How to Read Screw Sizes: Inch and Metric Guide
Quick answer: Screw size labels describe diameter, thread spacing and length. Inch examples such as #8-32 x 1 use a gauge number, threads per inch and length; metric examples such as M5 x 0.8 x 20 use diameter, pitch and length in millimeters.
What matters most
- A larger inch gauge generally means a larger major diameter.
- Threads per inch increases as inch threads become finer.
- Metric pitch is the distance between adjacent threads, so a smaller pitch is finer.
- Length is usually measured from under the head, except countersunk screws are commonly measured overall.
How to choose or use it
Always verify the applicable standard and measure with calipers and a thread gauge when fit matters. Do not infer interchangeability from similar dimensions: inch and metric threads can look close yet damage the mating thread.
Before buying, verify the screw diameter, length, thread, head style, drive, material and finish against the mating parts. A correct-looking screw can still be incompatible if its thread standard or corrosion performance is wrong.
Installation checklist
- Inspect the parts and confirm the required hole or mating thread.
- Use a correctly fitting driver and keep it aligned with the screw axis.
- Tighten only until the joint seats or to the specified torque; overtightening can strip threads or damage the workpiece.
- For structural, overhead or safety-critical joints, follow the approved drawing, code or fastener manufacturer’s data.