The lip is a stack of offsets
Measured from the centre line of the strike — which runs down the middle of the leaf thickness — the lip has to reach past everything the latch would otherwise hit. On a 1-3/4 leaf sitting flush with the jamb face, with casing standing a quarter inch proud and an eighth of lead-in, that is 7/8 plus 0 plus 1/4 plus 1/8, or 1-1/4 inches. Off a stock list of 3/4, 7/8, 1, 1-1/8, 1-1/4, 1-1/2 and 1-3/4, the shortest that reaches is 1-1/4 exactly.
Take the casing away and the same door wants 1 inch. That quarter inch of trim is the whole difference between two strikes, which is why the lip is the item most often ordered wrong on a set of openings that look identical from the schedule.
Standoff is the other half of the answer
The page prints how far the chosen lip stands off the jamb face — 1-1/4 of lip on a leaf whose face is 7/8 from the centre line leaves 3/8 sticking out. That projection is what a sleeve catches on, and it is also a dimension somebody may care about if the opening is on a route with a clear width requirement. The number is here; what is acceptable is a question for the authority having jurisdiction, not for this page.
The pocket is where jambs get ruined
Behind the plate, the latch pocket has to be deeper than the throw. A half inch throw with an eighth of clearance is 5/8 of pocket, and in 3/4 jamb stock that leaves 1/8 inch of wood at the back. That is thin, and it is the reason strike pockets so often break through into the shim space on a site-built jamb — the bolt bottoming out and a pocket cut with no clearance are the two ways the same jamb fails.
Where the arithmetic stops
It stops at the fire rating. Cutting a rated frame outside what its listing already permits voids the listing, and no dimension on this page changes that or works around it — the manufacturer of the assembly is the only source for what a rated frame can have done to it. Related pages: the lockset backset and stile fit calculator for the door side of the same prep, and the door hardware schedule calculator for the counts across a whole set.
Questions people ask
How is strike lip length measured?
From the centre line of the strike out to the outside edge of the lip, with the centre line running down the middle of the leaf thickness. That is why half the door thickness is always the first term: 7/8 for a 1-3/4 leaf, 11/16 for a 1-3/8 one. Everything after it is whatever stands between the door face and the point the latch has to clear.
What lip length do I need with casing in the way?
Add what the casing projects past the jamb face. A 1-3/4 leaf flush in its frame with a quarter inch of casing proud and an eighth of lead-in comes to 1-1/4, against 1 inch with no casing at all. The casing is doing a fifth of the total, which is more than most people expect from trim, and it is the usual reason a strike that worked on the last job is short on this one.
What happens if the lip is too short?
The latch reaches the trim before it reaches the plate. You get a bruised casing, a door that needs a shove to latch, and eventually a latch that stops closing cleanly at all. It shows up in the first week, which is at least honest — a lip that is too long fails the other way, standing out into the opening where it catches on things and where a projection into a route may matter to somebody inspecting it.
How deep should the latch pocket be?
Deeper than the throw, by whatever clearance keeps the bolt off the back. Half an inch of throw with an eighth of clearance is 5/8, and in 3/4 jamb stock that leaves an eighth of wood behind it. If the numbers you enter put the pocket through the back of the jamb, the page says so — that is a real outcome on site-built jambs and it is worth knowing before the chisel comes out.
Can I cut a strike into a fire-rated frame?
Not a question this page answers. Cutting a rated frame outside what its listing already allows voids the listing, and a voided assembly is replaced rather than repaired. What is permitted for that specific frame comes from the manufacturer of the assembly. No method or dimension for it is given here.