What Are Common Mortise Lock Cylinder Sizes?

A wrong cylinder size looks small on paper. It becomes expensive when doors cannot lock well. I see this problem in many batch orders.

Common mortise lock cylinder sizes are usually 60, 65, 70, 75, and 80 mm.¹ I do not choose them by length alone. I calculate the size from door thickness, cylinder cover height, 3–5 mm allowance, and the lock body position.²

Many buyers ask me one simple question: “What cylinder size should I buy?” I understand the question. I also know the risk behind it. A common size can still be the wrong size when the door thickness, cylinder cover, or lock position changes. So I always start with the door, not the cylinder.

How Do I Calculate The Correct Mortise Lock Cylinder Size?

A cylinder that is too short creates installation trouble. A cylinder that is too long looks poor and may be unsafe.³ I use one basic formula first.

The practical formula is: cylinder length = door thickness + inside cylinder cover height + outside cylinder cover height + 3–5 mm allowance.⁴ I use this method before I confirm common sizes for production or quotation.

I usually explain this formula to buyers before we discuss stock sizes. The door thickness is the base. The cylinder covers on both sides add extra height. The 3–5 mm allowance gives a small safe projection. This helps the key work smoothly and helps the covers sit well.⁵ I do not like choosing a cylinder only from a catalog list, because a catalog cannot see the real door.

Here is the simple calculation logic I use in factory specification work:

Item	What I Check	Why It Matters
Door thickness	40 mm, 45 mm, 50 mm, or other	It is the main base length
Inside cylinder cover height	Often 6–10 mm	It adds to the needed cylinder length
Outside cylinder cover height	Often 6–10 mm	It also adds to the needed cylinder length
Allowance	Usually 3–5 mm total	It prevents the cylinder from being too short
Lock body position	Centered or offset	It decides the split length

I have seen many cases where the buyer only told me “the door is 50 mm.” That is not enough. A 50 mm door with thin covers may use one cylinder length. A 50 mm fire-rated steel door with two thick stainless steel cylinder covers may need another. So I ask for the door thickness, cover height, door type, and lock body position before I confirm the cylinder size.

What Are The Most Common Mortise Lock Cylinder Lengths?

A buyer may feel pressure when a project has many door types. If the buyer picks only one size, some doors may fail during assembly. I keep the size list practical.

The most common mortise lock cylinder lengths are 60, 65, 70, 75, and 80 mm. These sizes are commonly used because factories, buyers, and warehouses can machine, purchase, stock, and supply them more easily.

I often see these sizes in door hardware procurement for wooden doors, steel doors, aluminum profile doors, and fire-rated doors. But I do not say one size fits all. I use these common lengths as a starting point. Then I match them to the actual door structure. In production, sizes are usually rounded to multiples of 5 mm.⁶ This makes machining easier. It also makes stock control easier. A buyer can also manage spare parts better when sizes follow this 5 mm pattern.

The table below shows how I normally explain common use cases:

Common Total Length	Common Split Examples	Typical Door Use	My Factory-Side Note
60 mm	30+30, 25+35	Aluminum profile doors, thin interior doors	I confirm cover height carefully
65 mm	32.5+32.5, 30+35	45 mm wooden doors	I check if the lock is centered
70 mm	35+35, 30+40	50 mm steel or fire-rated doors	I check door rebate and cover height
75 mm	35+40, 30+45	Thicker doors or offset lock cases	I confirm the outside projection
80 mm	40+40, 35+45	Heavy doors or special structures	I request door drawings when needed

I also remind buyers that cylinder length is not the same as mortise lock body size. It is also not the backset, center distance, or faceplate size.⁷ These are different specifications. If these numbers get mixed, the quotation may look correct, but the goods may not fit during final assembly.

How Do Door Thickness And Cylinder Covers Change The Size?

A small cover height can change the final size. I have seen buyers ignore this detail. The cylinder then arrives too short, and the installer cannot finish cleanly.

Door thickness alone does not decide the cylinder length.⁸ I add the inside cover height, outside cover height, and a 3–5 mm allowance. This is why two doors with the same thickness may need different cylinders.

I often use real examples because they make the sizing rule easier to understand. For a 50 mm steel fire-rated door with two 8 mm stainless steel cylinder covers, I usually calculate 50 + 8 + 8 + about 4 mm. The result is about 70 mm.⁹ So a 70 mm cylinder is commonly used. For a 45 mm interior wooden door, the cover height may be lower. A 65 mm cylinder is often common. For a 40 mm aluminum profile door with 8 mm cover height, a 60 mm cylinder is commonly used.

Here is a simple reference table:

Door Type	Door Thickness	Cover Height Example	Common Cylinder Length	My Comment
Steel fire-rated door	50 mm	8 mm + 8 mm	70 mm	I use this often for project supply
Interior wooden door	45 mm	Around 7–8 mm total per side range	65 mm	I still confirm handle rose and cover type
Aluminum profile door	40 mm	8 mm cover height	60 mm	I check the profile depth and lock case position
Thick entrance door	55–60 mm	8–10 mm each side	75–80 mm	I ask for a drawing or sample

I do not treat these examples as fixed rules. I treat them as common factory-side references. In batch procurement, the safest process is simple. I ask the buyer to confirm the door section drawing, cover height, and installation photo if available. This helps avoid wrong length, wrong split, and wrong finish. It also reduces after-sales cost after the goods arrive.

Why Are Cylinder Sizes Usually Rounded To 5 mm?

Odd sizes look exact on a drawing. They can create trouble in real purchasing. I often guide buyers toward standard 5 mm steps when the door allows it.

Mortise lock cylinder lengths are usually rounded to 5 mm steps, such as 60, 65, 70, 75, and 80 mm.¹⁰ This helps factory machining, material planning, stocking, batch packing, and future replacement.

In my factory work, I prefer standard steps because they reduce risk for both sides. A 68 mm cylinder may look perfect in theory. But if the door and cover system allow a small projection, 70 mm is usually more practical. The 3–5 mm allowance exists for this reason. It gives a safe range. It also makes the product easier to source later. A door brand may need the same cylinder again after six months. A standard size is easier to repeat.

I also see benefits in export orders. Importers and wholesalers often carry stock. They do not want too many tiny size differences. Standard sizes help them build a clear SKU system. A simple size system also helps their sales team avoid mistakes.

Size Method	Benefit	Possible Risk
5 mm standard steps	Easy production and stock control	Needs correct allowance check
Exact custom size	Can match special doors closely	Higher MOQ, longer lead time
One size for all doors	Simple purchase order	High fitting risk
Size by formula first	Safer configuration	Needs more project information

I still provide custom sizes when the door structure needs them. But I first check if a standard 5 mm size can solve the problem. This is usually better for cost, delivery time, and future supply stability.

What Is The Difference Between Total Length And Split Length?

A buyer may order “70 mm” and think the work is done. I do not stop there. A 70 mm cylinder can have different split lengths.

Total length is the full cylinder length. Split length shows how much cylinder extends to each side, such as 35+35 or 30+40.¹¹ A flush door often uses a symmetrical split. A rebated door may need an asymmetric split.¹²

This point is very important for bulk orders. A 70 mm cylinder does not always mean 35+35. For a flush door, the lock body is usually installed in the middle of the door leaf. So a symmetrical cylinder such as 70 mm = 35+35 is commonly used. The cylinder projection on both sides looks balanced. The handle set and cylinder cover also sit in a normal position.

For a rebated door, the situation may change. The lock body may not be exactly centered. The outside or inside side may need more length. In this case, 70 mm = 30+40 may be better than 35+35. The total length is still 70 mm, but the split is different. This can decide whether the cylinder sits correctly.

Door Structure	Lock Body Position	Common Split Logic	Example
Flush door	Usually centered	Symmetrical split	70 mm = 35+35
Rebated door	Often offset	Asymmetric split	70 mm = 30+40
Profile door	Depends on profile chamber	Confirm by drawing	60 mm = 25+35
Special project door	Depends on structure	Confirm by sample or section	75 mm = 35+40

I always ask for the split length in serious orders. If the buyer only writes total length, I ask again. This small check can prevent a large shipment from becoming hard to install.

How Should Buyers Confirm Cylinder Size Before Ordering?

A rushed order can save one day at the beginning and lose many days at installation. I prefer to slow down once and confirm the key data clearly.

Buyers should confirm door thickness, cover height on both sides, lock body position, door type, cylinder total length, and split length before ordering. Samples or drawings help the supplier configure the cylinder correctly.

When I work with door manufacturers and hardware brand buyers, I use a basic checklist. It is not complicated. It just prevents common mistakes. The buyer should tell me the door material first. A steel fire-rated door, wooden door, and aluminum profile door may all use mortise locks, but the cylinder size logic can change. Then I ask for door thickness. After that, I ask for the cylinder cover height on both sides. Some covers are flat and thin. Some stainless steel covers are higher. This difference directly affects the required cylinder length.

I also ask whether the lock case is centered. If the door is rebated, I ask for a section drawing. If the buyer has an old sample, I ask them to measure both sides from the screw hole center to each end. This helps confirm the split.

Here is the checklist I normally use:

Confirmation Item	Buyer Should Provide	My Reason
Door type	Wooden, steel, fire-rated, aluminum profile	Different structures change size
Door thickness	Exact mm value	Main base for calculation
Cover height	Inside and outside cover height	Directly adds to cylinder length
Lock position	Centered or offset	Decides split length
Total length	60, 65, 70, 75, 80 mm, or custom	Needed for quotation and stock
Split length	30+30, 35+35, 30+40, etc.	Needed for correct installation
Certification need	CE, fire-rated, project standard	Needed for market compliance

I believe this process is better than asking only for common size. It also fits batch procurement better. A buyer can reduce return risk, protect project delivery, and keep product finish consistent across the whole order.

Conclusion

I choose mortise lock cylinder size by formula, door structure, cover height, and split length, not by common length alone.

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