Shaft Size Conversion Chart
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 chart below converts between all three for the standard inch-series sizes from 1/2 inch to 4 inches, and includes the sixteenths suffix that inch-series mounted bearing part numbers use to state the bore — a -16 unit fits a 1-inch shaft, a -32 fits 2 inches. How that suffix fits into a full catalog number is covered in the Mounted Bearing Nomenclature guide.
Conversion table
Millimeter values use the exact conversion of 25.4 mm per inch, rounded to two decimals.
| Fraction (in) | Bore suffix (16ths) | Decimal (in) | Millimeters |
|---|---|---|---|
| 1/2 | 8 | 0.5000 | 12.70 |
| 9/16 | 9 | 0.5625 | 14.29 |
| 5/8 | 10 | 0.6250 | 15.88 |
| 11/16 | 11 | 0.6875 | 17.46 |
| 3/4 | 12 | 0.7500 | 19.05 |
| 13/16 | 13 | 0.8125 | 20.64 |
| 7/8 | 14 | 0.8750 | 22.23 |
| 15/16 | 15 | 0.9375 | 23.81 |
| 1 | 16 | 1.0000 | 25.40 |
| 1-1/16 | 17 | 1.0625 | 26.99 |
| 1-1/8 | 18 | 1.1250 | 28.58 |
| 1-3/16 | 19 | 1.1875 | 30.16 |
| 1-1/4 | 20 | 1.2500 | 31.75 |
| 1-5/16 | 21 | 1.3125 | 33.34 |
| 1-3/8 | 22 | 1.3750 | 34.93 |
| 1-7/16 | 23 | 1.4375 | 36.51 |
| 1-1/2 | 24 | 1.5000 | 38.10 |
| 1-9/16 | 25 | 1.5625 | 39.69 |
| 1-5/8 | 26 | 1.6250 | 41.28 |
| 1-11/16 | 27 | 1.6875 | 42.86 |
| 1-3/4 | 28 | 1.7500 | 44.45 |
| 1-13/16 | 29 | 1.8125 | 46.04 |
| 1-7/8 | 30 | 1.8750 | 47.63 |
| 1-15/16 | 31 | 1.9375 | 49.21 |
| 2 | 32 | 2.0000 | 50.80 |
| 2-1/8 | 34 | 2.1250 | 53.98 |
| 2-3/16 | 35 | 2.1875 | 55.56 |
| 2-1/4 | 36 | 2.2500 | 57.15 |
| 2-5/16 | 37 | 2.3125 | 58.74 |
| 2-3/8 | 38 | 2.3750 | 60.33 |
| 2-7/16 | 39 | 2.4375 | 61.91 |
| 2-1/2 | 40 | 2.5000 | 63.50 |
| 2-11/16 | 43 | 2.6875 | 68.26 |
| 2-3/4 | 44 | 2.7500 | 69.85 |
| 2-15/16 | 47 | 2.9375 | 74.61 |
| 3 | 48 | 3.0000 | 76.20 |
| 3-3/16 | 51 | 3.1875 | 80.96 |
| 3-1/4 | 52 | 3.2500 | 82.55 |
| 3-7/16 | 55 | 3.4375 | 87.31 |
| 3-1/2 | 56 | 3.5000 | 88.90 |
| 3-15/16 | 63 | 3.9375 | 100.01 |
| 4 | 64 | 4.0000 | 101.60 |
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.