Air Conditioner Size Calculator

The standard rule of thumb used by ENERGY STAR and HVAC professionals is 20 BTU per square foot of floor area. This baseline applies to a room with an 8-foot ceiling and average insulation. Adjustments are then made for sun exposure (add 10 percent for sunny rooms, subtract 10 percent for heavily shaded rooms), ceiling height above 8 feet (add roughly 10 percent per extra foot), and kitchen use (add 4,000 BTU because cooking appliances generate significant heat). These factors come from ENERGY STAR's room air conditioner sizing guidance.

Tonnage refers to the cooling capacity of an air conditioner expressed in tons of refrigeration. One ton equals 12,000 BTU per hour, a unit that originates from the amount of heat required to melt one ton of ice in 24 hours. A 1-ton unit delivers 12,000 BTU/hr, a 1.5-ton unit delivers 18,000 BTU/hr, and so on. Central air conditioning systems are commonly sized in half-ton increments from 1.5 to 5 tons. Window and portable units are typically described in BTU/hr rather than tons.

An undersized unit runs continuously without reaching the set temperature, resulting in high electricity bills, poor dehumidification, and accelerated wear. An oversized unit cools the room quickly but cycles off before removing enough humidity, leaving the space feeling clammy and cold. Short cycling also stresses the compressor and reduces the unit's lifespan. ENERGY STAR recommends sizing to the calculated BTU load rather than rounding up aggressively, and using a variable-speed or inverter system where possible to handle variation in heat load.

A taller room contains more air volume per square foot of floor area, which increases the cooling load. The standard baseline is an 8-foot ceiling. For each additional foot of ceiling height, add approximately 10 percent to the base BTU figure. A 10-foot ceiling in a 200 sq ft room (two extra feet above the 8-foot baseline) adds 20 percent to the base load. This adjustment is a simplification; precise calculation would involve air volume in cubic feet, but the per-foot rule is consistent with ENERGY STAR guidance for room-by-room sizing.

Cooking appliances, including ranges, ovens, and dishwashers, generate substantial heat that the air conditioner must overcome in addition to the solar and ambient heat gains it would face in any other room. ENERGY STAR adds 4,000 BTU to the cooling load calculation for rooms used as kitchens. This additional load accounts for the steady operation of burners and oven cycles during meal preparation. If the kitchen is only occasionally used or uses induction cooking, the actual additional load may be somewhat lower, but the 4,000 BTU adjustment is the standard conservative estimate.