“What size AC do I need” is the most-asked HVAC question in residential. The answer is in tons — the unit AC equipment is rated in and the unit your contractor will quote against. One ton equals 12,000 BTU/hr of cooling, the heat removed by melting one ton of ice over 24 hours. Here's how to size it without paying for a Manual J on a 1,500 ft² house.

The tonnage formula

The residential rule of thumb for cooling load:

tons = (ft² × 20 × sun_adj + occupant_extra) ÷ 12,000

Example: a 1,500 ft² ranch with normal sun and 4 occupants. Base = 1,500 × 20 = 30,000 BTU/hr. Occupant extra = (4 - 2) × 600 = 1,200. Total = 31,200 BTU/hr ÷ 12,000 = 2.6 tons. Round up to a 3.0 ton unit (the next standard residential size).

The AC tonnage calculator runs this and snaps to the nearest standard size — 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, or 5.0 tons.

Standard residential AC sizes

Residential AC units come in fixed sizes. There's no “2.7 ton” — the manufacturers ship in 0.5-ton increments:

  • 1.5 ton (18,000 BTU/hr): Apartments, small condos, 600-900 ft² zones
  • 2.0 ton (24,000 BTU/hr): Small ranch homes, 900-1,200 ft²
  • 2.5 ton (30,000 BTU/hr): Standard 3-bedroom, 1,200-1,500 ft²
  • 3.0 ton (36,000 BTU/hr): Larger 3-bedroom or modest 4-bedroom, 1,500-1,800 ft²
  • 3.5 ton (42,000 BTU/hr): 1,800-2,100 ft²
  • 4.0 ton (48,000 BTU/hr): 2,100-2,400 ft²
  • 5.0 ton (60,000 BTU/hr): 2,400-3,000 ft²

Anything over 5 tons in residential is unusual and almost always means two smaller units zoned (one upstairs, one down) instead of one big unit.

Why oversizing is worse than slight undersizing

DIYers and bad contractors over-size AC because they think bigger means better. It's the opposite. An oversized AC:

  • Short-cycles — runs 5-10 minutes, hits the thermostat target, shuts off, then turns back on 15 minutes later. Each cycle stresses the compressor and burns starting electricity.
  • Doesn't dehumidify — AC removes humidity primarily through long, steady runs that let the coil get cold enough to condense moisture. Short cycles leave the house cold and clammy.
  • Costs more upfront and to run — bigger compressor, bigger electrical service, more refrigerant, higher monthly bill.
  • Wears out faster — short-cycling cuts compressor life by 30-50%.

A correctly sized unit runs 70-80% of the hottest day, dehumidifies properly, and lasts 12-18 years. An oversized unit runs 30-40% of the same day and dies at 8-10 years.

When the rule of thumb breaks

The 20 BTU/ft² rule assumes a typical post-1990 house with 8 ft ceilings, average insulation, and standard window count. Adjust if your house is unusual:

  • Cathedral or vaulted ceilings: Add 10-20% — more air to cool
  • Heavy west-facing glass: Add 10% beyond the “sunny” setting
  • Old, leaky 1950s house: Add 15% — uncontrolled infiltration
  • Tight modern house with foam insulation: Subtract 10-15%
  • Kitchen-only zone: Add 1,200 BTU for major appliances

For any of these or for whole-house systems above 3 tons, an ACCA Manual J load calculation by an HVAC contractor is the right answer. The Manual J accounts for actual envelope U-values, window orientation, infiltration, and internal loads. Cost is typically $200-500 — pays for itself if it changes the equipment size by half a ton.

What pros do differently

Run Manual J on jobs over $5,000. Anything where the equipment cost justifies a $300 calc, get the calc. Wrong sizing is the #1 reason residential AC dies early.

Match the air handler.The condenser tonnage must match the air handler's rated airflow (typically 400 CFM per ton) — mismatched components run inefficiently and void the manufacturer warranty.

Verify ductwork can handle the airflow. A new 3-ton unit needs about 1,200 CFM through the ducts. Old ductwork designed for a 2-ton unit will starve a 3-ton install. Run the duct CFM calculator against the existing ducts before upgrading equipment.

Quick FAQ

What size AC for 2,000 ft²? ~3.5 tons (42,000 BTU/hr) at normal sun and standard occupancy. Can be 3.0 if shaded and tightly insulated; 4.0 if very sunny.

How many BTU per ton?12,000 BTU/hr per ton. A “2-ton AC” is 24,000 BTU/hr.

Can I use a smaller AC and run it longer? Up to about 10% undersized, yes — and the dehumidification improves. Past that, the unit can't reach setpoint on the hottest days and runs continuously.

Heating side too? Run the furnace size calculator for the heating output. Cooling and heating loads are different — most houses don't need the same tonnage on both sides of the year.