AC TONNAGE
About this calculator
This AC tonnage calculator returns the cooling capacity needed for a room or whole house, expressed in tons (the unit HVAC contractors quote and that residential AC equipment is rated in). 1 ton = 12,000 BTU/hr of cooling. The math is the standard residential rule of thumb — 20 BTU per square foot of conditioned area, adjusted for sun exposure and occupant count, divided by 12,000 to convert to tons. For a tighter spec on equipment over 3 tons or houses with unusual envelope construction, an HVAC contractor should run a full Manual J load calculation. Tonnage sizing follows ACCA Manual J (load) and Manual S (equipment selection).
How to use this calculator
Enter the conditioned floor area in ft². Pick sun exposure (heavily shaded north-facing areas need less cooling; south/west-facing with big windows need more). Set typical occupant count — every person above 2 adds 600 BTU/hr to the cooling load.
The calculator returns cooling capacity in tons (1 ton = 12,000 BTU/hr) plus the next standard equipment size (1.5, 2, 2.5, 3, 3.5, 4, 5 ton). Use this for whole-house central AC sizing planning. For individual rooms or window units, use the BTU calculator instead. For tight specs on systems over 3 tons or modern tight homes, get an HVAC contractor to run a full Manual J.
Worked example
For a 1,500 ft² home with normal sun exposure and 4 occupants:
Base load: 1,500 × 20 = 30,000 BTU/hr. Sun: no adjustment. Occupants: (4 - 2) × 600 = 1,200 BTU. Total: 31,200 BTU/hr = 2.6 tons.
Suggested unit: 3.0 ton (next standard size up). Equipment cost: $3,500-$6,000 for the AC condenser alone, $5,500-$9,500 installed with new line set and electrical.
For a 2,500 ft² home with very sunny exposure (top floor, west-facing) and 5 occupants:
Base: 2,500 × 20 = 50,000. Sunny multiplier ×1.10: 55,000. Occupants: (5-2) × 600 = 1,800. Total: 56,800 BTU = 4.7 tons → 5 ton.
For a 3,500 ft² home: 70,000 BTU base = 5.8 tons. Above 5 tons triggers a "consider zoning" recommendation — most residential AC tops out at 5 ton, larger homes typically use two systems or a multi-zone setup.
Common mistakes & waste factors
Oversizing. The 20 BTU/ft² rule was developed for 1980s-era leaky homes. Modern code-built tight homes often need 600-1,000 ft²/ton — half the cooling capacity. Get a Manual J done before buying.
Ignoring sun exposure. A south-facing 1,500 ft² home with floor-to-ceiling windows can need 30-40% more cooling than a north-facing version. The "sunny" adjustment in the calculator is a 10% bump — push higher for extreme exposure.
Forgetting the kitchen. Kitchens add 4,000+ BTU/hr from cooking, fridges, dishwashers. Whole-house tonnage that ignores kitchen heat-load gets cool-bedroom/hot-kitchen problems.
Matching equipment to design temp instead of typical conditions. Sizing for the 99% design temp (the worst 4 days a year) means oversized equipment that short-cycles 360 days a year. Right-sized equipment runs 80-90% of design hours and dehumidifies properly.
Rules of thumb
20 BTU/hr per ft² is the legacy rule. Modern tight homes: 12-15 BTU/ft².
12,000 BTU/hr = 1 ton of cooling.
Standard residential sizes: 1.5, 2, 2.5, 3, 3.5, 4, 5 ton. Above 5 ton, two units or zoning are typical.
Sun adjustment: ±10% standard, ±20% for extreme south/west exposure or top-floor units.
Occupants: +600 BTU/hr per person above 2.
Kitchen heat: +4,000 BTU/hr.
A properly-sized AC runs 80-90% of design hours during peak season and dehumidifies adequately. Oversized AC short-cycles and leaves the air clammy.
Common questions
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