ThreadSizeGuideThreads · Drills · Fasteners

Fastener fundamentals

Bolt Head and Wrench Sizes: Why Thread Size Is Not Enough

Choose a wrench from the measured width across flats and the fastener’s head standard—not from thread diameter alone.

Thread diameterIdentifies thread size
Across flatsSelects wrench/socket
Head heightAffects envelope
Head styleChanges dimensions

Hex bolts, hex cap screws, socket-head screws and flange bolts can share a thread size while requiring different tools and clearances.

Across flats versus across corners

A hex wrench or socket fits the distance across opposite flats. Across-corners diameter is larger and matters for surrounding clearance, but it is not the wrench label.

Avoid universal head-size assumptions

Product standards and head styles differ. Measure an existing fastener or consult the exact manufacturer drawing or governing standard before designing a counterbore or tool-access envelope.

Important limitation
A wrench that fits does not identify the bolt grade, thread or safe torque.