Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate central air/heat system size in tons and BTU/hr from your home's square footage.
Central heating and cooling systems are conventionally sized in "tons" (a unit borrowed from ice-melting capacity, historically), and a common rule-of-thumb sqft-per-ton ratio gets you in the right ballpark before a professional Manual J load calculation confirms the exact size. This tool also directly covers "AC size" questions, since central cooling sizing uses the same tonnage rule of thumb.
Enter your home or room's area and a sqft-per-ton ratio -- roughly 500 for hot/humid climates, 600 for moderate climates, and 700+ for mild climates or very well-insulated homes.
Tons = Area ÷ Sqft per Ton
BTU/hr = Tons × 12,000
1 ton of cooling capacity equals exactly 12,000 BTU per hour, by definition.
This rule-of-thumb sizing is a starting estimate, not a substitute for a professional Manual J load calculation, which accounts for insulation R-values, window count/orientation, ceiling height, local climate design temperatures, and air infiltration -- an undersized or oversized system both cause real comfort and efficiency problems.
The ratio you enter is the biggest lever in this tool. A smaller number means a smaller area per ton, so more capacity: 500 gives a larger system than 700 for the same house. Choose a lower number if the home has lots of west-facing glass, a poorly insulated attic, high ceilings, or several occupants and heat-producing appliances. Choose a higher number if the house is tight, well insulated, shaded, or in a mild climate. Since the ratio is a rule of thumb, it is smart to run the calculator at a couple of values and look at the range instead of a single answer.
Hypothetical example: a 1,800 sq ft home. At 500 sq ft per ton the result is 3.6 tons (43,200 BTU/hr). At 600 it is 3.0 tons (36,000 BTU/hr). At 700 it is 2.57 tons (30,857 BTU/hr). The spread is wide, which is exactly why a professional load calculation is valuable before buying equipment.
Residential equipment is sold in fixed sizes, often in half-ton steps, so a result like 3.33 tons will not match a real unit exactly. Do not round up automatically. A modestly smaller unit that runs longer cycles usually removes humidity better than a larger one that shuts off quickly. Discuss the choice with the contractor, who will use load numbers to decide between the two nearest sizes.
Use this tool to sanity-check a quote. If a contractor proposes a unit far larger than your result at a reasonable ratio, ask how the load was calculated and whether a Manual J was performed. A good contractor will explain their sizing in writing. If you are only cooling a single room or addition, this whole-home approach is not suitable, and the BTU calculator for a room is a better fit.
This tool reports cooling capacity in tons and BTU/hr. Heating needs depend on winter temperatures and insulation, so heating equipment may be sized differently, particularly for a furnace or a heat pump in a cold region. Ask an HVAC professional to size both for your climate.
Climate severity, insulation quality, window area, and ceiling height all significantly affect real cooling/heating load per square foot -- there's no single ratio that fits every home.
No -- an oversized system cools or heats too quickly and cycles on/off frequently (short-cycling), which hurts both comfort (poor humidity control) and long-term equipment life. Proper sizing matters in both directions.
Not automatically. Oversized equipment short-cycles and can leave the home humid. Discuss the nearest sizes with a qualified contractor.
It is meant for whole-home or large-area estimates. For one room, the BTU calculator is more appropriate.
One ton of cooling equals 12,000 BTU per hour.
Only as a rough starting point. Heat pump sizing depends on heating load as well, so get a professional load calculation.