removes humidity, and leaves the room cold and clammy while wasting energy. Undersize it and it runs flat out and never reaches setpoint on the hottest afternoon. The fix is to size on the actual heat gain.
What makes up the cooling load
Cooling load has two parts — sensible (raises air temperature) and latent (adds moisture):
| Source | Type | Typical gain |
|---|---|---|
| Walls & roof (conduction) | Sensible | Q = U × A × CLTD |
| Glass — solar | Sensible | up to ~400–500 W/m² (unshaded W/E) |
| People (office, seated) | Both | ~75 W sensible + ~55 W latent each |
| Lighting | Sensible | connected W × 1.25 |
| Computers / equipment | Sensible | ~150–300 W per workstation |
| Fresh air / infiltration | Both | 1.23 × L/s × ΔT (sensible) |
Rule-of-thumb tonnage (sanity check)
| Space | Approx. area per TR |
|---|---|
| Bedroom / low-gain room | 130 – 160 sq ft/TR |
| Living room / general office | 100 – 130 sq ft/TR |
| Glass-heavy / top floor / kitchen | 80 – 100 sq ft/TR |
| Server / IT room | size on equipment kW (≈ 0.28 TR per kW IT + margin) |
Indicative for Indian design conditions; always confirm with a proper heat-gain calculation for anything beyond a single small room.
Worked example — a 200 sq ft office
Rule of thumb: 200 / 110 ≈ 1.8 TR Heat-gain check: Envelope + glass solar ....... ~2.6 kW 2 people (2 × 130 W) ......... 0.26 kW Lighting (400 W × 1.25) ...... 0.50 kW 2 computers (2 × 200 W) ...... 0.40 kW Fresh air ..................... ~0.5 kW Total ≈ 4.3 kW = 4.3 / 3.517 ≈ 1.2 TR sensible + latent & margin → select a 1.5–2 TR unit
Notice the two methods disagree — the rule of thumb over-reads because this room is not glass-heavy. That gap is exactly why a proper HVAC load calculation pays for itself.
Convert load to tonnage
1 TR (ton of refrigeration) = 3.517 kW = 12,000 BTU/h Tonnage = Total cooling load (kW) / 3.517
Round up to the next standard size (0.8, 1.0, 1.5, 2.0 TR…), but only just — jumping two sizes brings back the short-cycling problem.
FAQ
How many tons of AC for 200 sq ft? Roughly 1.5–2 TR for an Indian office, depending on glass, orientation and occupancy — confirm with a heat-gain calculation.
What is 1 ton of AC? The cooling equal to melting one ton of ice in 24 hours — 3.517 kW or 12,000 BTU/h of heat removal.
Sensible vs latent load? Sensible cools the air; latent removes moisture. Humid coastal rooms need a higher latent share, which means colder coils, not just more tonnage.
Does a bigger AC cool better? No — an oversized unit short-cycles, controls humidity poorly and costs more to run. Right-sizing beats over-sizing.