Home / Mechanical & electrical maintenance

Tap drill size

Pick the thread and the material to see which drill to use for a cutting or forming tap, how much thread you will get and how deep to drill a blind hole.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Check
Thread
Drill
Depth
—Drill for cutting tap
—

Thread engagement versus drill size

Cutting tap engagement for each drill diameter. The shaded band is the allowed minor diameter (6H or 2B); the orange line is the forming tap drill.

Tap the chart to see the values.

Hole depth breakdown

Usable thread, tap chamfer, chip room and 118° drill point.

Tap the chart to see the values.

Drill table for this system

Next step

Take this page’s results to another calculator with the fields already filled in.

Bolt tightening torque →

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Engagement: % = (D − d_drill) ÷ (1.299 · P) × 100 (UN: % = 76.98 · n · (D − d_drill), inches)
Forming tap drill: d = D − 0.0068 · P · % (P in mm, or 1/n in inches)
Internal minor diameter: D1 = D − 1.082532 · P; 6H: D1 to D1 + TD1(6) (ISO 965-1); 2B: D1 to D1 + 0.25P − 0.4P² (≥ 1/4 in) or 0.05P^(2/3) + 0.03P/D − 0.002 (< 1/4 in)
Blind hole: full-diameter depth = usable thread + chamfer (2.5P plug/form C; 1.75P bottoming/form E; 4.5P taper/form B) + chip room (threads × P); point = 0.3 · d_drill (118°)

Assumptions and limits

  • Recommended drills per ISO 2306 and DIN 336 (metric) and the usual 75% thread charts (UN). A real drill cuts slightly oversize (0.001 to 0.004 in); in stainless steel and titanium, aim for 60% to 65% engagement.
  • Minimum thread lengths by material are rules of thumb so the bolt breaks before the internal thread strips (steel 1 × D, cast iron 1.5 × D, aluminum 2 × D); for critical joints use VDI 2230 or FED-STD-H28.

Standards and technical documents

  • ISO 261:1998: ISO general purpose metric screw threads — General plan
  • ISO 724:1993: ISO general-purpose metric screw threads — Basic dimensions
  • ISO 965-1:2013: Tolerances — Principles and basic data
  • ISO 2306:1972: Drills for use prior to tapping screw threads
  • DIN 336: Bohrer-Durchmesser für Gewindekernlöcher
  • ASME B1.1-2019: Unified Inch Screw Threads (UN, UNR and UNJ thread forms)
  • Machinery’s Handbook, 31st ed.: Tap drill sizes and percentage of thread

U.S. regulations

  • OSHA 29 CFR 1910.212: general requirements for machine guarding
  • OSHA 29 CFR 1910.133: eye and face protection

Frequently asked questions

What drill size for a 1/2-13 tap?

Use a 27/64 in (0.4219 in) drill for a cutting tap, which gives about 78% thread. For a forming tap, use about 0.466 in (a 15/32 in drill is close).

How do I find the tap drill for any inch thread?

Subtract one pitch from the major diameter for about 77% thread: 3/8-16 needs 0.375 − 0.0625 = 0.3125 in, a 5/16 in drill. For a forming tap at 65%, subtract 0.0068 × 65 ÷ TPI: 0.375 − 0.442 ÷ 16 = 0.347 in.

How deep should a blind tapped hole be drilled?

Add the tap chamfer (2 to 3 threads on a plug tap), chip room and the drill point to the usable thread. For 3/4 in of 1/2-13 thread with a plug tap, drill about 1.10 in at full diameter, 1.22 in including the point.

Why not tap 100% thread?

Going beyond 75% adds only about 5% strength and doubles tapping torque. At 60% to 75%, the bolt breaks before the internal thread strips in most steels.