Identify the thread type: metric, unified, or pipe
Measure the outside diameter of the male thread or inside diameter of the female thread
Count the number of threads per inch for imperial threads
Measure the distance between threads for metric threads
Use a thread pitch gauge to match the thread spacing
Compare the measurements to a thread size chart
Check whether the thread is tapered or straight
Verify the thread standard and class if needed
Confirm the size with a known matching nut, bolt, or fitting