Have you ever bought a new cam lock for a cabinet, mailbox, or piece of furniture, only to find it doesn’t fit? It’s a common problem. The world of cam locks can be confusing, with lots of numbers and terms. But finding the right size is easier than you think.
This guide will walk you through the entire process of how to measure a cam lock. You don’t need to be a locksmith or a carpenter. With a few simple tools and this easy-to-follow guide, you’ll be able to measure any cam lock correctly the first time. Let’s get started!
What Is a Cam Lock?
First, let’s talk about what a cam lock is. You’ve definitely seen one before. They are the most common lock used on cabinets, lockers, desks, mailboxes, and even some toolboxes. The name “cam” comes from the rotating part inside that locks the door.
You use a key to turn the lock. This turning action spins a metal arm, called the cam, which swings into place to hold the door shut. It’s a simple, effective, and affordable locking mechanism. Because they are used on so many different things, they come in many different sizes. That’s why knowing how to measure them is so important.
The Parts of a Cam Lock
To measure a cam lock properly, you need to know the names of its three main parts. Once you know these, the measuring steps will make perfect sense.
The Barrel (or Body)
This is the main, long cylindrical part of the lock. It’s the piece that you push through the hole in your door or drawer. Think of it as the “body” of the lock. Its job is to house the locking mechanism and provide a solid base.
The Cam
The cam is the flat, often L-shaped, metal arm that sticks out from the back of the barrel. When you turn your key, the cam rotates. It’s this rotating action that locks and unlocks your door. The cam swings behind the door frame or into a metal strike plate to hold everything securely.
The Collar (or Rose)
The collar is the round, flat plate that sits at the top of the barrel, right where the key goes in. When the lock is installed, this collar rests flat against the outside of your door or drawer. Its job is to cover the hole you drilled and give the lock a finished look.
Why Accurate Measurement is Crucial
Getting the measurements right is the difference between a successful repair and a frustrating waste of time and money.
- A Barrel That’s Too Long will stick out the back of your door, looking bad and preventing the door from closing.
- A Barrel That’s Too Short won’t reach through the door, meaning the cam can’t engage properly, and the lock is useless.
- A Barrel That’s Too Wide simply won’t fit into the existing hole, forcing you to drill it larger, which can damage your furniture.
- A Cam That’s the Wrong Size might not reach the strike plate to lock, or it might be too long and hit the inside of the cabinet.
Measuring correctly the first time ensures you buy a lock that fits perfectly, works smoothly, and keeps your belongings secure.
Tools You Will Need
You don’t need a fancy toolbox for this job. You only need one or two simple tools:
- A Ruler or a Tape Measure: A standard ruler that shows both inches and millimeters (mm) is perfect.
- A Caliper (Recommended for Best Results): A small, inexpensive digital caliper from a hardware store will give you the most precise measurements, especially for the barrel diameter. But a ruler will work just fine if that’s all you have.
Step-by-Step Measurement Guide
Now for the main event. Follow these four steps in order to get all the measurements you need.
Step 1: Measure the Barrel Length
This is the most critical measurement. It tells you how thick a door the lock can fit into.
- Place your cam lock on a flat table with the barrel pointing up and the collar resting on the table.
- Take your ruler and measure from the back of the collar straight down to the very end of the barrel.
- Crucial Tip: Make sure you are not including the cam in this measurement. You are only measuring the solid, cylindrical barrel itself.
This measurement is almost always given in millimeters (mm). Common sizes you’ll see are 20mm, 22mm, 25mm, 30mm, 35mm, and 40mm.
Step 2: Measure the Barrel Diameter
This measurement tells you how wide the hole in your door needs to be.
- Look at the barrel. You need to measure how wide this cylinder is.
- If you have a caliper, this is very easy. Just clamp the jaws of the caliper around the barrel and read the number.
- If you are using a ruler, line up the start of the ruler with one side of the barrel and see where the other side lands. Try to be as precise as you can.
The two most common barrel diameters are 13mm (which is very close to 1/2 an inch) and 16mm (which is very close to 5/8 of an inch).
Step 3: Measure the Cam
The cam needs to be the right size and shape to work with your furniture.
- Cam Length: Measure from the center of the hole where it attaches to the barrel to the far end of the cam.
- Cam Offset (or Drop): This is how far the cam “reaches” out. Look at the cam—it’s often bent. Measure from the center of the hole to the tip of the locking part of the cam.
- Cam Handedness (Right-Hand or Left-Hand): This determines which way you turn the key to lock it.
- Hold the lock with the barrel facing you and the cam pointing down.
- If turning the key clockwise makes the cam move to the right to lock it, it’s a right-hand cam.
- If turning the key counter-clockwise makes the cam move to the left to lock it, it’s a left-hand cam. Most cams are right-hand.
Step 4: Measure the Faceplate/Collar Diameter
This is mostly for looks, but it’s still good to know. Simply measure across the circular collar from one side to the other. This ensures the new lock’s collar will cover any marks or the old hole completely.
Standard Cam Lock Size Chart
The following table outlines different cam lock sizes.
| Size Category | Typical Barrel Length | Typical Barrel Diameter | Common Uses |
| Small / Short | 15mm – 22mm | 13mm (½”) | Thin Panels: Filing cabinets, thin metal lockers, some mailbox doors, small drawers. |
| Medium / Standard | 25mm – 30mm | 13mm (½”) or 16mm (⅝”) | Most Furniture & Cabinets: This is the most common size range. Used for kitchen cabinets, wooden desks, standard drawers, and many toolboxes. |
| Large / Long | 35mm – 40mm+ | 16mm (⅝”) or larger | Thick Doors: Large lockers, thick wooden cabinets, some industrial equipment, and heavy-duty tool chests. |
FAQs
What if I don’t have the old lock to measure?
Don’t worry! You can still find the right size. Carefully measure the thickness of the door or drawer where the lock will go. Your barrel length needs to be slightly less than this thickness. Next, measure the diameter of the existing hole. That will be your barrel diameter.
Are cam lock measurements in inches or millimeters?
While you might find some in inches, the vast majority of cam locks, even those sold in the US, use metric measurements (millimeters). It’s always safest to assume you are working in mm.
The cam on my new lock doesn’t reach the strike plate. What went wrong?
This usually means the “cam offset” or “cam drop” measurement was too short. When you order a new lock, double-check that this measurement is correct for your cabinet. You may need a cam with a longer reach.
Can I change the cam on a lock?
On many cam locks, the cam is removable. It is often held on by a small screw or simply snaps into place. This is useful if you need a different style or length of cam.
My measurement is between two standard sizes. What should I do?
For barrel length, always round down. For example, if your door is 27mm thick, choose a 25mm barrel lock, not a 30mm one. A slightly shorter lock will still work fine, but a longer one will not fit at all.
Conclusion
Measuring a cam lock might seem tricky at first, but it’s really a straightforward process. By breaking it down into the three main parts—the barrel, the cam, and the collar—and taking your time with a ruler, you can confidently find the perfect replacement lock.
Remember, the barrel length is the most important number to get right. Now that you have this knowledge, you can tackle that locking cabinet, mailbox, or desk drawer with confidence. No more guessing games at the hardware store.
