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What size AC, and why you must not buy on this number

Almost every sizing calculator on the internet multiplies square footage by a BTU figure — and almost every one also tells you, further down, that the correct method is ACCA Manual J. Both statements cannot be load-bearing. This page runs the rule of thumb so you can see whether a proposal is in the neighbourhood.

3.0 tons 2,000 sq ft, mixed climate, average envelope 18 BTU/sq ft × 2,000 × 1.00, rounded to the nearest half ton. Two houses of identical footprint on the same street routinely differ by a full ton once orientation, glass, leakage and ceiling height are counted. That difference is Manual J.

On the defaults a 2,000 sq ft house in a mixed climate with an average envelope lands at 3.0 tons (36,000 BTU/hr), with a neighbourhood of 2.5 to 3.5 tons. That is a starting point for judging a proposal, not a specification. If somebody quotes 5 tons and this says 3, that gap is the conversation. Oversizing cools the air fast, satisfies the thermostat, and shuts off before it has removed moisture.

The neighbourhood check

Square footage, climate, envelope. Then do not buy on it. Nothing is emailed.

Cooled floor, not the footprint including garage.

Moves the answer as much as a whole climate zone.

— Rounded size
— Load (rule of thumb)
— Neighbourhood
— Effective BTU per sq ft

What this assumed —

This is not a Manual J. Do not buy equipment on it. HyreHVAC does not size or install systems. Ceiling height, glass and orientation are invisible here — that is the point.

Questions this calculator answers

What size air conditioner do I need?
For a first pass, roughly 16 to 22 BTU per square foot depending on climate, adjusted for how well the house is built. A 2,000 sq ft house in a mixed climate with average insulation lands near 3 tons. That is a starting point, not a specification.
What is a ton of cooling?
12,000 BTU per hour. Residential systems are commonly sized between 1.5 and 5 tons, in half-ton steps.
Why does everyone say to get a Manual J instead?
Because square footage is one input out of dozens. Manual J accounts for orientation, window area and type, shading, insulation, air leakage, ceiling height, occupancy and internal gains.
Is it better to go bigger to be safe?
No. An oversized system short-cycles and dehumidifies poorly. Slightly undersized is the less damaging error on design days.
Can I use this to buy equipment?
No. Use it to tell whether a contractor recommendation is in the right neighbourhood.

Sources and methodology

Figures dated 2 August 2026. Last reviewed .

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