0.125 in · 3.18 mm nominal diameter.
Roll Pin Size Converter
Convert roll pin and spring pin sizes from inch fractions to decimal inches and millimeters, or enter a measured pin diameter to find the nearest common size. Handy for repairs, fixtures, shafts, handles, hinges, shop tooling, and small machinery.
Convert a roll pin size
Pick a standard pin size or enter a measured diameter from calipers.
A 1/8 inch roll pin is a common spring pin size for handles, light shafts, fixtures, and general shop repairs. Choose the length and pin style as carefully as the diameter.
How roll pin sizes convert
Roll pins are slotted spring pins that compress as they are driven into a hole. Hardware listings usually name the nominal outside diameter, such as 1/8 inch or 3 mm, plus the pin length. This converter translates the diameter into decimal inches and millimeters so you can compare inch and metric catalogs.
If you are measuring an old pin, use calipers on the relaxed outside diameter and compare it to the nearest standard size. A worn or crushed pin can read slightly smaller than the catalog diameter.
Choosing a roll pin hole
Unlike a loose cotter pin, a roll pin depends on spring pressure. The drilled hole is commonly near the nominal pin diameter or slightly under it, depending on material, pin duty, and manufacturer tolerance. The range shown here is a quick shop reference, not an engineering specification.
For firearms, brakes, steering, lifting equipment, press tooling, or production assemblies, use the part drawing or the pin manufacturer's recommended hole tolerance.
Roll pin size chart
| Pin size | Decimal inch | Millimeters | Reference hole range | Common use |
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Roll pin size FAQ
What size is a 1/8 roll pin in millimeters?
A 1/8 inch roll pin is 0.125 inch in nominal diameter, which converts to about 3.18 mm.
Is a roll pin the same as a spring pin?
Usually, yes. Roll pin is a common shop name for a slotted spring pin. Coiled spring pins are related but have different construction and may use different hole recommendations.
Should the hole be bigger or smaller than the roll pin?
A roll pin normally compresses into the hole, so the acceptable hole is often near the nominal size or slightly smaller. Use manufacturer tolerance data for important parts.