Measure and choose the right mortise lock size

Mortise lock dimensions are the key foundation for selecting or replacing a door lock correctly. The most important measurements include backset, center distance, faceplate width, faceplate length, and the lock case width. Only when these dimensions are taken accurately will the mortise lock fit properly, operate reliably, and meet long-term safety and functional requirements. This guide explains step by step how to measure the essential dimensions, which standards apply, and how to choose the right lock size for residential doors, office buildings, or special-purpose doors - clearly, practically, and with a future-proof approach.

What are mortise lock measurements?

Mortise lock dimensions refer to the specific construction dimensions of a door lock, which must harmonize exactly with the respective door leaf. Only with exact dimensional coordination can it be guaranteed that the lock sits securely in the intended cut-out, that its function is reliably guaranteed and that there are no disturbances in daily operation.

The most important mortise lock dimensions include:

• Backset: Distance from the visible edge of the cuff to the center of the keyhole or cylinder opening. This dimension has a significant influence on how far the key is from the door leaf.
• Distance: Distance between the center of the trigger nut and the center of the keyhole or cylinder opening. The distance determines whether the door fittings and cylinders fit together.
• Cuff width and cuff length: Cuff is the narrow front plate that remains visible on the mortise lock. Width and length determine whether the mullion fits exactly on the intended milling.
• Lock Box Width (Mortise Lock Depth): Specifies how much space the lock housing takes up inside the door leaf.

These dimensions are crucial in each case so that the door lock can be integrated into the door in a standard-compliant, stable and permanently secure manner. They are standardized sizes throughout the construction and security industry, but they can vary depending on the application scenario and the age of the building.

The most important dimensions of the mortise lock at a glance

In order to be able to quickly compare for different doors and areas of application, here are the most common dimensions in a compact table and detailed explanation:

Worth knowing: Special doors such as fire protection, escape route or burglar-resistant doors may have completely different dimensions or additional requirements for the lock and cylinder. Therefore, always check the individual dimensions of the existing door before replacement!

How to Measure a Mortise Lock: Step-by-Step Guide

1. Measure the backset:
Place a measuring tape or caliper at the edge of the faceplate (the visible front plate) and measure to the center of the keyhole or cylinder hole. This dimension determines the correct alignment of the key or cylinder.

2. Determine the center distance (PZ distance):
Measure horizontally from the center of the spindle hub (where the door handle fits) to the center of the keyhole or cylinder. This ensures proper compatibility between the lock, door hardware, and cylinder.

3. Measure faceplate width and faceplate length:

• Faceplate width: Measure the visible width of the faceplate.
• Faceplate length: Measure the full length of the faceplate as it appears on the door edge.

4. Check the lock case width (case depth):
Measure the depth of the lock case that sits fully inside the door leaf. This dimension affects stability, ease of installation, and correct fitting.

5. Record your measurements:
Write down all dimensions carefully and compare them with the manufacturer’s specifications before ordering. Photos of the measuring points can also help with installation and when seeking advice from professionals or retailers.

Determining mortise lock dimensions: step-by-step instructions

To avoid errors and make the replacement efficient, you should record the dimensions systematically and with suitable measuring tools. The following procedure has proven to be particularly reliable:

1. Measure backset:
   Place the tape measure or a caliper on the edge of the mullion (visible front plate of the door) and measure exactly to the middle of the keyhole or cylinder hole. This is where it is decided whether the key can be optimally guided later.
2. Determine distance:
   Measure horizontally from the center of the handle nut (where the door handle is attached) to the center of the cylinder or keyhole. This distance is relevant for the interaction of lock, fitting and cylinder.
3. Determine the cuff width and cuff length:
   Cuff width: Measure the visible width of the lock's front plate.Cuff length: Measure the total length of the front plate, which is visible on the outer edge of the door.
4. Check the width of the lock box:
    Determine the depth at which the lock must be pushed into the door leaf. This value has a decisive influence on the stability and installation of the lock.
5. Documentation:
    Carefully record the measured values and compare them with the manufacturer's data of the desired products before each order.

Practical tip: Pay attention to right-angled measurements to avoid deviations and document the dimensions with photos – this will make it easier to install them later and get advice from specialist retailers.

Mortise lock dimensions for room doors and other types of doors

Most modern room doors, whether in residential complexes, offices or administrative buildings, follow clear standard dimensions. Particularly common for interior doors are:

• Backset: 55 mm
• Distance (handle to lock): 72 mm
• Cuff width: 20 mm
• Cuff length: usually around 235 mm

This standardization ensures that a mortise lock for a room door can be easily replaced in the vast majority of cases, provided that dimensions and standards are observed.

Exterior doors, front doors or doors with special security requirements, such as fire protection, on the other hand, are less standardized. Here, larger distances often occur in terms of backset or distance, e.g. backset 65 mm or distance 92 mm – a difference that is hardly visible to the naked eye. Especially with these doors, a particularly thorough dimensional check is essential before you choose a new lock or a modern access solution.

Special cases: soundproofing, fire protection and metal doors

Many object and special doors – for example in schools, hospitals or technical rooms – are subject to their own standards. These doors often have additional inserts, reinforced lock boxes or special mullion designs (such as stainless steel cuff, extra length or diagonally designed mullion plate). The dimensions for the mortise lock must be chosen exactly in order to maintain function and approval.

Mortise Locks – Old Standard

Mortise locks built to old standards are commonly found in older buildings and historic doors. Their dimensions often differ from today’s DIN specifications, which makes accurate measuring especially important. Typical deviations include:

• Backset: 60 mm, 65 mm, or 70 mm
• Center distances: often 78 mm, occasionally 85 mm or intermediate values
• Faceplate widths: e.g., 22 mm or special profiles (square-edged, rounded, reinforced)
• Faceplate lengths: custom lengths without any standard reference

When replacing an old-standard mortise lock, all dimensions must be taken precisely and compared with current manufacturer specifications. Many manufacturers and hardware suppliers still offer special sizes or adapter solutions, allowing historic doors to be preserved without major modifications. If you are unsure, consulting a locksmith or security specialist is highly recommended.

Practical tip:
Along with measuring, document the exact shape of the faceplate and the existing strike plate. This information makes it much easier to find a compatible replacement lock.

DIN and EN standards for mortise locks and cylinders

The world of mortise locks is characterised by a multitude of standards that guarantee installation safety, compatibility and quality. Among the most important are:

• DIN 18251: The central standard for mortise locks regulates dimensions, test procedures and areas of application.
• DIN 18250: Specifies the requirements and dimensions of locking cylinders.
• EN 12209 / EN 179 / EN 1634: European standards that define requirements for burglary protection, fire protection and emergency exits in addition to dimensions.

What are the benefits of complying with the standards?

• Secure interchangeability: locks with DIN/EN dimensions can be easily replaced.
• Quality guarantee: Standardised products are tested for durability, safety and compatibility.
• Legal certainty: Compliance with standards is often mandatory, especially in the commercial sector (e.g. fire protection).

When buying or replacing a new one, you should therefore preferably choose mortise locks that demonstrably comply with the relevant DIN and EN standards.

Useful tips for practical application

1. Selection according to purpose

First of all, define the requirements: Should interior doors (low risk potential), exterior doors or special equipment be secured? For more frequented or endangered areas, it is advisable to look for products with increased wear resistance and tested safety.

2. Compatibility with modern access technology

Invest future-proof: Many mortise locks today are designed in such a way that they can be easily retrofitted with electronic lock cylinders, motor cylinders or digital fittings such as those from BlueID. Check the dimensions and plan for the retrofitting at the same time. With solutions such as an rfid locking system, you maintain maximum flexibility, security and comfort.

3. Installation instructions for decision-makers and those responsible

For commercial or public buildings, attach importance to documented dimensional control, tested standard parts and specialist installation. Not only the passage dimensions, but also additional functions such as panic function, fire protection or access logging can already be decided on the choice of the right mortise lock.

Special requirements: Security, fire protection and access control

In sensitive areas – from administrative corridors to laboratories to technical rooms – mortise locks are often the focus of sophisticated security concepts. The most important additional requirements include:

• Burglar resistance: Special reinforced lock types that make manipulation and break-in more difficult.
• Fire protection: Only tested and approved lock models are allowed in fire doors.
• Escape route security: Locks with panic function (e.g. according to EN 179) that ensure quick opening in the event of danger. Further information and systems can be found in our article on escape door control.
• Digital locking systems: Compatibility with modern access solutions such as NFC or Bluetooth fittings, including plug-in locks that can be integrated.

High-quality locking technology such as BlueID's is designed for precisely these requirements: It offers scalable, future-proof and networked options for access control – from individual doors to higher-level building management systems.

Conclusion: How to ensure the perfect fit in the long term

Choosing the right mortise lock starts with the precise determination of the relevant dimensions. Only those who know the backset, distance, mullion and lock box dimensions as well as the applicable standards can ensure the function, security and sustainability of the locking technology in the long term. Consider the possibilities of modern access technologies and usable synergies when selecting and replacing them – this will keep your doors equipped for future requirements.

Give us a call or configure your digital locking system.

FAQ: Frequently asked questions about mortise lock dimensions

How do I measure a mortise lock correctly?

Use folding rule or caliper to accurately record backset, distance, cuff width, cuff length and lock case width. Document the dimensions in writing, photographically if necessary.

What does backset mean for mortise locks?

The backset describes the distance between the gauntlet edge of the door and the center of the keyhole. It is a central value for the selection of suitable locking technology.

Are mortise lock dimensions for room doors standardized?

Yes, dimensions such as backset 55 mm and distance 72 mm are now standard for interior doors – this simplifies replacement considerably.

What to do in case of rare or old measurements?

Determine the dimensions particularly carefully; contact specialist dealers for special or old building sizes. Special solutions and adapters are widely available.

What role do standards and certificates play?

Standards such as DIN 18251 or EN 12209 give you security, quality and compatibility when purchasing and replacing mortise locks.

What do I have to pay attention to in public or commercial buildings?

In addition to suitable dimensional accuracy, fire protection, burglary protection and, above all, the connection to digital locking systems are of particular importance here - specialist advice is recommended.

How can the switch to digital access systems succeed?

When choosing mortise locks, make sure they are compatible with electronic fittings and cylinders. Rely on expandable solutions such as BlueID for maximum flexibility.