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About this tool
Size your air conditioner correctly from room dimensions, sun exposure, and city climate, then estimate the monthly running cost.
The AC Tonnage Calculator sizes a room air conditioner by adding up the actual heat load in BTU rather than applying a rule of thumb: floor area at 34 BTU per sq ft scaled by ceiling height, 600 BTU per person, 165 BTU per sq ft of sunlit glass (55 if shaded), 150 BTU per window for leakage, appliance watts at 3.412 BTU each, plus 28 BTU per sq ft if you are on the top floor. The subtotal is multiplied by a climate factor for your city — 1.15 hot and humid, 1.10 hot and dry, 1.05 composite, 0.92 moderate — then divided by 12,000 BTU to give tons and rounded up to the nearest standard size. It then estimates the monthly running cost at 3, 4 and 5 star for both inverter and fixed-speed units.
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Enter Length, Width and Height in ft, then People in the room, Number of windows and Total window glass area, and switch on whichever of the Top floor, Direct sunlight on windows and West-facing wall toggles apply
Pick Your city from the dropdown, which selects the matching climate zone (Hot & dry ×1.1, Hot & humid ×1.15, Moderate ×0.92 or Composite ×1.05), then choose Insulation quality and tap the appliance chips such as TV + set-top box · 120 W
Recommended size and the Load breakdown table in BTU/hr update live with no Calculate button; set Star rating, Hours per day and Electricity tariff to read the Monthly bill, then use Copy report or Download for ac-tonnage-sizing-report.txt
Every contribution — fabric, people, glass, leakage, appliances, roof, orientation, insulation — is listed in BTU so you can see what is driving the size.
City climate zone, top floor, west-facing wall and insulation quality each apply their own factor, which is why the same room needs different units in Mumbai and Pune.
The same load feeds an ISEER-based bill estimate for 3, 4 and 5 star inverter and fixed-speed units, so capacity and operating cost are decided in one pass.
A 144 sq ft room with average construction usually lands between 1 and 1.5 ton, but the deciding factors are the extras: a top-floor slab adds about 28 BTU per sq ft, direct sun on 15 sq ft of glass adds around 2,500 BTU, and a hot-humid city multiplies the whole load by 1.15. The same room can need 1 ton in Bengaluru and 1.5 ton in Delhi's top floor.
No — oversizing hurts comfort. An oversized unit cools the air quickly, hits the setpoint and shuts off before it has removed much moisture, so the room feels cold and clammy, and a fixed-speed compressor short-cycles, which wastes power and wears it out. Sizing to the calculated load is what gives both temperature and humidity control.
Yes, and often by a whole step. A directly sunlit roof adds about 28 BTU per sq ft of floor area, so a 144 sq ft top-floor room picks up roughly 4,000 extra BTU — a third of a ton — and a west-facing wall adds around 8% of the fabric load for the afternoon peak.
Cooling capacity in watts is divided by the unit's ISEER to get the electrical draw, then multiplied by your daily hours, 30 days and your tariff. Inverter models are assumed at roughly 3.7, 4.2 and 4.8 ISEER for 3, 4 and 5 star and fixed-speed at about 3.2 to 3.6, so an inverter's advantage grows the longer you run it. Check the ISEER printed on the model you are buying, since BEE revises the star bands periodically.
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