The nominal tool opening matches the selected size.
WORKSHOP CONVERTER
What size are you starting with?
PRACTICAL FIT
Tool recommendations
NOMINAL MEASUREMENTS
What the size marking actually measures
Nominal size is the stated standard size before manufacturing tolerance, coatings and wear are considered.
10 mm means the tool opening is nominally 10 mm between opposite parallel flats.
3/8 AF means 3/8 inch directly across the flats - exactly 9.525 mm.
3/8 SAE uses the same 3/8-inch across-flats opening as a 3/8 AF tool.
3/8 BSF identifies a 3/8-inch nominal thread. Its classic spanner jaw is about 0.600 inch across flats.
3/8 W identifies a 3/8-inch nominal thread. Its classic spanner jaw is about 0.710 inch across flats.
Before you put force on it
A small amount oversize. Use moderate force and inspect for wear.
A loose substitute. Avoid high torque, damaged heads and open-ended spanners.
Important: AF means “across flats”, not a thread family. Whitworth and BSF tool markings usually name the bolt thread diameter, not the jaw opening. The British values in this guide use the classic paired W/BSF spanner series; reduced-head, post-war and other variants can differ. Actual fasteners also vary by standard, head style, wear and manufacturing tolerance. Measure the head or recess when uncertain.