Find the right AC size (in tons) for a room from its area, floor level and sun exposure — so you don't under- or oversize the unit.
Rounded up to the next standard AC size sold in Pakistan (0.75 through 5 ton) — never down, so the unit isn't undersized. Based on the standard Pakistani planning rule of roughly 1 ton per 100-120 sqft, adjusted for room type, floor and sun exposure.
Some general HVAC guides quote a much lower baseline (~20 BTU/sqft) meant for milder, temperate climates. Pakistan's summer heat load is far higher, so the figure actually used by AC dealers and installers here works out closer to 100 BTU/hr per square foot for a normally-shaded room — this calculator uses that local baseline, not the temperate-climate one.
A top-floor room sits directly under the roof, which absorbs and radiates heat all day — add about 20% to the load. A room with direct west or south sun exposure through its windows gains extra heat in the afternoon — add about 15%. Combine both if they apply.
This is a fixed, universal HVAC conversion — not an approximation. 'Tons' in air conditioning describe cooling capacity, not weight, dating back to how many tons of ice would need to melt in 24 hours to produce an equivalent cooling effect.