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How to Measure Thread Size

June 12, 2024

How to Measure Thread Size

Thread size measurement is a crucial task in various applications, ranging from manufacturing to repairs. Accurately determining thread size ensures compatibility and proper function, which is vital for mechanical assemblies. This guide provides a step-by-step approach to measuring thread size, covering both internal and external threads.

Understanding Thread Parameters

Before diving into the measurement process, it's important to understand the key parameters of threads:

  1. Diameter: The major diameter (external threads) or minor diameter (internal threads).
  2. Pitch: The distance between threads.
  3. Thread Angle: The angle between the threads.

Tools Required

Measuring External Threads

Step 1: Measure the Major Diameter

Use a caliper to measure the major diameter of the external thread. Place the caliper jaws on the crest of the thread and note the measurement. Ensure the caliper is perpendicular to the thread axis for accuracy.

Step 2: Determine the Thread Pitch

Using a thread pitch gauge, match the gauge teeth to the thread. When the gauge fits snugly into the thread grooves without gaps or overlaps, you've found the correct pitch. Record this value.

Step 3: Check the Thread Angle

Most threads have a standard angle, such as 60° for metric threads. If necessary, use a thread comparator to verify the thread angle matches the standard.

Measuring Internal Threads

Step 1: Measure the Minor Diameter

For internal threads, use a caliper to measure the minor diameter. Insert the caliper inside the threaded hole and expand it until it touches the thread roots. Ensure accurate placement for a precise measurement.

Step 2: Determine the Thread Pitch

As with external threads, use a thread pitch gauge. Insert the gauge into the internal threads until it fits perfectly. Note the pitch value.

Step 3: Verify Thread Angle

Internal threads typically adhere to standard angles. Use a thread comparator or consult thread specifications to confirm the angle.

Tips for Accurate Measurements

Common Thread Standards


Measuring thread size accurately is essential for ensuring proper fit and function in mechanical assemblies. By using the right tools and following systematic steps, you can determine thread size with confidence. Whether working with external or internal threads, understanding the key parameters and standards will aid in achieving precise measurements.