AC Tonnage Calculator

Why “one ton per 100 square feet” misses

It is the rule every showroom quotes, and it is wrong in both directions at once. A shaded ground-floor bedroom in Pune and a top-floor living room in Chennai with a west window can have the same floor area and cooling loads that differ by more than double. Area is one term of five.

The four that the rule ignores are the ones you can do something about. An exposed roof adds roughly 30 %. West-facing glass gains three times what shaded north glass does, square foot for square foot. Four people in a room add 2,400 BTU/hr — a fifth of a ton — and a rack of equipment adds its full wattage as heat, because every watt drawn ends up in the room.

Read the split, not just the total

The bar showing where the heat comes from is the more useful output. If glass is a third of your load, an external shade or a solar film is cheaper than the next size up and it keeps saving after the unit is in. If the envelope dominates on a top floor, roof insulation pays back across every summer the building stands.

Oversizing is a real fault, not a safety margin

An air conditioner that is too large cools the room quickly and switches off before it has removed the moisture. The result is a space that is cold and clammy, with the compressor short-cycling — which wears it out and wastes energy on every restart. Humid coastal locations suffer this badly.

This is why the tool reports the exact tonnage alongside the standard size, and how far the rounding pushed you over. If a 1.0 ton unit is 40 % more than the room needs, look at whether 0.8 ton fits before accepting the round number.

The electrical side, which is what actually gets specified wrong

A 1.5 ton fixed-speed unit draws around 1,700 W, about 8 A at 230 V — and roughly 40 A for a second or two every time the compressor starts, because it is thrown directly across the line and pulls locked-rotor current until it reaches speed.

That transient is what decides three things the tonnage figure never mentions: the breaker curve, the cable size, and whether an inverter or generator can carry the unit at all. It is the most common reason an installation that looked correct on paper trips whenever the compressor cuts in. Feed the running figure into the load calculator to see it against the rest of the house, and the breaker sizing calculator for the protection.

An inverter compressor ramps instead of slamming, so its surge is nearer twice running current than five times. That, rather than the star rating, is usually the stronger argument for one on a weak supply.

How far to trust this

The envelope figures are per-square-foot heat gains calibrated against Indian design conditions and accepted practice, not a full ISHRAE or ASHRAE hourly cooling-load calculation. For one room and one split unit that is the right level of detail, and it will land you on the correct standard size. Anything centrally air conditioned, or any building where the plant is a capital decision, needs a proper hourly load study with real wall constructions and a weather file.