How to Measure Bolts For Your Bike - A Simple Guide
If you're thinking of upgrading your bolts, getting the right size bolt is essential. In this guide, we’ll walk you through how to measure bolts correctly, including regular and countersunk bolts.
1. Measuring Bolt Length
a. Regular (Button/Cap/Tapered Head) Bolts
For standard bolts, measure from under the head to the tip of the threads.
Image credit: https://jdcustomsusa.com
📏 Example: A bolt that measures 29.91mm from just under the head to the tip is considered a 30mm length bolt.
b. Countersunk (Flat Head) Bolts
With countersunk bolts, you measure the full length from the top of the head to the tip.
📏 Example: A countersunk bolt that measures 30.19mm from top to tip is considered a 30mm bolt.
🔧 Why? Because countersunk heads sit flush with the surface, so the full length matters for fit.
2. Measuring Bolt Diameter
Bolt diameter is the width of the threaded part, not the head.
- Use a caliper to measure across the threads.
- Don’t measure the bolt head — that varies by style.
📏 Example: A bolt that measures 5.88mm in diameter on the threaded part is considered a 6mm diameter bolt or M6.
🧩 Common bike bolt diameters:
- M5 (5mm) – common for water bottle cages, and small fittings
- M6 (6mm) – common for stems, seatpost clamps
- M8 (8mm) – used for heavy-duty applications like cranks
✅ Tip: "M" stands for metric, and the number is the thread diameter in millimeters.
3. Thread Pitch
Most bike bolts use standard thread pitches, but some may vary.
Image Credit: https://www.omnicalculator.com/construction/thread-pitch
- Use a vernier caliper or ruler to measure the length of the thread, and count the number of threads in that length. The thread pitch is the length divided by number of threads.
- Or, use a thread pitch gauge to match the threads
🔍 Example: M6 x 1.0 x 35mm means a 6mm diameter bolt, 1.0mm thread pitch, 35mm length.
4. Bolt Head Width
Measure the widest part of the bolt head. This measurement is less commonly required, usually for specific applications where the bolt head must fit within a certain slot size.
5. Naming Convention
Screws are specified using the following convention:
Thread Diameter x Thread Pitch x Screw Length - Head Type
Examples:
M6x1.0x30mm - Countersunk
This means a Countersunk head 6mm diameter bolt, 1.0mm thread pitch and 30mm length bolt from bolt head to thread end. (Because it's a countersunk bolt remember? The length includes the bolt head.)
M4x20mm - Cap Head
This means a 4mm diameter bolt, 20mm in length. Thread pitch not specified here. Most cycling screws have a standard thread pitch, so it's not absolutely necessary to determine the pitch for normal use screws.
6. Links
Download a 1-page copy of how to measure your bicycle bolts/screws HERE
Measured your bolts? Shop our titanium bolt collection HERE!