M Key Size Chart: Complete Guide for Beginners ( 2025 )

When you are working with bolts, nuts, or screws, you will often notice something like M6, M8, or M10 written on the label.
But what does that “M” actually mean? If you have ever stood in front of a hardware shelf wondering which size to pick, you are not alone — it confuses a lot of beginners and even some professionals.

In this guide, I will explain what M key sizes are, how to understand them, and share a complete M key size chart so you will never have to guess again.

What Does “M” Mean in Screw or Bolt Sizes?

The letter “M” stands for “Metric”, which means the bolt or screw size is based on the metric system (millimeters).
For example:

• M6 means the bolt diameter is 6 millimeters.

• M10 means the bolt diameter is 10 millimeters.

The number after the “M” always refers to the outer diameter of the bolt’s thread, not the length, not the wrench size, but the actual thickness of the bolt itself.

So, when you see something like M8 × 1.25 × 30, it means:

• M8 → Thread diameter = 8mm

• 1.25 → Thread pitch = Distance between threads (in mm)

• 30 → Length of the bolt = 30mm

It’s that simple once you get used to it!

Why Understanding M Key Sizes Matters

If you pick the wrong M size, your bolt might:

• Not fit the nut properly

• Get stripped easily

• Cause mechanical failure in your project

Whether you are building furniture, repairing an engine, or assembling metal parts, choosing the right size ensures:

• Tight, secure fittings

• Proper torque control

• Longer life for tools and threads

Even a 1mm difference can make a huge impact when dealing with metal components.

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The M Key Size Chart

Below is a quick reference chart that shows the most common M sizes and their corresponding hex key (Allen key) or spanner/wrench size.

This chart makes it super easy to know:

• Which Allen key to use with which screw or bolt

• What spanner/wrench size fits which bolt head

M Key Size Chart PDF:

Choosing the right M key size can be tricky, especially with so many bolts and screws in different projects. This M Key Size Chart PDF is designed to make your life easier. It shows the thread diameter, pitch, Allen key size, and wrench size for each metric bolt, helping you quickly pick the correct tool every time.

M_Key_Size_Chart PDF

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Quick Story: When 1mm Caused a Big Problem

A few months ago, I was helping a friend fix his motorcycle exhaust system. Everything looked fine until we reached the last bolt — an M8 socket bolt that just wouldn’t tighten properly.

We thought the thread was damaged, but later realized we had accidentally used an M10 wrench instead of an M8 one.
That tiny 2mm difference stripped the bolt head slightly, and we had to replace it.

It taught us both a simple but important lesson:
Always double-check your M key size before tightening anything.

How to Measure an M Key or Bolt Size

Measuring an M key or bolt size is easier than it looks, and once you know how, it becomes second nature. The goal is to find out the bolt’s diameter, thread pitch, and the key or wrench that fits perfectly. With a few simple tools, you can identify the correct size accurately.

1. Use a Vernier Caliper or Ruler

The most accurate way to measure a bolt size is with a Vernier caliper. Place it around the outer edges of the bolt’s thread to get the exact diameter in millimeters. If it measures about 6mm, it’s an M6 bolt. In the absence of a caliper, a regular millimeter ruler can give you a close estimate.

2. Check the Thread Pitch

After measuring the diameter, the next step is identifying the thread pitch — the distance between two adjacent threads. A thread gauge makes this easy by matching the teeth spacing. If you don’t have one, compare the bolt visually with another known M-size bolt to find a close match. Consistent thread spacing ensures proper fitment.

3. Match the Allen Key

Now test different Allen keys (hex keys) to find one that fits snugly into the bolt’s socket. The correct size should slide in smoothly without any looseness or wobble. Never force a key that doesn’t fit; doing so can strip the bolt head. A perfect fit ensures smooth tightening and safe torque application.

Different Types of M Key Bolts

Not all “M” bolts are the same — their heads, uses, and tools can differ quite a bit. Understanding each type helps you choose the right bolt for your specific task, whether you’re building furniture or fixing machinery. Below are the most common M bolt types you’ll encounter in real-world applications.

1. Hex Head Bolts

Hex head bolts have a six-sided head that’s tightened using a spanner or wrench. They are strong and durable, making them ideal for heavy-duty applications like engines, industrial machines, and automotive assemblies. Their large head provides excellent torque grip, ensuring a secure fit even under high pressure or vibration.

2. Socket Head (Allen) Bolts

Socket head bolts are designed to be tightened using an Allen key (hex key). You’ll find them everywhere — from motorcycles and bicycles to office chairs and furniture kits. Their cylindrical heads allow them to fit into tighter spaces, providing a clean, flush appearance while maintaining strong and reliable fastening.

3. Button Head Bolts

Button head bolts have a smooth, rounded top that gives them a sleek, finished look. They’re often used in visible areas where appearance matters, such as custom furniture, decorative fixtures, or hobby projects. Despite their low-profile design, they’re still tightened with an Allen key for easy and precise control.

4. Countersunk Bolts

Countersunk bolts have a flat, conical head that sits completely flush with the surface once tightened. This design is perfect for metal sheets, wood panels, and machine covers where a smooth surface is required. They prevent snagging or interference with moving parts, making them both functional and aesthetically pleasing.

5. Flanged Bolts

Flanged bolts include a built-in washer beneath the head, which spreads the load evenly across the surface. This prevents damage and helps keep the bolt from loosening under vibration. You’ll commonly see them in automotive, construction, and mechanical equipment because they save time — no separate washer needed.

Each of these bolt types still follows the same M-size standard. For example, an M6 socket head bolt has the same thread diameter as an M6 hex head bolt — only the head design and tightening tool differ. Understanding these variations helps you choose the right bolt every time.

Common Uses of Each M Size

To make it even easier, here’s a general idea of where each size is commonly used:

So, if you’re assembling a piece of IKEA furniture, you will likely use M4 or M5 bolts. But if you are repairing your car engine, you will probably deal with M8, M10, or M12 bolts.

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Tips for Working with M Key Bolts

Working with M key bolts seems simple, but using them correctly can make all the difference between a secure, long-lasting joint and one that fails too soon. Here are some practical tips that every mechanic, DIY enthusiast, or hobby builder should know before tightening their next bolt or screw.

🔹 1. Always Use the Right Key Size

Never use a smaller or larger Allen key “just because it fits a little.” A slightly loose key can round off the bolt head, making it difficult or impossible to remove later. Always double-check your key size against the M bolt chart before tightening to prevent damage and ensure a perfect fit.

🔹 2. Apply Proper Torque

Each bolt size is designed to handle a specific torque level. Applying too much force can strip the threads or even snap the bolt, while too little torque leaves the joint weak. Always refer to the recommended torque specifications, especially in automotive or machinery work, to maintain safety and precision.

🔹 3. Keep Your Tools Clean

A bit of oil, grease, or dust on your Allen key or wrench can cause it to slip inside the bolt head. This not only wears out the bolt but can also injure your hand if the tool slips suddenly. Regularly clean your tools and keep them dry for consistent grip and control.

🔹 4. Store Bolts Separately

Bolts of different M sizes can look similar at first glance. Storing them together often leads to mix-ups that cause frustration during assembly. Keep clearly labeled containers for M4, M5, M6, and other sizes. This simple habit saves time, prevents errors, and keeps your workspace organized and efficient.

🔹 5. Use Anti-Seize Grease for Reuse

If you plan to remove and reinstall bolts frequently, applying a thin coat of anti-seize grease on the threads is a smart move. It prevents rust, galling, and thread damage, making future disassembly much easier. This trick works especially well for outdoor or high-temperature environments where corrosion is common.

Allen Key vs. Hex Wrench — What’s the Difference?

People often mix up these terms. Here’s the quick distinction:

Allen Key = Small L-shaped tool that fits inside the bolt head.

Hex Wrench or Spanner = Used externally on hexagonal heads.

Both serve the same function, tightening or loosening but they fit differently based on the bolt head design.

M Size Confusion: M8 vs M10

This is a common mistake among beginners.
At first glance, M8 and M10 bolts look quite similar.
However:

• M8 = 8mm diameter, fits 13mm wrench

• M10 = 10mm diameter, fits 17mm wrench

If you use an M10 wrench on an M8 bolt, it’ll slip or round off the edges. So, whenever in doubt, measure twice, tighten once.

M Key Sizes in Real Life Projects

If you are into DIY or mechanical repair, here are some real-life examples:

• M4 or M5 → Common in computer cases and 3D printers

• M6 → Bicycle seat clamps, small engine parts

• M8 → Automotive applications (like motorcycle frames)

• M10–M12 → Engine mounts, suspension systems

• M16+ → Heavy machinery and structural bolts

The metric system makes it super convenient, you can easily find compatible nuts, washers, and keys once you know the M number.

Conclusion

Understanding the M key size chart isn’t just for engineers — it’s for anyone who wants to assemble or repair something confidently. Once you learn that “M” simply stands for millimeters, and that each size follows a standard diameter and key fit, everything starts to make sense.

Whether you are fixing your car, building furniture, or tinkering with a machine, keep this M key size chart handy.
It’ll save you time, prevent damage, and make every project feel much smoother.

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