Imperial 1/4 inch = metric 6.35 mm, Imperial 1/2 inch = metric 12.7 mm. Specifications of the standard: the front 1/4 or 1/2 is the handle, the back is the size of the knife size specification.

Part No. Size(inch) D(mm) R(mm) B(mm)  Is a semicircular knife 1/4 X 1/4 23.5 3.17 6.35  Is a semicircular knife 1/4 X 5/16 25.0 3.96 7.93  Is a semicircular knife 1/4 X 3/8 26.5 4.76 9.52  Is a semicircular knife 1/4 X 1/2 29.5 6.35 12.7  Is a semicircular knife 1/4 X 5/8 33.0 7.96 15.9  Is a semicircular knife 1/4 X 3/4 36.0 9.52 19.05  Is a semicircular knife 1/4 X 7/8 39.2 11.1 22.2