# AC Unit Size Calculator — What Size Air Conditioner (Tonnage) Do I Need? > Free AC tonnage calculator: enter your home's square footage and climate zone to size a cooling-only central air conditioner in tons, BTU/h and kW, see the nearest standard unit, and sanity-check a contractor's quote for oversizing — metric or imperial, in your browser. What size central air conditioner do you need? Enter your home's area for a cooling-only, whole-home tonnage estimate — a rule-of-thumb starting point, not a Manual-J load calculation. - **HTML page:** https://siteometry.com/en/tools/ac-unit-size-calculator - **Category:** [Decorating & Interiors](https://siteometry.com/en/tools/category/decorating-interiors) - **Figures live on the page:** tables, diagrams and worked figures the text refers to ("the table below") are drawn by the page and are not reproduced here. Each section below links to its anchor on the page. - **Runs in the browser:** Siteometry calculators need no account and send no input anywhere — every number below is computed on the visitor's own device. ## How to size a central AC with this calculator 1. **Enter your area and units** — Pick metric or imperial, then enter your home's total conditioned floor area — the single biggest driver of the size. 2. **Set your climate zone** — Choose your IECC climate zone. Hotter zones raise the cooling BTU/ft²; this is the lever a plain BTU calculator leaves flat. 3. **Adjust ceiling, sun, insulation and occupants** — Tune ceiling height, sun exposure, insulation quality, occupants and whether there is a kitchen, and watch the contribution bar show what drives the load. 4. **Read the tonnage and check a quote** — Read the recommended size in tons, BTU/h and kW with its nearest standard central unit, and enter a contractor's quoted tonnage to check it for oversizing. _On the page: https://siteometry.com/en/tools/ac-unit-size-calculator#howto_ ## AC unit size calculator The size of a central air conditioner is measured in tons — one ton equals 12,000 BTU/h of cooling. Getting the tonnage right matters: an undersized unit never keeps up on the hottest days, while an oversized one short-cycles, cools unevenly and leaves the home humid. This calculator sizes a whole-home, cooling-only central AC from your floor area and climate zone, with adjustments for ceiling height, sun, insulation, occupants and a kitchen. It returns the load in tons, BTU/h and kW, the nearest standard central size, and a check for a contractor's quote — all in your browser. For a room-by-room breakdown or heater sizing, use the BTU calculator instead. _On the page: https://siteometry.com/en/tools/ac-unit-size-calculator#ac-guide-intro_ ## Tons of AC per square foot The quickest rule of thumb is around one ton of cooling per 400 to 600 square feet, tighter in hot climates and looser in mild ones. Because that spread is almost entirely about climate, this calculator starts from a climate-adjusted cooling rate in BTU per square foot rather than a single flat number. The table below shows the editable default rate for each climate band and the floor area a single ton covers at that rate. Change your climate zone above and the base rate — and the recommended size — moves with it. _On the page: https://siteometry.com/en/tools/ac-unit-size-calculator#tons-per-sqft_ ## How the tonnage is calculated The cooling load is built additively so every factor is a visible part of the total: a climate-adjusted base-area load, plus a ceiling-volume delta above 8 ft, plus sun and insulation percentages on the base, plus flat adders for extra occupants and a kitchen. Dividing the load by 12,000 gives tons. _On the page: https://siteometry.com/en/tools/ac-unit-size-calculator#cooling-formula_ ## Tons, BTU/h and kW Air conditioners are rated three ways for the same capacity: tons, BTU per hour and kilowatts. One ton is 12,000 BTU/h, and one kilowatt is 3,412 BTU/h, so a 36,000 BTU/h load is 3.0 tons or about 10.5 kW. Use the reference table below to convert between them for any standard central size. _On the page: https://siteometry.com/en/tools/ac-unit-size-calculator#tons-and-btu_ ## Choosing a standard size Central AC is sold in fixed capacities — 1, 1.5, 2, 2.5, 3, 3.5, 4 and 5 tons — so the calculator snaps your load up to the nearest available size, never below it, because undersizing is the worse failure on a design day. It then flags how far above the load that size sits, using a guard tighter than ACCA Manual S so you don't drift into a unit that is far too big. _On the page: https://siteometry.com/en/tools/ac-unit-size-calculator#sizing-ladder_ ## What changes the size Six things move the cooling load beyond raw floor area. Change any one in the calculator and the matching segment of the contribution bar and the tonnage update immediately, so you can see exactly what is driving the size. _On the page: https://siteometry.com/en/tools/ac-unit-size-calculator#factors_ ## Oversizing and contractor quotes Bigger is not better with air conditioning. An oversized unit reaches the thermostat setpoint quickly and shuts off before it has run long enough to pull humidity out of the air, so the house feels cold and clammy, the compressor wears from frequent starts, and efficiency drops. Contractors sometimes round up 'to be safe', which is how a home that needs 3 tons ends up with 4. Enter a quoted tonnage in the quote-check panel and the tool compares it with your estimated load, flagging a quote that runs well above it. Treat a half-ton of headroom as normal and a full ton or more as a question worth asking. _On the page: https://siteometry.com/en/tools/ac-unit-size-calculator#oversizing-and-quotes_ ## Worked examples Two estimates at the default constants — the same 2,000 ft² home in a moderate and a hot climate. Your own numbers will differ once you set the zone, insulation and factors to your space. _On the page: https://siteometry.com/en/tools/ac-unit-size-calculator#worked-examples_ ## Frequently asked questions ### What size air conditioner do I need? As a starting point, size central AC at roughly one ton per 400–600 square feet, depending on your climate — closer to 400 ft² per ton in a hot zone and 600 in a mild one. Enter your home's area and climate zone above and the calculator returns the load in tons, BTU/h and kW and snaps it up to the nearest standard central size. It is guidance, not a Manual-J calculation. ### How many tons of AC per square foot? About one ton (12,000 BTU/h) per 500 square feet is a common rule of thumb at 20 BTU/h per ft², but the real figure depends on climate, ceiling height, sun, insulation and occupancy. This tool starts from a climate-adjusted BTU/ft² — 23 in hot zones down to 18 in cold ones — and adjusts for the rest, rather than using a single flat number. ### Is a bigger air conditioner better? No. An oversized AC short-cycles: it cools the air fast, shuts off before it removes much humidity, then restarts, leaving the home cold and clammy while wearing the compressor and wasting energy. Right-sizing — the smallest unit that still covers the peak load — is the goal, which is why the calculator snaps up only to the next standard size and warns when a quote runs well above your load. ### Is this a Manual-J load calculation? No. This is a rule-of-thumb estimate to give you a starting point and sanity-check a contractor's quote. A full ACCA Manual-J accounts for window area and orientation, air leakage, duct losses and local design temperatures. Verify with an HVAC professional before purchasing equipment. ### Does it work in metric? Yes. Switch units at the top and enter your area in square metres; the tool reads kW first, with BTU/h and tonnage alongside. Tonnage is shown in both systems, since air conditioners are sold by the ton worldwide. _On the page: https://siteometry.com/en/tools/ac-unit-size-calculator#faq_ ## Related calculators - [BTU Calculator — AC Tonnage & Heater Sizing](https://siteometry.com/en/tools/btu-calculator) - [Paint Coverage & Cost Calculator](https://siteometry.com/en/tools/paint) - [Tile & Flooring Calculator](https://siteometry.com/en/tools/tile) - [Flooring Calculator — Laminate, Vinyl Plank & Hardwood](https://siteometry.com/en/tools/flooring-calculator)