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HVAC Sizing Calculator

Estimate the heating and cooling capacity a home needs before you get equipment quotes.

The house

Conditioned floor area

sq ft

Count only heated and cooled space. Leave out an unconditioned garage, attic or crawlspace.

Average ceiling height

Use a floor-area-weighted average if some rooms are vaulted.

IECC climate zone

Find your county on the IECC climate zone map if none of the cities listed are close.

Occupants

Each person past the first two adds about 600 BTU/h to the cooling load.

Envelope

Insulation quality

Air tightness

Window area

LowUnder 10% of floor areaAverageRoughly 12–15% of floor areaHighOver 18% of floor area, or large glass walls

Sun exposure

Mostly shadedMature trees or north-facing glassAverageMixed orientation, partial shadeFull sunUnshaded west or south glass

This is a screening estimate built on BTU-per-square-foot factors by climate zone. It tells you whether a quote is in the right neighborhood. It does not tell you what to buy.

Estimated cooling capacity

3.5tons

40,000 BTU/h of cooling and 54,000 BTU/h of heating at design conditions.

Nearest nominal AC sizeEquipment is built in these steps only: 3.5 ton
Heating loadOutput the furnace or heat pump must deliver: 54,000 BTU/h
Furnace input at 80% AFUENominal 70k input model: 67,500 BTU/h
Furnace input at 95% AFUENominal 60k input model: 56,842 BTU/h
Cooling load densityRule-of-thumb range is 400–700 sq ft per ton: 540 sq ft/ton

Get a Manual J before you buy equipment

Nothing on this page is a load calculation. An ACCA Manual J measures your actual wall and attic R-values, window U-factor and solar heat gain by orientation, infiltration and duct losses, and it routinely lands a half ton to a full ton away from a square-footage estimate. Insist on one — many jurisdictions require it with the permit, and a contractor who sizes by square footage alone is telling you something about the rest of the work.

Oversizing makes cooling worse, not safer

An oversized air conditioner reaches setpoint before it has run long enough to pull moisture out of the air, then shuts off. You get a house that is cold and clammy, short cycling that wears out the compressor, and a higher bill. If a proposal comes in more than about half a ton above this estimate, ask what load calculation produced it.

How each input moves the load
Adjustment	Effect	Cooling BTU/h	Heating BTU/h
Baseline, climate zone 4	22 / 30 BTU per sq ft	39,600	54,000
Ceiling height 8.0 ft	×1.00 cool, ×1.00 heat	39,600	54,000
Insulation — average	×1.00 cool, ×1.00 heat	39,600	54,000
Window area — average	×1.00 cool, ×1.00 heat	39,600	54,000
Sun exposure — average	×1.00 cool, ×1.00 heat	39,600	54,000
Air tightness — average	×1.00 cool, ×1.00 heat	39,600	54,000
Occupants — 3 in the house	+600 BTU/h cooling, no heating credit	40,200	54,000
Design load	3.5 tons cooling	40,000	54,000

Sensitivity — the same house one step better or worse
Scenario	Cooling	Heating
As entered	3.5 tons	54,000 BTU/h
Tight and well insulated instead	2.5 tons	32,500 BTU/h
Leaky and poorly insulated instead	4.5 tons	84,500 BTU/h
Spread between those two	2.0 tons	160%

What this assumes

This is a load approximation, not an ACCA Manual J calculation.
Climate zone factors follow the IECC climate zone map.
Assumes an average building with no unusual solar gain, air leakage or zoning requirements.

Important

A proper equipment selection requires an ACCA Manual J load calculation, Manual S equipment selection and Manual D duct design. Use this to sanity-check a quote, not to order equipment. If a contractor sizes a system by square footage alone, that is a warning sign.

Understanding the result

Oversized equipment performs worse

An oversized air conditioner cools the air to setpoint before it has run long enough to remove humidity, then shuts off. The result is a house that is cold and clammy, with short cycling that wears the compressor and raises the bill. Bigger is actively worse in cooling.

The rule of thumb and why it fails

The old rule — roughly 500 to 600 square feet per ton — ignores insulation, window area and orientation, air leakage, ceiling height and climate. A well-sealed new house and a 1950s house of identical size can differ by more than a full ton.

What Manual J adds

A Manual J calculation measures your actual building: wall and attic R-values, window U-factor and solar heat gain coefficient by orientation, infiltration, duct location and internal gains. Reputable contractors run one before quoting a replacement, and many jurisdictions require it with the permit.

Common questions

How many tons of AC do I need for 2,000 square feet?

Typically between 2.5 and 4 tons depending on climate zone, insulation and window area. That spread is exactly why square-footage rules of thumb are unreliable and why a Manual J calculation is worth insisting on.

Should I replace my system with the same size?

Not automatically. The existing system may have been oversized originally, and if you have added insulation or replaced windows, your load is lower now. Have the load calculated rather than matching the old nameplate.

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The next decision usually follows directly from this one.

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