Examples abound of instances where you must decode a key in the field or in your shop. At its most basic level you may simply want the bitting from a key. You may need to decode a key to determine which key bitting specification it uses so that you know how to pin cylinders or cut additional keys to match the manufacturer’s requirements. Whatever the case we all must decode keys at some point.

decode v. to determine a key combination by physical measurement of a key and/or cylinder parts

bitting n. 1. the number(s) which represent(s) the dimensions of the key 2. the actual cut(s) or combination of a key

key bitting specifications n. pl. the technical data required to bit a given (family of) key blank(s) to the lock manufacturer’s dimensions

An example of a key bitting specification, Schlage's Classic.

An example of a key bitting specification, Schlage’s Classic.

Decoding keys may seem straight forward to any of you that regularly interface with a specific manufacturer’s key bitting specification. Schlage’s Classic key bitting specification, found commonly in commercial and residential settings, comes to mind. There are times, however, where we must service keys and/or cylinders belonging to a key bitting specification that we are unfamiliar with.  Unless you regularly service Corbin Russwin’s various key bitting classes and depth systems, for example, they might cause great confusion without the ability to decode depths from a working key. 

With all this in mind, the goal of this article is to show you various methods of decoding a key and how to utilize the information gathered. 

Methods of Decoding Keys

Direct and Blind Codes 

The simplest method of decoding a key is when a code is stamped on the key bow. This code can be a direct code or a blind code.

code n. 1. a designation assigned to a particular key combination for reference when additional keys or cylinders may be needed.

direct code n. a designation assigned to a particular key which includes the actual combination of the key

blind code n. a designation, unrelated to the bitting, assigned to a particular key combination for future reference when additional keys or cylinders may be needed

Direct Codes

A direct code is essentially what you input into code cutting equipment to produce a key to manufacturer’s specifications. Direct codes correspond to bottom pin lengths belonging to the key bitting specification. The benefits of decoding a key using a direct code is that it is quick and straight forward. A direct code will not clue you into the key bitting specifications, however. You must either know that information, know how to derive it, or know where to find it to make use of a direct code. 

An example of a direct code.

An example of a direct code.

Blind Codes

Blind codes are very popular for wafer locks but there are instances of their use in other platforms. To derive a key’s combination from a blind code you must have access to code books/software. You reference blind codes against one of these sources which in turn provides you with the direct code. Nearly all code books and code software contain either full or partial key bitting specification information; some even provide the information to produce a key with the combination(s) using various code cutting equipment. This greatly assists decoding as well as the ability to service additional, related keys and/or cylinders. 

An example of a blind code.

An example of a blind code.

Blind codes are often alphanumeric although there are times where the blind codes can be numbers only. These numbers cannot be confused with direct codes, however, because they will nearly always be less than the actual number of cuts found in the key. It’s also important to note that you should not confuse markings from a master key system with blind codes. The standard key coding system, SKCS, must be learned, understood, and practiced so that information stamped on the key isn’t confused with a blind code.  

standard key coding system n. an industry standard and uniform method of designating all keys and/or cylinders in a master key system. The designation automatically indicates the exact function and keying level of each key and/or cylinder in the system, usually without further explanation.

Key Gauge  

You can also decode a key using a key gauge.

key gauge n. a usually flat device with a cutaway portion indexed with a given set of depth or spacing specifications.

Examples of key gauges.

Examples of key gauges.

A key gauge allows you to insert a key into cutaway portion and move the key within it until the key comes to a stop at or near an index marker. At its most basic level this index marker will be a whole number that corresponds with a depth/cut within that system. There are key gauges capable of decoding more than cuts in a key, such as angles or Aft and Fore cuts with Medeco keys, depending on the system, but that is a story for another article. There are also key gauges with measurements on them that function much like a caliper. While not as precise as a caliper they can be very effective and quick. 

Keep in mind that you must know the appropriate key gauge for the key; there isn’t one key gauge that works for all key bitting specifications. Utilizing the wrong key gauge on a key will accomplish nothing more than wasted time.  


Digital calipers.

Digital calipers.

Calipers allow you to take actual measurements of the key, usually with accuracy of ± .001″. These measurements can then be used to determine the key bitting specifications out right. The process to decode using a caliper is rather straight forward. In fact, there is a formula for this process that you should commit to memory.

Cylinder Math

Effective Plug Diameter – Root Depth = Bottom Pin Length. 

effective plug diameter n. the dimension obtained by adding the root depth of a key cut to the length of its corresponding bottom pin which establishes a perfect shear line. This will not necessarily be the same as the actual plug diameter.

root depth n. the dimension from the bottom of a cut on a key to the bottom of the blade

The effective plug diameter varies by manufacturer but this information is readily available. You can find the effective plug diameter of multiple manufacturer’s on our Key Bitting Specifications page. On LAB universal pin kits, LAB lists this measurement on each pinning chart and labels it E.D., or effective diameter. Keep in mind that effective plug diameter is not the same as the actual plug diameter itself. The effective plug diameter accounts for tolerances, plug diameter does not. Avoid confusion by also committing this to memory. 


The root depth is amount of material between the bottom of the key blade and the bottom of the key cut. You measure the root depth with your calipers.

Measuring root depth with calipers.

Measuring root depth with calipers.

By taking this measurement and subtracting it from the effective plug diameter, or what is needed to create the shearline, we are determining the correct length of the bottom pin. All measurements are taken within a thousand of an inch so you will need to compare the results of this formula against the manufacturer’s key bitting specification to determine the bottom pin/cut assigned to it. Bottom pins correspond to the cut, which simplifies things for everyone involved. For example, if your formula produces a difference of .270” and it is the Schlage classic key bitting specification then you can safely assume you have a 7 cut which utilizes a 7 bottom pin.  

Measuring with a caliper and comparing/deriving is usually slower than other decoding methods but it is the most accurate method. An added benefit of a caliper is that they are multipurpose. You can use calipers to measure other things, such as pins. They are also very helpful when calibrating key machines as they can provide precise measurements throughout the process. 

Visual Decoding 

Once you become very proficient with a key bitting specification or specifications, you can begin to visually decode keys. This process allows you to determine key cuts by using visual clues of the key. It takes some skill to be proficient with visual decoding. Once you are proficient with visual decoding you decode keys almost as fast as if it were a direct code (as long as the keys are accurate!). 


I utilize all options and I would advise you to as well. Keep key gauges and a caliper on the truck and in the shop. Make sure you have access to code books or software. The situations at hand will determine the best, or perhaps only, method of decoding. You best bet is to become proficient at all of them.

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