AC Size Calculator

The calculator first estimates your cooling load in BTU/hr, then converts that load to AC tonnage and rounds to the nearest half ton — the increment equipment is actually sold in. Rather than a single flat square-foot ratio, it uses a per-climate-zone BTU rate adjusted for insulation, sun, ceiling height, and occupants. For a deeper dive on the conversion step, see the tonnage calculator.

Not sure whether your existing system is the right size? Read what size AC do I need for a plain-English walkthrough of the trade-offs.

The table pairs each AC size with the home it typically serves at average insulation. The mild column reflects IECC zones 4–5; the hot column reflects zones 1–2, where the same square footage needs more capacity. Use it to sanity-check the calculator's result above.

Most sizing errors come from trusting a rule of thumb too far or from padding the number "to be safe." The symptoms of an oversized unit are easy to miss until you live with them — see oversized AC symptoms if your rooms feel cold but humid.

• Sizing on square footage alone — it ignores climate zone, which alone can shift the answer by 30% or more.
• Oversizing 'just to be safe' — the unit short-cycles, fails to dehumidify, and costs more to buy and run.
• Forgetting ceiling height — a 10-foot ceiling adds ~25% volume over an 8-foot baseline.
• Ignoring sun and glass — large unshaded west-facing windows can add 10–20% to the cooling load.
• Sizing to the record-hot day instead of the design day — that forces permanent oversizing for 8,600+ hours a year.
• Replacing a unit 'with the same size' without checking — the original may have been oversized too.

The "ton" in central air conditioning is a unit of heat-removal rate, not weight: one ton equals 12,000 BTU/hr, the energy needed to melt a ton of ice over 24 hours. A 3-ton system therefore moves 36,000 BTU/hr of heat out of your house on the design day. That rated capacity is published by AHRI (the Air-Conditioning, Heating, and Refrigeration Institute) for a specific outdoor and indoor condition, so two units with the same nominal tonnage can deliver slightly different real-world capacity depending on coil match and refrigerant charge.

Tonnage is closely tied to airflow but is not the same thing. The long-standing design target is roughly 400 CFM of air per ton at the evaporator coil — so a 3-ton system is engineered around about 1,200 CFM. If the duct system, blower, or filter cannot move that air, the coil's rated capacity never reaches the room, which is why ACCA Manual D duct design matters as much as picking the right tons. Sizing the box correctly and then starving it of airflow produces the same complaints as buying the wrong size in the first place.

A central system is two pieces working as one: the outdoor condenser and the indoor evaporator coil (paired with a furnace or air handler). You should never buy a condenser and an indoor coil at random. Instead, choose an AHRI-matched system — a combination that AHRI has tested together and listed with a certificate number. The matched listing is what determines the published tonnage and the efficiency rating, and most manufacturer warranties and utility rebates require it.

Efficiency is now expressed as SEER2, the seasonal rating measured under the U.S. Department of Energy's 2023 M1 test procedure, which uses higher external static pressure than the old SEER test and so produces slightly lower numbers for the same hardware. SEER2 affects your operating cost, not your tonnage: a 3-ton 14.3-SEER2 unit and a 3-ton 18-SEER2 unit remove the same 36,000 BTU/hr, but the higher-rated one uses less electricity to do it. The federal minimum SEER2 differs by region, with the South and Southwest held to stricter floors than the North, so confirm the current minimum for your area before comparing quotes. EPA ENERGY STAR tiers sit above those minimums for buyers who want to pursue tax credits.

• Verify the outdoor unit and indoor coil share an AHRI certificate — mismatched coils can lose 5–10% of rated capacity.
• Compare equipment at the same tonnage first, then weigh SEER2 for running cost — never trade comfort sizing for an efficiency number.
• Ask for the Manual J load and the Manual S equipment-selection sheet, not just a model number.
• Check the regional minimum SEER2 and any ENERGY STAR / DOE rebate tiers before signing.

The single most consequential decision in this whole process is whether to size honestly or pad "for headroom." ACCA Manual J produces a calculated cooling load; ACCA Manual S then selects equipment whose AHRI capacity at your design condition is within a tight window above that load — generally no more than about 15% over for a cooling-only system. Padding beyond that is where comfort problems begin. An oversized condenser satisfies the thermostat's temperature setpoint in a few minutes, shuts off, and never runs long enough to wring moisture out of the air, so you get short cycling and the cold-but-clammy feeling that drives people to set the thermostat even lower.

Short cycling also wears the compressor through repeated starts, widens room-to-room temperature swings, and can raise bills despite a higher SEER2 rating, because the equipment rarely reaches steady-state efficiency. A right-sized system, by contrast, runs long, steady cycles on hot afternoons — often 80–90% of the hour on the design day — which is exactly what keeps indoor humidity in the comfortable 45–55% range.

The inputs you entered above interact, which is why a flat square-foot rule fails: more conditioned area raises the load, but a hotter climate zone, weaker insulation (lower R-value walls and attic), higher-U-value windows, and strong west-facing sun each multiply it, while shade and tight construction pull it back down. Two 2,000-square-foot homes can legitimately need a half ton apart. Use this estimate to budget and to challenge an oversized quote, then confirm the final number with a room-by-room Manual J. If you only remember one thing, remember the conversion: divide the load by 12,000 to get tons, round to the nearest half ton, and resist the urge to round up "to be safe." The tonnage calculator handles that last step, and oversized AC symptoms shows what padding actually feels like once you live with it.