Shaft Size Conversion Chart & Calculator

Mounted bearings, bushings, and couplings all identify their bore by shaft size, but the same size appears in three notations depending on where you read it: a fraction on a print, a decimal on a micrometer, and millimeters on metric equipment. The calculator and chart below convert between all three, and include the sixteenths suffix that inch-series mounted bearing part numbers use to state the bore — a -16 unit (the Sealmaster NP-16 or Browning VPS-216, for example) fits a 1-inch shaft, a -32 (the Sealmaster NP-32) fits 2 inches. How that suffix fits into a full catalog number is covered in the Mounted Bearing Nomenclature guide.

Shaft size calculator

Standard inch shaft sizes

Standard inch shaft sizes from 1/4 to 5 inches: each shaft diameter as a fraction, its bore suffix, the decimal, and millimeters, using the exact conversion of 25.4 mm per inch, rounded to two decimals. Each row names parts bored for that shaft. Dimensions by shaft size are in the pillow block sizes, Q-D bushing sizes, split taper bushing sizes, and V-belt sheave sizes guides.

Standard metric shaft sizes (mm)

Standard metric shaft sizes in millimeters, from 10 mm to 300 mm: the sizes our bearings and bushings are bored for, with each one's decimal-inch equivalent and the nearest inch size. A metric shaft and its nearest inch size are never the same part: 30 mm is 0.16 mm smaller than 1-3/16 in, far outside a bearing's shaft tolerance, so order by the system the shaft was made in.

Measuring a shaft you cannot identify

Measure the shaft diameter with calipers at an unworn section — near the bearing seat but outside any polished or grooved wear band. Inch-series shafting is machined slightly under nominal, so a reading of 0.998 inches means a 1-inch shaft, not a metric 25 mm one; a reading close to a clean millimeter value (25.02 mm, for example) points to metric shafting instead. When a measurement falls between the two systems or the shaft is heavily worn, send us the measurement and the machine details through the support form and we will confirm the bore before you order. Ready to buy? Browse mounted bearings by housing type and filter by bore, or go straight to pillow blocks, Q-D bushings, or split taper bushings.

A mounted ball bearing wants its shaft at nominal size or slightly under it, never over. The recommended shaft tolerance for Sealmaster mounted ball bearings is below; the calculator checks a caliper reading against it. An ER insert states its shaft size the same way as the housed unit it goes in.

Fraction to decimal chart

Every 64th of an inch as an exact decimal and in millimeters. Sixteenths, the steps shaft sizes are made in, are in bold.