Garage BTU Calculator

About Garage BTU Calculator

A garage needs substantially more capacity per square foot than a finished room of the same size, and sizing it like a bedroom is the most common reason a unit runs constantly without ever reaching temperature. This calculator uses 35 BTU/h per square foot for cooling rather than the 25 used for an insulated interior room. The difference comes from three things a garage almost always has: little or no wall insulation, an uninsulated sectional door that is often the largest single surface in the room, and no ceiling insulation above it if the space is under a roof void. Many garages also have a concrete slab with no edge insulation, which pulls heat out in winter and holds it in summer. Enter the floor area and ceiling height, and adjust the sunlight setting if the door faces direct sun for much of the day, since an uninsulated metal door in full sun behaves like a radiator. If you are heating rather than cooling the space, switch the mode toggle or use Heating BTU Calculator, because heating a garage uses a different and higher rate again. For the general case see BTU Calculator.

How to use Garage BTU Calculator

  1. Enter room area in m² or ft²
  2. Set ceiling height, sunlight exposure, and number of occupants
  3. See recommended BTU, kW, and AC unit type

Frequently Asked Questions

How many BTU do I need for a 2 car garage?
A typical two car garage is around 400 to 600 square feet. At 35 BTU/h per square foot that is roughly 14,000 to 21,000 BTU/h for cooling, before adjusting for ceiling height and sun exposure. Enter your actual area above rather than relying on the range.
Why does a garage need more BTU than a room the same size?
Insulation. Garage walls are usually uninsulated or lightly insulated, the sectional door is a large uninsulated panel, and there is often no ceiling insulation. Heat moves through those surfaces far faster than through a finished wall, so the equipment has to work harder to hold a temperature.
Does insulating the garage reduce the BTU I need?
Substantially. Insulating the walls, ceiling and especially the door can bring the requirement close to the standard room rate. If you are planning to insulate before installing anything, size the equipment for the insulated state rather than the current one.
Should I use the garage setting for a workshop?
Yes if the building fabric is the same. If the workshop has significant equipment running, add capacity beyond the result, since motors and machinery contribute heat the way computers do in an office.
Is this accurate enough to buy equipment from?
It is a rule-of-thumb estimate suitable for choosing between unit sizes. For a permanent installation, particularly a ducted or multi-zone system, a proper Manual J load calculation accounts for local climate, orientation and construction in a way any per-square-foot rate cannot.

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