Reference
Door and Lock Measurements
Most lock returns are measurement problems wearing a compatibility costume. Five numbers decide what will physically fit your door, and all five can be taken in ten minutes with a tape measure.
The five numbers
Doors in American homes are broadly standardized and not completely standardized, which is the worst of both worlds. Two doors that look identical can take different locks. Take these measurements from the actual door you intend to change, write them down, and check them against the manufacturer specification for the exact model you are considering rather than against a general rule.
| Measurement | How to take it | What it decides |
|---|---|---|
| Door thickness | Open the door and measure the edge, from face to face, at the height of the lock. | Whether the lock body and spindle are long enough to assemble at all. Thick doors and thin doors both cause trouble. |
| Backset | From the edge of the door to the center of the existing bore hole. | Whether the latch reaches the edge. Two backsets are common in the US and a lock made for one will not sit right in the other. |
| Bore diameter | Across the large round hole in the face of the door. | Whether the new lock covers and fills the existing hole, or whether you are looking at filling and re-boring. |
| Cross bore | The smaller hole drilled in from the door edge, where the latch or bolt sits. | Whether the bolt assembly fits without enlarging the edge of the door. |
| Swing direction | Stand outside. Note whether the door pulls toward you or pushes away, and which side the hinges are on. | Handedness for handle sets, and whether an inward-only device such as a flip latch is even an option. |
| Strike screw length | Take one screw out of the existing strike plate and measure it. | Whether the strike is anchored in the thin jamb or in the framing behind it. This is the single cheapest thing to change on most doors. |
Backset, in plain terms
Backset is the distance from the edge of the door to the center of the hole the lock sits in. Two values are common in American residential doors, and a lock supplied for one can often be adjusted to the other — but only if the listing or the manufacturer says so. If you buy a lock whose backset does not match and cannot adjust, the latch will not reach the strike, and there is no clever fix for that at the door.
Measure to the center of the hole, not to the near edge of it. This is the mistake that gets made most often, and it is out by half the bore diameter, which is more than enough to buy the wrong lock.
Where the strength actually is
A deadbolt is a piece of metal that crosses the gap between the door and the frame. When a door fails under force, the metal is usually not what gives. What gives is the wood: the jamb the strike plate is screwed to, which in a lot of houses is a thin board nailed to the framing behind it, held by screws barely long enough to reach through the board itself.
This is why the strike plate and its screws get their own upgrade page on this site, and why they sit above lock choice in the order of spending. Longer screws that reach past the jamb into the framing behind it change what the fixing is anchored to. That is a real change to the Delay layer, and it costs less than a takeaway.
Single cylinder and double cylinder
A single-cylinder deadbolt has a key outside and a thumbturn inside. A double-cylinder deadbolt takes a key on both sides, which is sometimes suggested for doors with glass nearby. It also means that anybody inside the house needs a key to get out through that door, which is exactly the property the escape routes page warns about.
Whether double-cylinder locks are permitted on a dwelling’s exit door depends on the code your jurisdiction has adopted, and in some places they are restricted for this reason. That is a question for your local building official, not for a comparison table. If you are considering one, read the escape page first and then ask locally.
What an ANSI/BHMA grade is, and what it is not
ANSI/BHMA grades are a published performance classification for builders hardware, developed under the American National Standards Institute process and administered by the Builders Hardware Manufacturers Association. A product earns a grade by being tested against a written standard. The grade belongs to that tested product, and it is published.
That last word is why this site does not print a grade next to any lock on a gear page. If a manufacturer has published a grade for a specific model, that is a fact you can look up on the manufacturer’s own documentation. If a listing says a product is certified without publishing what it was certified to, then what you have is a marketing sentence, and repeating it as a grade would be inventing information. Where a listing states something about certification, we report the listing as saying it, and stop there.
- A grade cannot be inferred from weight, finish, price or the word heavy-duty.
- A grade on one model in a range does not carry to the other models in that range.
- Comparing two locks by grade only means something when both grades are published.
- Load figures printed on a box — pounds of force, and similar — are the seller’s claim about their own test, not a classification.
Before you order anything
- Take all six measurements above and write them on your phone, not on a scrap of paper by the door.
- Photograph the edge of the door with the latch visible, and the inside face of the frame.
- Find the manufacturer specification sheet for the exact model number you are considering.
- Check thickness, backset and bore against that sheet, not against the listing bullet points.
- If two of the numbers disagree with each other, measure again before assuming the door is unusual.
The lock fit checker on this site runs those numbers and tells you which shapes of lock are physically possible, which listings here state that they suit them, and what you still need to confirm with the manufacturer. It does not tell you that a specific lock will fit your specific door, because no tool that has not seen your door can honestly say that.