Chisel and hinge mortise

Five and ten, not halfway

A butt hinge on a passage door is not spaced off the door's height — the top and bottom hinges sit at fixed setbacks that have nothing to do with how tall the door is, and only a third or fourth hinge, if the door is tall enough to need one, gets spread evenly between them.

The setbacks are asymmetrical on purpose. The bottom rail of a panelled door is wider than the top rail, and a hinge screwed into the joint between rail and stile would be biting end grain. Five and ten put both leaves in solid stile instead.

Size comes off thickness and width, not height

The leverage trying to pull a hinge's screws out of the jamb grows with how far the door's weight sits from the hinge pin, which is a function of the door's thickness and width, not how tall it is. A standard 1 3/8" interior door takes a smaller hinge than a 1 3/4" solid-core or exterior door, and a wide door on either bumps the size up again.

Count is the other half, and it does come off height: one hinge for roughly every 30" of door, with two as the floor even on a short one.

The mortise is one number

Depth is exactly the leaf thickness, and that is the whole of the decision — not the hinge's overall width, not a round number that looks close. Cut shy of it and the door will not close, because the two leaves bind on each other before the door reaches the jamb. Cut deep and the hinge binds the other way, straining its screws every time the door shuts.

The direction to err, when the gauge line is a hair uncertain, is shy rather than deep: a hinge sitting slightly proud can be shimmed out with cardboard behind the leaf, and a mortise cut too deep cannot be un-cut.

What this page does not tell you

• —Convention and manufacturer practice, not code. A fire door, a commercial opening or anything on a closer is governed by its own listing and hardware schedule.
• —Cabinet doors on concealed European hinges are a different problem and need no mortise at all — see the cabinet hinge calculator, not this one.
• —A router with a hinge template and guide bushing cuts a cleaner, more consistent mortise than a chisel alone on a run of doors, but the depth rule and the five-and-ten layout are identical either way — the tool changes, the numbers do not.