Tap Drill Size Calculator
Find the exact drill bit for any tap. Covers UNC/UNF machine screws, ISO metric threads and NPT pipe threads – with full reference charts and the formula behind every size.
Browse by thread system
Families of threads – each with its own chart and its own set of size pages.
Reference guides
Practical workshop references: tapped holes, die work, wood screws, unit conversion and prescription safety glasses.
Tapped Hole Guide
Sizing, callouts, blind vs through
ConceptTap & Die Chart
Die markings + drill for every tap
ChartPilot Holes
Wood screws #4-#14 + lag screws
WoodBSPT vs BSPP vs NPT
British pipe threads, full chart
PipeMM to Inches
Metric bit & measure conversion
ConverterPrescription Safety Glasses
SPH, CYL, axis and prescription notation
Workshop guideWhat is a tap drill size?
The tap drill is the hole you drill before running a tap to cut threads. Drill it too small and the tap breaks or jams; too big and the threads strip under load.
SAE: drillØ = majorØ − (pitch × %thread) · Metric: drillØ = majorØ − pitch
For NPT pipe threads this rule doesn't apply – they're sized to the pipe taper, which is why NPT taps use a different, fractional drill.