What is a Key Gauge?

A key gauge is a flat, precision-marked measuring tool used by locksmiths to identify the cut depths and spacing on a key, or to measure pin heights during lock rekeying. Rather than guessing or relying solely on a key machine's built-in code chart, a key gauge lets a locksmith physically measure and confirm cut depths, which is especially useful when working from an unfamiliar or worn key.

Key gauges are a core part of a locksmith's rekeying and key-cutting toolkit, used alongside pinning kits, key machines, and pin tweezers to ensure pins and key cuts match up accurately to the correct depth specifications for a given lock brand and keyway.

How Does a Key Gauge Work?

A key gauge works by using a series of precisely cut notches, slots, or a sliding caliper-style mechanism to measure the depth of each cut on a key blade. The key is placed against or into the gauge, and each cut is compared to the marked depth increments until a matching measurement is found, allowing the locksmith to read off the corresponding depth number or letter for that position.

On pin-based rekeying jobs, key gauges are also used in reverse — locksmiths measure the depth of a pin chamber or an existing key cut to determine which size pin needs to be installed, cross-referencing the measurement against a manufacturer's pinning chart to select the correct key pin and driver pin combination.

Key Components of a Key Gauge

A key gauge typically includes a few core features depending on its style:

• Measuring notches or slots – precisely spaced increments used to compare against key cuts

• Depth scale/markings – numbered or lettered indicators corresponding to standard depth specifications

• Caliper-style sliding arm (on adjustable gauges) – allows continuous measurement rather than fixed notch comparison

• Brand-specific templates – some gauges are manufactured for a specific lock brand's depth and spacing system (e.g., Kwikset, Schlage)

• Durable metal construction – ensures measurement accuracy holds up under repeated shop use

Locksmiths often keep multiple key gauges on hand, since depth and spacing specifications differ between lock manufacturers.

Key Gauge Sizes

Key gauge sizing refers to the depth increments a gauge is built to measure, which vary by lock brand and keyway system:

• Kwikset (KW1) – typically uses a 5-depth system, with depths ranging from the shallowest to deepest cut positions

• Schlage (SC1) – typically uses a 9- or 10-depth system, offering finer depth increments than Kwikset

• Automotive keyways – often use their own depth and space charts specific to the vehicle manufacturer

• High-security and restricted keyways – may use non-standard depth systems requiring a dedicated gauge from the lock manufacturer

Because depth and spacing systems aren't universal, using a gauge matched to the correct brand and keyway is essential for accurate results — a Kwikset gauge won't correctly read a Schlage key, and vice versa.

How to Use a Key Gauge

Using a key gauge accurately typically follows these steps:

1. Identify the key or lock brand to select the correct gauge (e.g., Kwikset, Schlage, or a specific automotive gauge).

2. Clean the key of any dirt or wear that could affect measurement accuracy.

3. Align the key's shoulder against the gauge's reference point, ensuring consistent positioning for each cut.

4. Compare each cut position against the gauge's notches or sliding scale, reading off the depth value at each position from the bow to the tip.

5. Record each depth reading in order to build the full key code (e.g., 3-1-4-2-5), which can then be used to cut a duplicate key or select the correct pins for rekeying.

6. Cross-reference the readings with the manufacturer's depth and pinning chart if the gauge is being used for a rekeying job rather than direct duplication.

Taking careful, consistent measurements at each cut position is the most important part of using a key gauge accurately, since even a small misread can result in an incorrectly cut or pinned lock.