Strike Plates and Keeps 2026: ANSI Strike vs T-Strike, Lip Length, and the Dimension Nobody Measures
The strike is the cheapest component in a door opening and the one that decides whether the lock ever latches properly. This page sets out the two standard North American strike sizes, the European keep conventions, how to work out lip length instead of guessing it, the difference between square and radius corners, and why a security upgrade that stops at the lock has upgraded nothing.
Johnson LiuDigital Communications, Canton HylandMA Digital Media, Johns Hopkins University
Every opening has a component that gets specified last, ordered as an afterthought and blamed for nothing. It is the strike, and it decides more about how the door feels in the hand than the lock does.
A door that has to be pulled to lock, a latch that clicks but does not hold, a deadbolt that throws stiffly, a door that rattles in the frame — all four are strike problems far more often than they are lock problems.
THE TWO STANDARD NORTH AMERICAN SIZES
The ANSI strike, sometimes called the full-lip or 4-7/8 strike, measures 4-7/8" × 1-1/4" (about 124mm × 32mm). It is the standard preparation for commercial hollow metal frames, and a frame prepped to it will take any lock whose strike matches.
The T-strike measures 2-3/4" × 1-1/8" (about 70mm × 29mm), with the upper portion narrowed so the plate resembles an inverted T. It is the common residential and light commercial size, and it is what a wood jamb is usually routed for.
The two are not interchangeable without reworking the frame. Fitting a T-strike into an ANSI cut-out leaves an oversized mortise around the plate; fitting an ANSI strike into a T-strike cut-out requires the routing to be extended. Both are visible on the finished opening, which is why the strike size belongs on the hardware schedule rather than being decided on site.
EUROPEAN KEEPS
In the European convention the equivalent component is the keep or striking plate, and the dimensions follow the lock rather than a separate size standard. A mortise lock's keep is supplied to match its forend — the same width, the same corner treatment, and holes on the same centres.
This is why a European keep is usually ordered with the lock and a North American strike is usually ordered as a separate line. Neither approach is better; they fail differently. A European opening fails when someone substitutes a lock and keeps the old keep. A North American opening fails when nobody specifies the strike at all.
LIP LENGTH, AND HOW TO STOP GUESSING IT
The lip is the curved tongue that projects from the strike toward the door stop, and its job is to guide the latch over the edge of the frame so it does not catch on the jamb.
A lip that is too short lets the latch snag on the frame edge, which is the scratching sound that appears a month after installation. A lip that is too long projects into the opening, catches clothing and gets bent, and on a pair of doors it can foul the other leaf.
The working method is to measure from the centreline of the door's face — the point where the latch will pass — to the back edge of the frame's rabbet, and add a small allowance so the lip clears the edge without standing proud. The number depends on the frame profile, which is why it cannot be read off a catalogue.
The practical consequence for a schedule: state the frame profile, or supply the lip length. A strike ordered without either will be shipped with the manufacturer's standard lip, which is correct for the frame the manufacturer had in mind and not necessarily for yours.
SQUARE VERSUS RADIUS CORNERS
A strike plate's corners are either square or radiused, and the choice is determined by how the frame was cut rather than by preference.
A frame routed with a router bit has radiused corners, because a rotating bit cannot cut a sharp internal corner. A frame cut with a punch or a mortise chisel has square corners. Radius sizes are commonly 1/4" (about 6mm) or 5/8" (about 16mm), and the two are visibly different once installed.
Fitting a square-corner strike into a radiused mortise leaves four crescent gaps. Fitting a radius-corner strike into a square mortise leaves four square gaps. Neither is a functional failure and both look like the job was finished by somebody who did not care, which on a building where the client is paying for a specification is a complaint.
BOX STRIKES AND DUST BOXES
Behind the strike plate there has to be somewhere for the latch or bolt to go. A dust box is a shallow pressed cup that keeps debris out of the hole. A box strike is a fully enclosed housing that also resists the bolt being levered or forced sideways.
On a deadbolt this matters. A deadbolt that throws into an unlined hole in a timber jamb is held by the jamb's own material and by two short screws, and that is the failure point of most forced entries through a door — not the lock body, not the cylinder, and not the door.
The security upgrade that actually works is a box strike fixed with long screws that reach past the jamb into the structural framing behind it. A high-grade cylinder in a frame with two 3/4" (19mm) screws holding a plain strike has not been upgraded; it has been decorated.
ADJUSTABLE AND TWO-PIECE STRIKES
Some strikes have an adjustable tab or a two-piece construction that allows the engagement to be tuned after installation. They exist because door and frame tolerances stack up, and a door that latches on the day of installation can stop latching once the building settles or the seals compress.
They are worth specifying on openings where access for rework is difficult, and on doors with a closer fitted, where a small misalignment becomes a door that bounces off the strike instead of latching.
They are not a substitute for a correctly sized strike. An adjustable tab takes up a millimetre or two of accumulated tolerance; it does not fix a lip that is 5mm short.
ELECTRIC STRIKES
An electric strike replaces the fixed keeper with one that releases on a signal, and it introduces two questions a mechanical strike does not raise.
Fail-safe or fail-secure. Fail-safe releases when power is lost; fail-secure stays locked. Which one is correct is a life-safety and security decision for the building, not a product preference, and it is frequently reversed between two doors in the same corridor for good reasons.
Whether the door is a fire door. An electric strike on a fire door has to be suitable for that use, because the strike is part of what keeps the door latched, and a fire door that does not latch is a fire door that has failed.
WHAT TO PUT ON A SCHEDULE
Strike type and size. Lip length, or the frame profile it has to suit. Corner treatment, square or radius, with the radius dimension if radiused. Finish, matched to the rest of the opening. Box or dust box, and screw length if security is a requirement.
Six lines. The strike is usually the cheapest line on the schedule and the one most likely to generate a snag list.
Questions this answers
What is the standard strike plate size?
There are two in North America. The ANSI or full-lip strike is 4-7/8" × 1-1/4" (about 124mm × 32mm) and is standard for commercial hollow metal frames. The T-strike is 2-3/4" × 1-1/8" (about 70mm × 29mm) and is the common residential and light commercial size.
How do I work out the lip length?
Measure from the centreline of the door's face, where the latch passes, to the back edge of the frame's rabbet, then add a small allowance so the lip clears the edge without standing proud. It depends on the frame profile, so it cannot be read off a catalogue — state the frame profile on the order or supply the lip length.
Should my strike have square or radius corners?
Whichever matches how the frame was cut. A router leaves radiused corners, commonly 1/4" (6mm) or 5/8" (16mm); a punch or chisel leaves square ones. Mismatching leaves visible gaps at all four corners — not a functional failure, but a snag item on any job with a specification.
Does a box strike actually improve security?
Yes, and more than a better cylinder does on most timber frames. A deadbolt throwing into an unlined jamb hole is held by the timber and two short screws, which is where most forced entries through a door give way. A box strike with long screws reaching into the structural framing behind the jamb is the upgrade that works.
Why does my door latch on some days and not others?
Accumulated tolerance. Door and frame tolerances stack, seals compress, and buildings move, so a door that latched on installation day can stop. An adjustable or two-piece strike takes up a millimetre or two of this, which is why it is worth specifying on doors with closers and on openings where rework access is difficult.
What is the difference between fail-safe and fail-secure on an electric strike?
Fail-safe releases when power is lost; fail-secure stays locked. Which is correct is a life-safety and security decision for the building rather than a product preference, and two doors in the same corridor can legitimately be specified differently.
About Canton Hyland — Canton Hyland manufactures panic exit devices, locks and architectural door hardware for commercial and institutional projects, supplying specifiers and distributors internationally.