The AC BTU room size calculator estimates how much cooling capacity an air conditioner needs to keep a single room comfortable. Capacity is measured in British thermal units per hour (BTU/hr), and choosing a number close to the room's real cooling load is the single most important decision when buying a window unit, a portable air conditioner, or a ductless mini-split.

Instead of guessing, this tool turns a handful of measurable inputs — room length and width, ceiling height, sunlight exposure, the number of people, kitchen heat, equipment load, and your climate — into a recommended cooling capacity and a sensible range to shop within. It supports both metric and imperial measurements, so you can enter meters or feet without converting anything by hand.

The result is a planning estimate built on widely published rules of thumb from sources such as ENERGY STAR and the U.S. Department of Energy. It gets you into the right size class quickly, which is exactly what you need when comparing products online or sanity-checking a quote. It is not a substitute for a contractor's detailed load calculation, and the tool says so clearly in its output.

The calculation begins with the most established rule of thumb for room air conditioners: roughly 20 BTU per hour for each square footof floor area, assuming a standard 8-foot (about 2.4 m) ceiling. From that base, the tool applies multipliers and fixed additions for the conditions that change a room's heat gain.

• Ceiling factor = ceiling height ÷ 8 ft. A 10-foot ceiling gives a factor of 1.25 because there is 25% more air volume to cool.
• Sunlight factor = 0.9 for heavily shaded rooms, 1.0 for average exposure, and 1.1 for very sunny rooms.
• Climate factor = 0.9 (mild), 1.0 (moderate), 1.1 (hot), or 1.2 (very hot and humid).
• Occupant BTU = 600 for each person beyond the first two who regularly use the room.
• Kitchen BTU = 4,000 when the unit cools a kitchen, to offset cooking and appliance heat.
• Equipment BTU = 0, 1,000, or 3,000 depending on the heat from electronics and appliances in the room.

The metric path converts your meters to feet and your square meters to square feet internally, so the same proven coefficients apply regardless of the unit you choose.

Cooling equipment is rated in different units depending on the type and the market, which makes side-by-side comparison confusing. This calculator reports the same capacity three ways so you can compare any product.

• BTU/hr — the standard rating for window and portable air conditioners in North America.
• Tons — one ton of refrigeration equals 12,000 BTU/hr. Central systems and mini-splits are often sold by the ton, so a 1.5-ton unit is 18,000 BTU/hr.
• Kilowatts (kW) — common outside North America. Multiply BTU/hr by 0.000293 to get cooling kW. A 9,000 BTU/hr unit is about 2.6 kW of cooling.

Do not confuse cooling kilowatts with the electrical power the unit draws. A modern unit's efficiency rating (SEER, EER, or CEER) determines how many watts of electricity it uses to deliver each unit of cooling.

This calculator deliberately uses conservative, transparent rules of thumb. Those rules are excellent for a quick estimate, but they cannot see everything that affects a real room's heat gain.

• It does not model specific insulation R-values, window glazing, shading devices, or air leakage.
• It treats climate as a broad multiplier rather than using your location's design temperatures and humidity data.
• It assumes a single, reasonably enclosed room rather than open-plan spaces with cross-loading from adjacent areas.

For an actual purchase — especially for central air or whole-home systems — have a licensed HVAC professional run a Manual J load calculation. Manual J is the industry standard that accounts for construction details, orientation, duct losses, infiltration, and local climate to size equipment precisely. Use this tool to get into the right ballpark and to question quotes that look far too large or too small, then let a professional confirm the final number.