How much to cut off a prefit door
Written for the bench, not the office. Fractions throughout, in sixteenths.
Prefit is the trade's word for a slab made to fit a frame that is already on the wall. Nothing is supplied to hang it in — the jamb exists, and the panel is cut down to suit. That single word changes what the shop does: no jamb install, no strike mortise, and a trim whose size somebody has to work out.
The arithmetic
Start from the opening — the width inside the jamb, not the rough opening in the wall. Take out what stands between the leaves, take out the gaps, and divide what is left.
| Opening inside the jamb | 47-7/8 |
| Less the T-astragal | − 5/8 |
| Less three 1/8 gaps | − 3/8 |
| Left for two leaves | 46-7/8 |
| Each leaf finishes at | 23-7/16 |
| The slab arrives at | 23-3/4 |
| Off each leaf | 5/16 |
The three gaps are the ones you can point at on the job: jamb to leaf, leaf to leaf, leaf to jamb. The astragal is the strip that closes the joint between the two leaves. The slab in this example is a 2/0 fiberglass, and it turns up 23-3/4 wide — which is the next thing worth knowing.
Two things that catch people out
A slab does not arrive at its call size. Fiberglass, steel and metal doors come a quarter of an inch under: a 2/0 is 23-3/4, not 24. Wood arrives full size. Measure from what turns up, not from what was ordered, or every leaf comes out a quarter narrow.
Which astragal. The T-astragal is 5/8 thick on an exterior door and 1/2 on an interior one — an eighth of an inch that lands straight in the answer. And the handing does not tell you which you have: RH out is a swing, not a statement about weather. The slab type is what decides it.
More than two leaves
The pattern holds: one gap at each jamb and one between each neighbouring pair, and an astragal in each joint. Two leaves means three gaps and one astragal; three leaves means four gaps and two astragals.
When the answer comes out negative
If the opening is wider than the panels plus their gaps, the sum goes below zero. That is not a cut — it is a problem to raise before anybody touches a saw. A negative number in a trim field is exactly the kind of thing that gets acted on, so it should never be presented as an instruction.
Where this came from
DoorPrep is shop software for door and millwork prep shops. It reads the sticker or the packing slip, works out the route each door needs, and puts the numbers above on the job itself — so the person at the bench is not doing this on the back of the slip.
Start a free 14-day trial — no card needed — or see it working first.