HVAC BTU Room Sizing Calculator
Size an air conditioner or mini-split to a room. Enter the room length and width, the ceiling height, choose the room's sun exposure, and add the number of regular occupants to estimate the cooling capacity in BTU per hour you need.
Last updated: May 2026
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About this calculator
This calculator estimates the cooling load of a room using a refined version of the standard square-foot rule. The formula is: BTU/hr = (Length × Width) × 20 × (Height ÷ 8) × (1 + Sun ÷ 100) + Occupants × 380. The base figure of 20 BTU per square foot is the widely used rule of thumb for a room with average insulation. Because that rule assumes 8-foot ceilings, multiplying by (Height ÷ 8) scales the load to the room's actual air volume — a 9- or 10-foot ceiling holds more air to cool. The sun-exposure factor adjusts the load by up to ±10%: a heavily shaded north-facing room needs less, while a sun-drenched room with large windows needs more. Finally, each regular occupant adds about 380 BTU/hr of body heat. The result tells you the minimum capacity to look for; oversizing an AC is counterproductive because it short-cycles, cooling the air quickly without running long enough to remove humidity, leaving the room cold and clammy.
How to use
Suppose you are sizing a unit for a 16 ft × 14 ft sunroom with 9 ft ceilings, a very sunny exposure (+10%), and 3 regular occupants. Step 1 — Floor area: 16 × 14 = 224 ft². Step 2 — Base load: 224 × 20 = 4,480 BTU/hr. Step 3 — Ceiling adjustment: × (9 ÷ 8) = × 1.125 = 5,040 BTU/hr. Step 4 — Sun adjustment: × (1 + 10 ÷ 100) = × 1.10 = 5,544 BTU/hr. Step 5 — Add occupant heat: + 3 × 380 = + 1,140 = 6,684 BTU/hr. You would choose a unit rated at roughly 7,000–8,000 BTU/hr, the nearest standard size at or above the requirement.
Frequently asked questions
How many BTUs do I need to cool a 16x14 room?
A 16 ft × 14 ft room is 224 square feet, which at the standard 20 BTU per square foot needs about 4,480 BTU/hr as a baseline. Adjusting for a 9-foot ceiling, a very sunny exposure, and three occupants raises that to roughly 6,700 BTU/hr, so you would pick a unit rated around 7,000–8,000 BTU/hr. Exact needs vary with insulation and climate, but matching capacity to the room rather than guessing prevents both weak cooling and the humidity problems that come with an oversized unit.
Does ceiling height affect the air conditioner size I need?
Yes. The common "20 BTU per square foot" rule quietly assumes a standard 8-foot ceiling. A room with 9- or 10-foot ceilings, or a vaulted ceiling, contains noticeably more air volume to cool, so it needs proportionally more capacity. This calculator scales the load by the ratio of your actual ceiling height to 8 feet, so a 10-foot ceiling adds 25% to the base requirement. Ignoring ceiling height is a common reason an AC that looked correctly sized on paper still struggles to keep a tall room comfortable.
Why is an oversized air conditioner a problem?
An oversized air conditioner cools the air temperature quickly and then shuts off, a pattern called short-cycling. Because it never runs long enough, it removes far less humidity than a right-sized unit, leaving the room cold but damp and clammy. The frequent on-off cycling also wastes energy, wears out the compressor faster, and creates uneven temperatures. Bigger is not better with cooling capacity — the goal is to match the unit to the room's actual load, which is exactly what this calculator estimates.