AC Tonnage Calculator
Estimate the central air-conditioning size your home needs. Enter conditioned square footage, climate zone and a few envelope details, and get the cooling load in BTU/hr and the recommended unit size in tons.
Recommended AC size
3.5 tons
- Calculated cooling load (tons)
- 3.23
- Total cooling load (BTU/hr)
- 38,700
- Base load (BTU/hr)
- 37,500
- Ceiling-height multiplier
- 1
- Insulation multiplier
- 1
- Sun-exposure multiplier
- 1
- Occupant adjustment (BTU/hr)
- 1,200
Cooling load = square feet × climate BTU/sqft × ceiling factor × insulation factor × sun factor, plus 600 BTU/hr for every occupant beyond two. Climate factors: 20 (cool), 25 (moderate), 30 (warm), 35 (hot) BTU/hr per sq ft. Ceiling factor is your ceiling height ÷ 8 ft (the residential baseline). Insulation: 1.15 poor, 1.00 average, 0.90 good. Sun: 0.90 shaded, 1.00 normal, 1.10 sunny. Divide total BTU/hr by 12,000 to get tons, then round up to the next 0.5-ton unit size (residential central AC ships in 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0 ton sizes). This is the ENERGY STAR / ACCA simplified sizing — use Manual J for a final installation.
How to use this calculator
Enter the conditioned floor area in square feet — that is the part of the home the AC actually cools (so exclude an unconditioned garage, attic or basement). A typical US single-family home is 1,500 to 2,500 sq ft. Pick the climate zone that best matches where the home is: cool for the Northern US and Canada, moderate for the mid-Atlantic and Midwest, warm for the Southern US, and hot for South Florida, South Texas and Arizona. Set the ceiling height in feet — 8 ft is the residential standard, but vaulted or coffered ceilings can be 10 to 12 ft and that meaningfully changes the load because cooling scales with conditioned volume, not just floor area. Pick the insulation quality: poor for older homes with original windows and minimal attic insulation, average for typical existing construction, good for a modern build with air-sealing and a current-code envelope. Pick the sun exposure: shaded for homes under heavy tree cover or with mostly north-facing glass, normal for the average house, sunny for homes with large west-facing windows and little shade. Enter the number of people who regularly use the space — the baseline assumes two, and every additional regular occupant adds 600 BTU/hr.
How the calculation works
Base cooling load is square footage × a climate factor in BTU/hr per sq ft: 20 for cool, 25 for moderate, 30 for warm, 35 for hot, from the Trane, Carrier, Lennox and ENERGY STAR public sizing tables (these roughly map to IECC climate zones 5–7, 4, 3 and 1–2 respectively). The base is then multiplied by a ceiling-height factor (your ceiling height divided by 8 ft, because cooling load scales with conditioned volume not floor area), an insulation factor (1.15 poor, 1.00 average, 0.90 good per the ASHRAE Handbook of Fundamentals residential cooling-load chapter), and a sun-exposure factor (0.90 shaded, 1.00 normal, 1.10 sunny, the same ±10% adjustment ENERGY STAR uses for room ACs). Finally, 600 BTU/hr is added for every regular occupant beyond two. The result is divided by 12,000 BTU/hr — the definition of one refrigeration ton (the heat absorbed melting a short ton of ice in 24 hours) — to get tons, and rounded up to the next 0.5-ton increment because residential split systems are sold in 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 and 5.0 ton sizes. This is the published rule-of-thumb sizing; for a final installation an HVAC contractor should run an ACCA Manual J calculation that accounts for window U-values, infiltration and duct losses.
Worked example
A 2,000 sq ft single-family home in Atlanta (warm climate), with 8 ft ceilings, average insulation, normal sun exposure and a family of four. Base load = 2,000 × 30 = 60,000 BTU/hr. Ceiling factor = 8 ÷ 8 = 1.00. Insulation factor = 1.00. Sun factor = 1.00. Envelope load = 60,000 × 1.00 × 1.00 × 1.00 = 60,000 BTU/hr. Occupant adjustment = (4 − 2) × 600 = 1,200 BTU/hr. Total = 61,200 BTU/hr ≈ 5.1 tons. Round up to the next 0.5-ton unit and you would specify a 5.0-ton system (the practical residential ceiling; anything larger usually becomes a two-stage or zoned install). That matches the rule-of-thumb 400 sq ft per ton you see in HVAC contractor guides for the warm climate band.
Frequently asked questions
Why is the AC tonnage calculator different from a BTU calculator?
They answer related but different questions. A BTU calculator is the ENERGY STAR room-AC sizing — you point it at one room and it sizes a window unit in BTU/hr, with adjustments for kitchen heat and a few extra occupants. An AC tonnage calculator sizes a central, ducted system for a whole home or zone in tons, with adjustments for climate, ceiling volume, envelope quality and overall sun exposure. The base BTU/sqft figure is higher for central systems because duct losses and whole-home infiltration are baked in. Use BTU for a window unit, tons for a central split system.
What does "1 ton" actually mean for an AC unit?
It is a unit of cooling capacity defined as 12,000 BTU/hr — the average rate of heat absorbed when one short ton (2,000 lb) of ice melts over 24 hours. It is a leftover from the days when commercial buildings were literally cooled with delivered blocks of ice, and the industry never bothered to switch. A "3-ton AC" can remove 36,000 BTU of heat per hour from a space, regardless of whether it actually weighs anything close to three tons.
Can I just use square feet ÷ 500 as a rule of thumb?
It is the rough national-average shortcut and gets you within ±0.5 ton of this calculator on a typical moderate-climate home with average insulation and an 8 ft ceiling. It falls apart at the extremes. In Phoenix the right rule is closer to sq ft ÷ 350, and in Minneapolis it is closer to sq ft ÷ 600. The full formula here applies a climate factor and accounts for ceiling volume, insulation, sun and occupants, so you get a closer-to-real answer in those edge cases. For a quick gut-check on a typical mid-Atlantic home, sq ft ÷ 500 is fine; for a final spec, run the proper calculation.
Is bigger always better when sizing AC?
No — oversizing is one of the most common HVAC mistakes. An oversized unit cools the air to thermostat quickly but does not run long enough to remove humidity, so the house feels clammy. It also short-cycles (turns on and off rapidly), which wastes energy and shortens compressor life. ACCA and ENERGY STAR both recommend sizing to within ±15% of the calculated load, not "round up to be safe". If this calculator says you need 3.0 tons, a 3.5-ton unit is fine but a 4.0-ton unit is probably too big.
Does this account for ductwork or installation quality?
Not directly. The climate BTU/sqft figures used here already include a typical duct-loss assumption (around 15% — standard practice for residential central AC sizing). If your ducts are routed entirely through unconditioned attic space, are leaky, or are undersized for the airflow, the real load can be 25 to 40% higher than this estimate. An ACCA Manual J calculation paired with a Manual D duct design is the right way to account for that on a real installation. Treat this calculator as the starting point for a contractor conversation, not the final spec.
How accurate is this estimate compared to Manual J?
For a typical existing home with reasonably matched inputs (correct square footage, honest climate zone, honest insulation), this calculator lands within roughly ±0.5 ton of a properly run ACCA Manual J calculation on most homes — close enough to choose between the standard 0.5-ton size increments residential systems ship in. It diverges from Manual J most on homes with very large glass areas, unusual orientations, very low or very high infiltration, or significant internal gains (home offices, server rooms, professional kitchens). For those, a real Manual J is worth the small fee an HVAC contractor charges.