Garage-door clearances — headroom, backroom, and side room around the opening

A garage door needs room around the opening, not just the opening. Headroom above (12″ standard, 14″ with an opener, 4–6″ with a low-headroom kit), backroom behind for the horizontal track (door height + 18″), and ~3.75″ of side room at each jamb. This is exactly the fit-check behind the calculator’s headroom result.

What this diagram shows

A side section through a garage opening showing the space a sectional door needs beyond the opening itself. HEADROOM is the clear space above the top of the opening up to the ceiling or joists — 12 inches for a standard door, 14 inches when a powered opener is added, and only 4 to 6 inches if a low-headroom bracket kit is used. BACKROOM is the space behind the opening that the horizontal track needs so the door can slide back and lie flat under the ceiling when open, equal to the door height plus about 18 inches. SIDE ROOM is roughly 3.75 inches at each jamb for the vertical track on a torsion-spring door. The closed door fills the opening; a dashed door shows the open position riding back on the horizontal track. Miss any of the three clearances and a standard door will not install.