Jamb depth is the front-to-back thickness of your wall where the window or door sits — from the interior finished surface to the exterior face. It decides the frame depth you order and whether extension jambs are needed to meet your drywall cleanly. It takes one minute and it's the most-forgotten number in window ordering.
Jamb depth runs through the wall: start at the interior finished surface (drywall or plaster face) and end at the exterior face where the window seats. Include the drywall and sheathing — don't include exterior siding or trim. It is not the width of the old window frame you can see; it's the wall itself.
Open the window or remove a screen so you can reach the frame's side. Hook your tape on the interior edge of the drywall or casing reveal, and run it flat along the existing jamb to the exterior stop or the outside face of the wall.
Measure at two or three spots — a consistent number means you're reading it right.
The Panes window frame body is 3-1/4″ deep — everything past that is made up with extension jambs. Most 2×4 walls with drywall and sheathing land right at 4-5/8″ (frame plus a 1-3/8″ extension); 2×6 walls land at 6-5/8″ (frame plus 3-3/8″). If your number is close to one of these, you've measured it correctly.
Extensions come in 1/4″ steps from 1-3/8″ to 7-3/8″, and deeper for thick masonry walls — whatever your wall measures, the frame is built to meet it.
If the wall is deeper than the window frame, factory extension jambs make up the difference so your interior trim lands flush — in PVC, clear pine, fingerjoint pine, or oak, sized in 1/4″ steps from 1-3/8″ to 7-3/8″ (plywood extensions run deeper for masonry walls, up to 14″). Give us your jamb depth with your order and the extensions arrive attached — measured once, finished right.