0.125 in · 3.18 mm nominal diameter.
Cotter Pin Size Converter
Convert cotter pin sizes from fractions to decimal inches and millimeters, or enter a measured pin diameter to find the nearest standard size. Useful for clevis pins, castle nuts, hitch pins, linkage hardware, trailers, farm equipment, and small repairs.
Convert a cotter pin size
Pick a common fractional pin size or enter a measured diameter.
A 1/8 inch cotter pin is a common retaining pin for light linkage, trailer, mower, and general hardware jobs. Match the pin length and material as well as the diameter.
How cotter pin sizes convert
Cotter pins are usually listed by nominal wire diameter and length. The diameter is often a fraction of an inch, such as 1/16, 3/32, 1/8, or 3/16 inch. This converter turns those sizes into decimal inches and millimeters for measuring with calipers or comparing metric hardware.
The listed size is the pin diameter, not the overall bent shape after the legs are spread. If you are replacing an old pin, measure the straight wire diameter near the head and choose the closest standard size.
Choosing a matching hole size
The pin should slide through the drilled hole without forcing, then be bent to retain the part. A practical clearance hole is often slightly larger than the pin diameter; this chart uses about 1/64 inch clearance as a quick reference.
For steering, brakes, towing, lifting, machinery guards, or any safety-critical assembly, follow the equipment manual or hardware specification instead of guessing from a chart.
Cotter pin size chart
| Pin size | Decimal inch | Millimeters | Reference hole | Common use |
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Cotter pin size FAQ
What size is a 1/8 cotter pin in millimeters?
A 1/8 inch cotter pin is 0.125 inch in diameter, which converts to about 3.18 mm.
How much clearance does a cotter pin hole need?
For a quick shop reference, a hole about 1/64 inch larger than the cotter pin diameter is often enough for easy insertion. Confirm the part drawing or manual when the joint is safety-critical.
Do cotter pin length and diameter both matter?
Yes. Diameter controls the fit through the hole; length controls how much leg remains to bend after the pin passes through the part. Replace both dimensions as closely as practical.