BTUFit › Garage mini split
Garage mini split calculator
A partly insulated 2-car garage, the 480 sq ft the form opens on, carries a 14,400 BTU/h load and wants a head rated 18,000 BTU. The identical 480 sq ft as an ordinary room comes to 12,000. A garage is not a room: no insulation above, a giant leaky door in front, and a slab that soaks heat all day. Sizing it like a bedroom is how people end up with a unit that never catches up.
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Why garages need more BTU per square foot
The calculator starts from the same room brackets as everywhere else on this site, then applies an envelope factor: +20% for a partly insulated garage, +45% for an uninsulated one, and +10% if the sun cooks the door. Those factors are editorial (garages vary wildly), but they match how installers size them in practice: a 480 sq ft room wants 12,000 BTU; the same 480 sq ft as an uninsulated garage wants 18,000.
Two upgrades shrink the number before you buy:
- Door insulation kit: the single biggest win, often a full unit size.
- Ceiling insulation if the attic above is open; heat pours down through an uninsulated lid.
If you're keeping a workshop at temperature year-round in a cold climate, treat this as the summer number and check winter with the heating calculator set to "older / poorly insulated".
Frequently asked questions
What size mini split for a 2-car garage?
A typical 2-car garage is 440–530 sq ft. Insulated, that's a 12,000 BTU head; uninsulated (bare studs, uninsulated door), the load jumps roughly 45% and you're looking at 18,000 BTU. Insulating the garage door first is usually cheaper than the bigger unit.
Can I heat my garage with a mini split in winter?
Yes, that's one of the most common garage installs. But heating capacity falls with the outdoor temperature, and garages leak. In a cold climate, pick a cold-climate (hyper-heat) model and treat the recommendation here as a floor, or run the numbers in the heating calculator with "older / poorly insulated".
Do I really need to insulate the garage first?
You don't need to, but every BTU you buy fights the envelope. An uninsulated metal garage door in the sun radiates like a griddle; a $200–400 door insulation kit routinely knocks the required unit down a full size, which saves more than the kit costs.
Will a window AC or portable work in a garage instead?
Sometimes. Garages rarely have windows, so that usually means a through-wall sleeve or a portable venting through a wall port. Both are less efficient than a mini split and most portables top out around 10,000 BTU SACC, which won't hold a hot 2-car garage. The mini split is the standard answer for a reason.