Measuring a lock cylinder: determine the correct dimensions in minutes

To measure a lock cylinder, first identify the center at the retaining/fixing screw (faceplate screw) in the lock faceplate and measure from there to both ends of the cylinder (A and B sides), each up to the outermost visible edge of the fitting/escutcheon. Select the cylinder in 5 mm steps so that it does not protrude beyond the level of the external security fitting (ideal: flush to a maximum of 1 mm projection).

Why accurate measuring is crucial

A precisely measured lock cylinder is the foundation for security, ease of use, and a perfect fit, both for classic mechanical profile cylinders and modern electronic knob cylinders. Cylinders that are too long can protrude and be vulnerable from the outside; those that are too short can bind in the fitting or be difficult to operate. For those responsible in companies, residential complexes, and institutions, correct measurements also matter to:

• Properly account for fittings, security roses, and door thicknesses.
• Reduce lead times and costs (avoid incorrect orders).
• Plan and implement a later digital retrofit with electronic cylinders.

Incorrectly sized cylinders can also cause significant follow-up costs. On security-relevant doors, they may even jeopardize insurance coverage if the locking system is not installed in compliance with standards. Professional facility managers and security experts know: investing a few minutes up front for correct measurements can prevent costly bad purchases and security risks in the long term.

Measuring a lock cylinder correctly -step by step

This guide works for Euro profile cylinders in the installed state (without removal):

1. Ideally open the door: This gives you a clear view of the cylinder and the door rabbet.
2. Determine the centerline: Locate the faceplate retaining/fixing screw that secures the cylinder. Its center marks the cylinder’s midpoint.
3. Measure the outside dimension (side A): Place a ruler or caliper at the centerline and measure to the outermost visible edge of the external fitting/security escutcheon. Do not measure to the door leaf edge; measure to the outer edge of the fitting/rose.
4. Measure the inside dimension (side B): Repeat the measurement on the inside to the outermost visible edge of the interior fitting.
5. Round to the 5 mm grid: Cylinder lengths are specified in 5 mm steps (e.g., 30/30, 30/35, 35/40).
6. Check projection: On the outside, the cylinder should be flush to a maximum of 1 mm projection. More than 3 mm projection on the outside is a security risk.
7. Define the type: Double cylinder (keyed both sides), knob cylinder (turn knob on the inside), half cylinder (e.g., for garages, utility rooms).

For an exact result, professionals recommend a thorough approach. It is important that the cylinder has the correct length to sit optimally in the door leaf and fittings. Careful alignment of the ruler or caliper at the retaining screw ensures precise measurements. If in doubt, it can help to measure multiple times and compare the results.

Measuring with removal (optional)

• Loosen the faceplate screw, turn the key slightly, and pull out the cylinder.
• Measure total length as a control by adding A and B.
• Advantage: highest accuracy. Disadvantage: more effort and not always practical with security doors.

Removal offers additional benefits: you can check the cylinder for wear and detect any damage. Note that some security doors may require special tools or a professional for removal. In such cases, plan sufficient time or consult a specialist.

Measuring a lock cylinder length

• A dimension: from the cylinder center (retaining/fixing screw) toward the outside.
• B dimension: from the cylinder center toward the inside.
• Total length: A + B.

Important:

• Never measure “through the air” to the door leaf; measure to the outermost point of the fitting (rose/security escutcheon).
• If new fittings are planned, include their thickness (e.g., thick security roses on front doors).
• For asymmetrical doors (e.g., entrance doors with a stronger outside): common combinations are e.g., 35/45, 40/50, etc.

The A/B dimensions are critical for ordering the correct cylinder. Even small measurement inaccuracies can cause significant installation issues. A deviation of only a few millimeters can be the difference between a perfectly functioning locking system and a security-compromising projection or a door that cannot be operated properly.

Lock cylinder measuring template - easier with a guide

A measuring template for lock cylinders (e.g., cardboard or a manufacturer download) saves time and prevents errors:

• Place the template at the center of the retaining screw.
• Read the outside up to the fitting edge (A), and the inside (B).
• Transfer the result to the 5 mm grid.

Tip: A DIY template is quick to make. Draw a zero line (center/retaining screw) and mark millimeter or 5 mm intervals to the left and right. This lets you read A/B at a glance, especially helpful when dealing with many doors in a facility.

Using a template is especially recommended for facility managers or property managers who regularly measure or replace lock cylinders. A durable template made of plastic or metal lasts significantly longer than paper versions and provides consistent results for repeated measurements.

Some manufacturers offer professional cylinder measuring tools that, in addition to determining length, also capture data such as door leaf width or fitting type. These specialized tools speed up the process and minimize human measurement errors, which saves time and costs on large projects.

Measuring lock cylinder sizes table (ABUS orientation)

Brands use a 5 mm grid. Typical A/B combinations are comparable across manufacturers. Below is a compact orientation table (A/B in mm):

• 30/30 = 60 mm total length
• 30/35 = 65 mm
• 30/40 = 70 mm
• 35/35 = 70 mm
• 35/40 = 75 mm
• 35/45 = 80 mm
• 40/40 = 80 mm
• 40/45 = 85 mm
• 40/50 = 90 mm
• 45/45 = 90 mm
• 45/50 = 95 mm
• 50/50 = 100 mm

How to use the table:

• Determine A and B as described and choose the nearest matching 5 mm combination.
• If you’re between two values, prioritize slight or no projection on the outside.

Note: Manufacturers such as ABUS provide detailed size tables. The grid logic remains the same: A/B in 5 mm steps, total length = sum.

While standard cylinder sizes are sufficient for most applications, manufacturers also offer special sizes if needed. These custom options are particularly relevant for historic buildings, special locking systems, or very thick doors. Note that custom sizes usually have longer lead times and cost more than standard versions.

Measuring a front door lock cylinder - specifics for entrance doors

Front doors often have thicker fittings and security roses on the outside. Pay attention to:

• Outside flush: The cylinder should not protrude on the outside (max. 1 mm) to make attacks more difficult.
• Account for the security fitting: Include its thickness in the measurement, not just the door leaf.
• Emergency function: Recommended so it remains lockable from the inside even if a key is inserted outside - important in multi-family homes and care facilities.
• Weather exposure: Consider corrosion-resistant versions (V2A/V4A) and models optimized against cold bridges.

Front doors increasingly use special security cylinders. These offer additional protective features such as enhanced drill resistance, anti-pull protection, or integrated anti-tamper mechanisms. Such high-security cylinders may impose special requirements on the installation situation, which is why exact measurements are particularly important here.

Modern apartment entrance doors often feature multi-point locking systems. In such cases, the cylinder must be compatible with the multi-point mechanism. A cylinder that is too long or too short can impair the functionality of the entire security system. If in doubt, consult a professional to verify the interaction of all components.

Common measuring mistakes and how to avoid them

• Measured to the door leaf by mistake: Always measure to the visible edge of the fitting/rose.
• Ignored projection: Outside should be flush at mosttoo long is a security risk.
• New fittings not considered: For replacement projects, first determine the final fitting thicknesses.
• Wrong cylinder type: Distinguish between double, knob, and half cylinders, as well as special profiles (e.g., Swiss round, Scandinavian oval profiles).
• Fire and panic doors: Only install approved cylinders; for panic doors, ensure compatible cam position/function.

Another common error is insufficient consideration of the cylinder’s vertical position. With certain door types, especially profile or specialized security doors,vthe cylinder’s vertical alignment is crucial to its function. A deviation can cause problems when turning the key or lead to increased wear.

Do not underestimate the importance of the profile type. While Euro profile cylinders are predominant in Germany, international buildings or older properties often feature other profiles such as Swiss round, Scandinavian oval, or British oval profiles. Misidentifying the profile inevitably leads to incorrect orders and project delays.

Special cases and pro tips

• Half cylinders: Only one side is measured (the outer or inner side). The total length corresponds to the single dimension plus the cam area follow manufacturer specifications.
• Knob cylinders: Measure the inside dimension (knob side) according to fitting thickness so the knob can turn freely.
• Thick doors (>70 mm): For special doors (historic buildings, acoustic, fire-rated), combinations like 45/55, 50/60, etc., are common re-measure before ordering.
• Changing roses: If upgrading to security roses, A (outside) and possibly B (inside) will change, take final measurements after the roses are installed.
• Installation: After inserting the cylinder, align it centrally, tighten the faceplate screw, and perform a function test with and without door seal compression.

Asymmetrical door constructions, common in historic or heritage-protected properties, require special attention. Unusual size combinations outside standard grids may be necessary. In such cases, a custom build or a modular cylinder may be the best solution.

For complex locking systems with several hundred cylinders, create detailed measurement documentation. Besides A/B dimensions, include information on door type, fitting, security requirements, and any special features. Such documentation facilitates not only the initial installation but also future maintenance or replacement work.

From mechanical to digital- accurate measurements for electronic retrofits

If you are switching from a mechanical system to a digital one (e.g., electronic knob cylinders or smart fittings), precise dimensions pay off twice:

• Electronic cylinders are ordered in the same A/B grids.
• For outsides with security fittings, “flush to minimal projection” remains the standard.
• In many facilities, electronic door fittings with integrated readers are an alternative to a cylinder. In this case, cylinder length becomes less important; door leaf and lever set dimensions are decisive.
• For doors with panic function or high security requirements, planning and approval by a specialist partner is recommended.

Transitioning to electronic locking systems brings numerous benefits, including flexible access assignment, detailed access logs, and the option to integrate with higher-level security and building management systems. However, electronic components often have different installation requirements than their mechanical counterparts.

With electronic knob cylinders in particular, watch for obstructions around the door. The knob needs sufficient clearance to turn, and in some cases, electronic components may have a slightly larger form factor than purely mechanical systems. Carefully assessing the installation conditions before ordering helps avoid surprises during installation.

Planning a digital retrofit? BlueID supports you from site survey and selection of suitable electronic cylinders and fittings through commissioning, including secure, cloud-based access management.

Technological aspects of digital lock cylinders

When retrofitting to digital locking solutions, consider technical aspects in addition to physical dimensions:

• Power supply: Electronic cylinders are battery-operated. For easy battery replacement, the knob should be readily accessible.
• Radio range: With Bluetooth- or RFID-based systems, building conditions can affect signal strength.
• Environmental conditions: Electronic components have specific temperature and humidity requirements—especially relevant for exterior doors.
• Integration options: Digital systems can be integrated into higher-level building management or security systems, which should be considered during planning.

Planning a digital retrofit? BlueID supports you from site survey and selection of suitable electronic cylinders and fittings through commissioning, including secure, cloud-based access management.

Short checklist for measuring a lock cylinder

• Found the center of the retaining/faceplate screw?
• Measured A to the outside fitting edge?
• Measured B to the inside fitting edge?
• Chosen the 5 mm grid and checked exterior projection?
• Determined the cylinder type (double, knob, half)?
• Considered special cases (front door, panic, fire rating, new roses)?

Conclusion

To measure a lock cylinder, always determine A and B from the center of the retaining screw to the outermost edges of the fittings, not to the door edge. Round to 5 mm steps and avoid projection on the outside. This way you order with a perfect fit, increase security, and are well prepared for a future digital retrofit.

Careful measurement of lock cylinders is essential for the long-term security and functionality of doors. Whether for new installations, renovation projects, or digital retrofits, precise dimensions are the foundation of a successful project. The small time investment for precise measurements pays off many times over through avoided misorders, optimal security, and long-term reliable operation.

Would you like to check multiple doors in your facility or switch directly to electronic cylinders? Request a non-binding consultation or a product demo from BlueID, we support you from measurement to rollout.

FAQ – measuring a lock cylinder

Can I measure a lock cylinder without removing it?

Yes. Measure from the center of the retaining screw to the fitting edges (outside and inside). That is sufficient in most cases. Removing the cylinder increases accuracy but is not strictly necessary.

What do the A and B dimensions mean?

A is the distance from the cylinder center to the outside, B to the inside. The total length is A + B in millimeters. Cylinders are specified in 5 mm steps (e.g., 35/45).

How far may a cylinder project on the outside?

Ideally flush to a maximum of 1 mm. More than 3 mm projection increases the risk of forced attacks (e.g., snapping) and should be avoided.

Is there a size table like ABUS provides?

Yes, a 5 mm grid is used across manufacturers. You can use A/B combinations such as 30/30, 35/40, 40/50, etc. The key is that A + B equals the total length and there is no projection on the outside.

How do I measure a lock cylinder on a front door?

Include the thick external security fitting and measure to its outermost edge. Ensure minimal or no projection on the outside and consider the emergency function.

What should I bear in mind for half cylinders?

Only one side is measured (A or B, depending on installation orientation). The total length is based on this dimension; additionally check the cam position per the manufacturer.

I’m planning a digital retrofit. Do I still need to measure?

Yes. Electronic knob cylinders follow the same A/B grids. Alternatively, you can use electronic door fittings, where other dimensions are decisive. In both cases, project-specific planning is recommended.

Differences between mechanical and electronic cylinders when measuring?

The measuring principle remains the same (A/B from the cylinder center). For electronic cylinders, also consider requirements such as outside flushness, battery access, and compatibility with panic and fire doors, professional advice is recommended.

What should I do with unusual door thicknesses or special sizes?

Most manufacturers offer custom builds for exceptionally thick or thin doors. Document the exact A/B dimensions and consult the manufacturer about available options outside the standard grid