What "jamb size" means

A jamb is the wooden frame your door hangs inside. Its width (the size you're picking here) is how deep that frame is — the distance straight through your wall, from one face to the other. It needs to match your wall thickness, so the finished frame sits flush (perfectly even) with the wall on both sides.

We make the jamb in two widths — here they are:

The shorter board is 4 5/8"; the taller one is 6 5/8". (Shown in raw MDF; primed pine is white, but the widths are the same.) Which one you need comes down to just one thing — how your wall is built.

Standard 2×4 wall → choose 4 5/8" (most common)

Most interior walls are built from 2×4 lumber — the upright wood pieces inside the wall, called studs. Add a layer of drywall (the flat panel that forms the wall surface) on each side, and the wall comes out about 4½" thick. So you'd pick the 4 5/8" jamb. This fits the large majority of homes.

The picture above is a view looking straight down through the wall. The jamb (on the left) reaches all the way across: the stud in the middle (about 3½") plus about ½" of drywall on each side — about 4½" altogether, and the 4 5/8" jamb spans it edge to edge.

Thicker 2×6 wall → choose 6 5/8"

Some walls — often exterior walls or custom builds — are framed with thicker 2×6 studs. A wall like that is deeper, so it needs the wider 6 5/8" jamb instead:

Same idea as before — just a deeper wall, with a bigger stud (about 5½") in the middle, so the jamb has to be wider to reach flush on both sides.

Not sure which wall you have?

Easy — at the doorway, measure straight through the wall, from the surface on one side to the surface on the other:

• About 4½" → choose 4 5/8"
• About 6½" → choose 6 5/8"

And don't worry if the jamb sticks out a hair past the wall — that's completely normal. The casing (the trim that frames the doorway) covers that small gap once it's installed.