Unified Tap Drill Reference
Common UNC sizes
- #6-32 → #36 (.106 in)
- #8-32 → #29 (.136 in)
- #10-24 → #25 (.1495 in)
- 1/4-20 → #7 (.201 in)
- 5/16-18 → F (.257 in)
- 3/8-16 → 5/16 in
- 1/2-13 → 27/64 in
Rule of thumb
Tap Drill ≈ Major Diameter − (1.0825 / TPI)
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Tap drill sizes for common Unified inch threads at about 75 percent thread.
Try it in the calculatorSpeeds and feeds by size: tap RPM and feed · drill RPM and feed
Formula
Tap Drill = Major Dia − (1.0825 / TPI)Units: inch
Machine-readable reference
Structured metadata for formulas, tags, canonical status, and future diagram support.
Slug
tap-drill-un
Category
Threads & Fasteners
Canonical
Yes
Updated
4/23/2026
Units
inch
Tags
tap, drill, UNC, UNF
Source
Machinery's Handbook, 31st ed., Threads
Tap Drill ≈ Major Diameter − (1.0825 / TPI)
Source: Machinery's Handbook, 31st ed., Threads