Metric screws use a Thread Pitch whereas US screws use 'Threads Per Inch' (TPI) to calculate the different thread types.
Metric Thread Pitch is calculated in millimeters (mm.). It measures the distance between two thread peaks. For Thread pitch, the smaller the number the finer the threads. Many US and Metric screws have both Coarse and Fine thread versions of the same screw size, making it frustrating to track down the needed screw. Below we hope to help with that.
Metric Fasteners – Thread Pitch
Thread pitch calculates the difference between two threads for metric fasteners. In the above example, that is distance is 1.25mm.
To measure thread pitch, use a caliper and measure from the peak of one thread to the next. Generally, the distance between threads is so small that measuring the distance between 10 threads and then moving the placeholder one place is much easier and more accurate.
Common metric screw notation example
M8-1.25 x 40mm
Since we are dealing with a metric screw, all figures will be in mm (millimeters).
M8 - Diameter of the bolt (or 8mm)
1.25 - Thread Pitch (1.25 millimeters between the top of each thread)
40mm - Length of the bolt
US / Inch fasteners – TPI / Threads Per Inch
The term TPI relates to how many threads there are in an inch. For bolts longer than an inch, this is easy to do as we mark off an inch of the screw and count the number of threads in that inch, as shown above. It may be easier to see the threads if you take a photo with your phone and zoom in. Counting threads becomes highly problematic when measuring shorter screws (shorter than an inch) or fasteners such as nuts-- if necessary, you can measure fractions of an inch and convert back to an inch by multiplying by the denominator used (ie., 16 threads in 1/2", then multiply 16 x 2 (denominator) to get 32 threads per inch)
Common US screw notation example
#10-32 x 6″
- #10 - Diameter or size of the bolt
- 32 - Threads per inch
- 6″ - Length of the bolt
US - Screw sizes in inches
Fastener Information - Sizing, Specifications and General Help