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HVAC Size Calculator

Rough AC sizing by square footage and climate zone — get BTU load and tonnage as a planning estimate before you call a contractor.

HVAC Size Calculator

Rough AC sizing by square footage and climate zone — get BTU load and tonnage as a planning estimate before you call a contractor.

This free HVAC size calculator gives a rule-of-thumb cooling estimate: 24 BTU per sq ft in hot climates, 20 in moderate, 17 in cool. It converts the load to tons and the nearest standard half-ton unit size. This is a planning number only — real sizing requires a Manual J load calculation.

Rough estimate only. This uses a square-footage rule of thumb. Correct sizing requires an ACCA Manual J load calculation by a licensed contractor — insulation, windows, orientation, and ductwork all matter.
Disclaimer: Rough planning estimate only. Correct HVAC sizing requires an ACCA Manual J load calculation performed by a licensed contractor — insulation, windows, orientation, and ductwork all change the answer.

How to use this calculator

  1. Step 1: Enter your home square footage (conditioned space only — do not count the garage).
  2. Step 2: Pick your climate zone — hot (South FL, TX, AZ), moderate, or cool/northern.
  3. Step 3: Click Calculate to see the BTU cooling load, exact tonnage, and nearest standard half-ton unit.
  4. Step 4: Use the number to sanity-check contractor quotes, then get a Manual J before buying.

The formula

BTU = sq ft x 24 (hot) / 20 (moderate) / 17 (cool). Tons = BTU / 12,000, rounded to nearest half-ton.

Worked example

A 2,000 sq ft home in a moderate climate needs about 40,000 BTU, which is 3.33 tons — so a 3.5-ton unit. The same home in South Florida (hot zone) needs 48,000 BTU, exactly 4 tons.

Frequently Asked Questions

What does a ton mean in air conditioning?

One ton of cooling equals 12,000 BTU per hour — the heat needed to melt one ton of ice in 24 hours, a definition from the early days of refrigeration. It has nothing to do with the weight of the unit. A 3.5-ton system therefore removes 42,000 BTU of heat per hour.

Is it better to buy a bigger AC than I need?

No — oversized is worse than slightly undersized. An oversized unit cools the air so fast it shuts off before removing humidity, leaving you cold and clammy, and the constant short-cycling wears out the compressor. Slightly undersized just runs longer on the hottest days, which actually dehumidifies better.

What is a Manual J load calculation?

Manual J is the ACCA (Air Conditioning Contractors of America) method for properly sizing HVAC equipment. It accounts for insulation levels, window sizes and orientation, ceiling height, air leakage, ductwork, and local climate — not just square footage. Any reputable contractor should perform one before recommending a unit size.

Does insulation change what size AC I need?

A lot. The 20 BTU per sq ft rule of thumb assumes average construction. A well-insulated, tightly sealed home with good windows may need 15-25% less capacity, while a poorly insulated older home with single-pane windows can need 25% more. That is exactly why the rule of thumb is only a starting point.