letter drill · nominal diameter
X
drill bit size.
A X bit measures 0.3970 in across (10.084 mm).
The inch decimal is the published nominal designation; millimetres are a conversion.
Calibrated relative diameter · 1 in reference
X · letter
0.3970 in
10.084 mm DesignationXletter
Inch decimal0.3970nominal
Metric conversion10.084millimetres
Where this size sits from 0 to 1 inch X · 10.084 mm on a zero to 25.4 millimetre scale. Neighbor marks: 10.1 mm, 10 mm.
Measured neighbors
What sits beside X.
“Nearest” means nominal diameter only. It does not make a neighboring size acceptable for a tolerance, tapping, reaming, or fastener application.
Source record
ASME.
The standard text is paywalled. Number and letter series are cross-checked against public manufacturer charts; exact standard edition/page verification remains a release gate.
Open source record