Tap Drill Size Calculator

Direct answer. Enter a thread's major diameter and pitch (Metric) or major diameter and threads-per-inch (Unified), plus your target percent-of-thread engagement. The calculator returns the theoretical tap drill diameter for that engagement.

Method. For any 60-degree V-thread (both ISO metric and Unified inch threads share this included angle), the fundamental sharp-V triangle height is H = (sqrt(3)/2) x pitch - basic 60-degree triangle trigonometry, stated directly in ASME B1.1 as "1.000H = 0.86602540P". The long-standing American National / Sellers "percent of thread" convention used by published tap-drill calculators defines 100% thread as removing a diametral allowance of 2 x (3/4)H = 1.5H = (3 x sqrt(3)/4) x pitch, approximately 1.299038 x pitch, from the major diameter. Scaling that by your target percent gives: drill diameter = major diameter - 1.299038 x pitch x (percent thread / 100).

Worked example. M6 x 1.0 at 75% thread: 6.0 - 1.299038 x 1.0 x 0.75 is about 5.03 mm, close to the commonly published 5.0 mm tap drill for M6 x 1.0. A 1/4-20 UNC thread (pitch = 1/20 in) at 75% thread: 0.25 - 1.299038 x 0.05 x 0.75 is about 0.2013 in, close to the commonly published #7 drill (0.2010 in) for 1/4-20 UNC.

Definitions. Major diameter is the largest (crest-to-crest) diameter of an external thread, i.e. the thread's nominal size. Pitch is the axial distance between adjacent thread crests, in millimetres. TPI (threads per inch) is the inch-system equivalent - pitch (in) = 1 / TPI. Percent thread engagement describes how deep the generated thread profile is cut relative to the standard's full 100% double-depth reference; lower percentages cut a shallower, easier-to-tap thread with less contact area, higher percentages cut closer to full thread depth.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Thread standard

Metric (ISO)

Major (nominal) diameter

6

Thread pitch (Metric mode only)

1 mm

Threads per inch (Unified mode only)

20 TPI

Target thread engagement

75 %

Primary result: Tap drill diameter: 5.026 mm.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

Theoretical approximation, not an inspection or compliance tool. This calculator computes an idealized geometric drill diameter from thread geometry and your target engagement percentage. It never implies that the computed value, or any hole drilled to it, has been inspected, verified, or meets a dimensional-compliance, QA, or contractual thread-fit requirement (2B/3B, 6H/6g or otherwise). Verify critical or structural threaded joints with the applicable engineering print, thread gauge, or a qualified inspector.

Real twist drills commonly cut a hole slightly oversize (drill wander/runout), which increases actual thread engagement above the nominal drill diameter's theoretical value. This tool does not model drill oversize, tap class fits, material spring-back (notably in softer metals and plastics), blind-hole tap drill depth, or any specific tap or drill manufacturer's published chart - those can differ from this idealized result. Always round to your nearest available drill (fractional, number, letter or metric) and confirm fit in your actual material before cutting a production part.