AC sizing is one of those things people consistently get wrong in both directions. Pick a unit too small and the house never cools off on a 95°F day; pick one too big and it short-cycles, leaves the air humid, and burns out the compressor in a third of its rated life. Here is the rule of thumb that gets you close, plus when you need a real Manual J.

20 BTU per square foot — the starting point

The simplest cooling load estimate is 20 BTU per square foot of conditioned space for moderate-climate residential construction. A 200 ft² bedroom needs about 4,000 BTU/hr of cooling — call it a 5,000 BTU window unit or part of a 1-ton mini-split. A whole 1,500 ft² ranch needs about 30,000 BTU/hr → a 2.5 ton system.

The BTU calculator starts here and adjusts for sun and occupancy.

Adjustments to the 20-BTU rule

The base rule is calibrated for an average room with two occupants, normal sun exposure, and standard insulation. Real adjustments:

  • Heavily shaded room: drop 10%
  • Very sunny room (south or west exposure with minimal shade): add 10%
  • Each occupant above 2: add 600 BTU/hr (typical body heat output is 250-400 BTU/hr at rest; 600 covers active cooking, exercise, or kids)
  • Kitchen: add 4,000 BTU/hr if cooking regularly with the AC running (heat from the range, oven, dishwasher)
  • Ceilings over 8 ft: scale base BTU/ft² proportionally — 10 ft ceilings need 25 BTU/ft²

Tons in HVAC: 12,000 BTU/hr = 1 ton of cooling. Common residential sizes are 1.5, 2, 2.5, 3, 3.5, 4, and 5 ton.

Manual J — when the rule isn't enough

ACCA Manual J is the industry-standard residential load calculation. It accounts for:

  • Wall U-values and insulation R-values
  • Window areas, orientations, and glass types (low-E, triple-pane, etc.)
  • Air infiltration rates (tight new construction vs leaky 1950s ranch)
  • Internal heat gain (lights, appliances, occupants)
  • Local design temperature (the 1% peak summer day published by ASHRAE for your county)
  • Solar gain by hour and orientation
  • Latent load (humidity removal)

Most permits in production housing now require Manual J on file, and many high-efficiency rebate programs require it. If you're sizing a system for a heavily glazed modern home, an old leaky house, a humid coastal climate, or a dry mountain climate — get a Manual J done. The 20-BTU rule misses by 30-50% in those cases.

Bigger isn't safer

Oversizing is the most common HVAC mistake homeowners make. A unit that's too big:

  • Short-cycles (cools the air faster than it can pull humidity out — house feels clammy)
  • Wears compressors and contactors faster — every start is 3-5x the running current
  • Leaves "stratified" air, hot ceilings, cold floors
  • Costs more to install and run than a properly sized unit

Right-sized AC runs longer at lower speeds, pulling more humidity out and giving more even temperatures. Variable- speed equipment (inverters, two-stage compressors) makes this even more forgiving.

Common errors

Sizing only by square footage. A 200 ft² sunroom with three exterior walls and a south-facing glass wall is not equivalent to a 200 ft² interior bedroom. Sun exposure matters as much as size.

Forgetting humidity. AC pulls heat AND water out of the air. A unit that hits temperature in 5 minutes hasn't pulled the humidity out yet — house feels cold and damp. Humid climates need run time more than tonnage.

Using "cooling capacity" from a thermostat. Smart thermostats sometimes display estimated load. Those numbers come from runtime data on the existing system and are biased by whatever the existing unit happens to be — they don't tell you the right size, just the historical load.

Quick FAQ

What size AC for a 1,500 sq ft house? Roughly 30,000 BTU/hr (2.5 ton) for moderate climate, no unusual sun or insulation issues. Run a Manual J for anything tight or unusual.

Can a window unit cool a whole house? Only if the house is very small (under 600 ft²) and very open. Most window units max out at 25,000 BTU/hr (≈ 2 ton) and only cool the room they're in plus some bleed.

Is a heat pump sized the same as AC? Cooling-side yes. Heating-side no — heat pumps lose capacity as outdoor temperature drops, so cold-climate sizing requires a heat-pump-specific load calc and often auxiliary electric strip heat or backup furnace.

Estimate only. The BTU calculator uses the 20 BTU/ft² rule of thumb. For permits, rebates, and tight specs run a full ACCA Manual J calculation through a licensed HVAC contractor.