What Size AC Do I Need? Load and Capacity Checks

You need an air conditioner whose delivered capacity at your design conditions is within about 95% to 115% of the house's calculated cooling load, with its sensible and latent capacity each covering their share. Convert the load to tons by dividing BTU/h by 12,000, but treat tons as a unit of load, not a sizing rule: the DOE's two modeled 2,223 ft² houses needed 1.7 tons each, where a 500 ft² per ton chart would say 4.4.

If all you have is square footage

Area charts exist and are worth understanding, because they show how much a single assumption swings the answer. The shortcut is quoted two ways that describe the same division: 400 to 600 ft² per ton, or roughly 20 to 30 BTU/h per ft². Since a ton is 12,000 BTU/h, the two forms are linked.

The shortcut can mislead badly. The U.S. Department of Energy's Building America sizing guide models two one-story, 2,223 ft² houses built to the 2009 IECC and reports their Manual J cooling loads: 20,610 BTU/h in Chicago and 20,690 BTU/h in Orlando. That is 9.3 BTU/h per ft², about 1.7 tons each. A 500 ft² per ton rule would call for 2,223 ÷ 500 = 4.4 tons. Houses with older construction can run higher, but you cannot know which side yours falls on without the envelope.

How much the non-area inputs move a 1,600 ft² house

This site's simplified per-area method, the one behind the BTU calculator, applied to a 1,600 ft² house in IECC Zone 4 with 8 ft ceilings and three occupants, with only insulation and sun changing:

The corners are 25,800 and 60,600 BTU/h: 2.15 to 5.05 tons for one set of floor dimensions. Moving from poor to good insulation at average sun removes 50,600 − 34,600 = 16,000 BTU/h, more than a full ton. If a quote proposes 4 tons for this house, the first question is which row and column the contractor assumed. The tonnage calculator converts any of these loads to tons.

The two-system trap

Assign floor area to the system that actually serves it. For a 2,400 ft² house split evenly between an upstairs and a downstairs system, 500 ft² per ton on each 1,200 ft² gives 2.4 tons per system, 4.8 combined. Applying the whole 2,400 ft² to each system gives 4.8 tons each and doubles the combined figure to 9.6. Do not count an attic floor because ducts pass through it, and remember that a roof-exposed upstairs and a shaded downstairs rarely share one load per square foot.

Check a candidate unit against all three loads

Use the Chicago house's published Manual J result: 20,610 BTU/h total cooling, made of 17,370 sensible and 3,240 latent. Divided by 12,000 that is 1.72 tons of load, which is the starting point for a 2-ton search, not an instruction to buy a 2-ton unit. Manual S, as the DOE guide describes it, keeps cooling capacity from exceeding the total load by more than 15%, and designers commonly also hold it above about 95%. For this load that window is 19,580 to 23,702 BTU/h of delivered total capacity at design conditions.

A candidate inside the window can still fail, because it must also remove 17,370 sensible and 3,240 latent on its own. Take two candidates whose expanded manufacturer tables (assumed values for the arithmetic) read like this at your outdoor and indoor conditions:

The second unit needs a different coil or airflow selection, or a deliberate humidity-control plan. Heat pumps follow different limits, up to about 125% of the heating load in heating-dominated climates, which the heat pump sizing guide works through. The DOE guide also admits a real-world wrinkle: in energy-efficient houses, a manufacturer's next size up can exceed 15% (its own example package is 17% over at 750 CFM), and the larger unit should still be chosen so the peak load is met. If a 19,000 BTU/h load can be served only by 17,500 or 23,400 BTU/h units, the guide says to select the 23,400 and consider multispeed equipment for part-load hours.

Tons are not a rounding rule

If a calculation produces 31,200 BTU/h, its load is 31,200 ÷ 12,000 = 2.60 tons. Keep that figure through selection instead of rounding it to a purchase size: a nominal 2.5-ton model and a 3-ton model each deserve a performance-table check. A nominal 3-ton unit's actual capacity depends on outdoor temperature, entering air conditions, the matched coil, and airflow. Capacity and efficiency also answer different questions. SEER2 tells you cooling per unit of energy; it does not tell you how many BTU/h your house needs or how much moisture the unit removes. For other units, 1 W is about 3.412 BTU/h, and the unit converter covers the rest.

Ducts and airflow are part of the size

Ask for measured external static pressure and confirmation that the existing blower and ducts can deliver the proposed airflow before accepting a bigger unit. At standard air conditions, sensible transfer is about 1.08 × CFM × (room − supply temperature). A room with a 3,240 BTU/h sensible load and a 20°F difference needs 3,240 ÷ (1.08 × 20) = 150 CFM. The Chicago house's target is 750 CFM for a 2-ton nominal size, about 375 CFM per ton. A hot back bedroom is usually an airflow problem, and a duct fix often does more than a larger compressor.

• Matching the old unit. Windows, insulation, and sealing may have changed since it was installed, and it may have been too big.
• Rounding 2.6 tons up to 3. That is 3.0 ÷ 2.6 = 115% of the load before any capacity derating.
• Treating tons as floor area. A ton is 12,000 BTU/h of heat removal, nothing more.
• Buying capacity to fix a duct problem. Extra tonnage can leave the house colder, damper, and still uneven.

Oversized single-stage systems satisfy the thermostat quickly and then sit idle, and moisture removal depends on run time, so short cycles can leave a house cool but humid; see signs your AC is oversized. Undersizing is a separate risk, but long afternoon runtime alone is not proof of it; check airflow, coil, charge, duct leakage, and thermostat accuracy first.

What to ask for before you order

The Manual J input summary with room loads, the Manual S capacity comparison for the exact indoor and outdoor models, and an airflow plan. Before a site visit, gather measured rooms and ceiling heights, window sizes and orientation, window U-factor and SHGC labels, insulation and any blower-door or duct-leakage records, and notes on which rooms are hot or damp. The report checklist shows how to review what comes back. If a contractor quotes without a calculation, how to verify a quote explains how to test the number.

Sources: DOE Building America, Strategy Guideline: HVAC Equipment Sizing (Manual S limits, Chicago and Orlando houses); ACCA Manual S for the selection procedure.