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Measuring Fasteners: Length & Diameter
How to Measure Fasteners: Screw & Bolt Length, Diameter and Thread Pitch
Correctly measuring fasteners is essential when replacing screws, bolts or threaded components. Using the wrong size can lead to poor fitment, damaged threads or insecure assemblies.
This guide explains how to measure screws and bolts accurately before ordering replacements.
This guide explains how to measure screws and bolts accurately, including:
Thread length
Thread Size
Thread pitch or TPI
Head-type measurement rules
By following these steps you can identify the correct fastener size before ordering replacements.
How to Measure Screw or Bolt Length
Fastener length is measured differently depending on the type of head on the screw or bolt. The key rule is to measure the portion of the fastener that sits inside the material once installed.
Countersunk Head Screws
Examples include:
Because countersunk heads sit flush with the surface, the length is measured from:
Tip → Top of the head
This measurement includes the head because it becomes part of the installed length.
Non-Countersunk Head Screws
Examples include:
These heads sit above the material surface, so the length is measured from:
Tip → Underside of the head
The head itself is not included in the measurement.
Set Screws (Grub Screws)
Set screws normally have no head, meaning the entire fastener sits within the threaded hole.
Measure:
End → End
Shoulder Bolts
Shoulder bolts have two sections:
- The threaded portion
- The smooth shoulder
Length is measured across the shoulder section only, not the threaded portion.
How to Measure Fastener Diameter
The diameter of a screw or bolt refers to the major diameter of the thread, not the head.
This measurement determines which nut, washer or threaded hole the fastener will fit.
Metric Fasteners
Metric screws are labelled using the thread diameter in millimetres.
Example:
| Screw Size | Thread Diameter |
|---|---|
| M4 | 4mm |
| M6 | 6mm |
| M10 | 10mm |
An M6 screw will normally be used with an M6 nut and washer.
Imperial Fasteners
Imperial screws use inch measurements.
Example:
| Screw Size | Diameter |
|---|---|
| 1/4" | 0.25 inch |
| 5/16" | 0.3125 inch |
These sizes pair with matching imperial nuts and washers.
Measuring Thread Pitch or Thread Count
Two systems are used depending on whether the fastener is metric or imperial.
Metric threads are defined by pitch, which is the distance between threads measured in millimetres.
Example:
| Fastener | Pitch |
|---|---|
| M6 x 1.0 | 1mm between threads |
| M6 x 0.75 | 0.75mm between threads |
Lower pitch numbers indicate finer threads.
Imperial fasteners use Threads Per Inch (TPI).
Example:
| Screw Type | TPI |
|---|---|
| 1/4-20 UNC | 20 threads per inch |
| 1/4-28 UNF | 28 threads per inch |
To estimate TPI manually:
- Measure ½ inch of thread
- Count the threads
- Multiply by two
Common Fastener Measurement Mistakes
Incorrect measurements often occur when the wrong reference point is used.
Avoid these common mistakes:
Measuring the head diameter instead of the thread diameter
Fasteners are sized by the thread, not the head.
Measuring countersunk screws from under the head
Countersunk screws must be measured including the head.
Confusing metric and imperial fasteners
An M6 screw may appear close to a 1/4" bolt but they are not interchangeable.
Guessing thread pitch
Always confirm pitch or TPI when replacing a fastener.
Frequently Asked Questions
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