BTU / HVAC Sizing Calculator

How to estimate the BTU you need

A BTU (British Thermal Unit) measures heating and cooling capacity: more BTU means more power to move heat in or out of a space. HVAC equipment is rated in BTU per hour, and air conditioners are also rated in tons, where one ton equals 12,000 BTU per hour.

The starting point

Cooling load starts from the floor area times a rate that depends on climate: roughly 20 BTU per square foot in a cool northern region, up to about 35 in a hot desert one. That figure is then adjusted for the factors that change how hard the system works: a taller ceiling means more air to condition, a sun-drenched room gains more heat, extra occupants each add warmth, and a kitchen adds a large fixed load from appliances.

Why right-sizing beats oversizing

It's tempting to buy the biggest unit you can, but an oversized air conditioner is a common and costly mistake. It cools the air quickly and shuts off before pulling out enough humidity, so the room feels cold and damp, and the constant short cycling wastes energy and shortens the equipment's life. An undersized unit simply can't keep up on the hottest days. The goal is the right size, not the biggest.

Treat this as a ballpark

This calculator gives a quick estimate, useful for choosing a window unit or a mini-split, or for sanity-checking a contractor's quote. It is not a substitute for a Manual J load calculation, the industry-standard method that accounts for insulation R-values, window type and orientation, air leakage, and ductwork. For a permanent central system, have a professional run one.

This rule of thumb cannot see your insulation, which is one reason a Manual J load calculation exists. Our blown-in vs. batt insulation comparison covers what it costs to bring an attic up to target, and why doing that before sizing equipment can drop half a ton off the system you buy.

Deciding between ductless and ducted? Our mini-split vs. central AC comparison prices both by number of zones, covers the 20 to 30 percent that attic ductwork loses, and is clear that whether the house already has ducts decides this more often than any spec does.

Want the answer as a lookup rather than a form? Our AC size chart runs this same formula across eight house sizes and all four climate zones, and is explicit about what a square footage chart cannot see. Once you know the size, our AC replacement cost guide prices central air by tonnage, tier and efficiency.