How to Measure Screw Size and Pitch
How to Measure Screw Size: Calculating Threads per Inch
Andrew Prestridge | September 12, 2020
How to Measure Screws
Screws are defined by three measurements: diameter, pitch, and length. Measure the diameter across the threads and the length from underneath the head of the screws (unless it's a flat-head, then measure the whole length of the screw.) For the pitch, mark out 1 inch of threads and count the number of threads in that length.
Diameter and length are fairly straight-forward and call out the overall dimensions for the physical size of the screw while the last measurement, the pitch, defines the threads and ensures the screw can actually screw into the nut or threaded hole.
For example, a common imperial screw is the #6-32x1/2” and we can see the each of the measurements is clearly specified, although in a slightly cryptic manner. #6-32x1/2” means a #6 screw (which has major diameter of 0.138”), with 32 Threads Per Inch (TPI), that is 1/2” long. There are multiple methods of measuring pitch, and sometimes a thread pitch gage is the quickest method.
Calculating Threads Per Inch (TPI)
The Threads Per Inch (TPI) is the number of threads along one inch of the length of the screw, just as the name suggests. By simply counting the number of threads and dividing by the length you can easily calculate the TPI of a screw.
Metric screws convey the same information, but with slightly different terminology: the second number is the length between threads, not the threads per inch. For instance, an M6x1x20 screw has a diameter of 6mm (M6 means Metric, not a #6 imperial), a pitch of 1mm and length of 20mm. The pitch of 1 doesn’t mean that the screw has only 1 thread per inch, but rather that each thread is spaced apart by 1 mm. Since there are 25.4 millimeters in 1 inch, the M6x1.00 screw has an equivalent TPI of 25.4.
As the TPI increases for screws it means there are more and more threads in the same one inch, so the threads are getting smaller and smaller: a 6-32 screw has bigger threads than a 6-40 screw. By contrast, in metric screws as the pitch increases the individual threads take up more space and are increasing in size, so an M6x1.00 has smaller threads than an M6x1.50 screw - TPI and pitch are inversely proportional.
For good or for bad, there hasn't been a Whitworth-esque standardization movement for gears.
Gears and Pitch
This same relationship holds for gears, the imperial dimension is Diametral Pitch and the metric dimension is called Module. The Diametral Pitch is the number of teeth of a gear per inch of its pitch diameter (effectively the same as a screw’s TPI), while Module is more directly the pitch of the gear. Just like in screws, a gear with a Module of 1 has an equivalent Diametral Pitch of 25.4. As the Module increases, gear teeth increase in size, but as Diametral Pitch increases those gear teeth decrease in size in order to fit more teeth into the same inch of pitch diameter. If you ever need to convert, just use the following equations:
Diametral Pitch = 25.4 / Module
Module = 25.4 / Diametral Pitch
Figure from "A Treatise on Gear Wheels" by George Grant, 11th Edition, (Figure 31 graphical comparison of gear pitch - with edits) 1906
Measure the major diameter across the threads. Higher is larger is both inch and metric screws
Inch screws count the Threads Per Inch, while metric screws measure the length between threads
Measure the length beneath the head of the screw, except flat-heads measure the whole length