Free CNC Engineering Tool

Thread & Tap Drill Calculator

Quickly calculate tap drill sizes for common Metric, UNC and UNF internal threads. Select the thread size and desired thread engagement to see the calculated hole diameter and nearest standard drill size.

Tap Drill Size Calculator

Designed for common straight 60° threads using cutting taps.

Select the thread size shown on your drawing.
75% is a common reference value for cutting taps.
Calculated Tap Drill Diameter
6.78 mm
0.2670 in
Nearest Standard Drill
6.8 mm Standard metric drill
Thread M8 × 1.25
Major Diameter 8.000 mm
Pitch 1.250 mm
TPI 20.32
Engagement 75%

Reference only: final tap drill size may vary with material, tap geometry, thread tolerance, coating, lubrication and the tool manufacturer’s recommendation.

How to Use the Tap Drill Calculator

The calculator is designed to give a quick machining reference for common Metric, UNC and UNF internal threads. In most cases, you only need three pieces of information from the drawing.

1

Select the Thread Standard

Choose Metric for ISO metric threads, UNC for Unified National Coarse threads, or UNF for Unified National Fine threads.

2

Select the Thread Size

Select the nominal thread diameter and pitch or threads per inch shown on the engineering drawing.

3

Choose Thread Engagement

Select the desired thread percentage. The calculator then estimates the tap drill diameter and the nearest standard drill size.

Tap Drill Size Formula

For standard 60° cutting-tap threads, a simple workshop formula can be used to estimate the tap drill size before selecting the nearest available standard drill.

Metric Thread Quick Formula
Tap Drill ≈ Major Diameter − Pitch
This quick rule is commonly used as a practical starting point for standard metric cutting taps.
Example: M8 × 1.25
Major diameter 8.00 mm
Pitch 1.25 mm
Estimated drill diameter 8.00 − 1.25 = 6.75 mm
Common standard drill 6.8 mm
UNC / UNF Quick Formula
Pitch = 1 ÷ TPI
Once the pitch is known, subtract it from the nominal major diameter as a quick starting estimate.
Example: 1/4-20 UNC
Major diameter 0.250 in
Threads per inch 20 TPI
Pitch 1 ÷ 20 = 0.050 in
Estimated drill diameter 0.250 − 0.050 = 0.200 in
Common standard drill #7 = 0.201 in
The quick formulas above are useful for general reference, but the calculator in Section 1 also considers the selected thread engagement percentage. For production work, always verify the final pre-drill size against the tap manufacturer's data when available.

Common Metric Tap Drill Sizes

Use this chart as a quick reference for common ISO metric internal threads made with cutting taps. The listed drill sizes are typical workshop values and may be adjusted for material, thread engagement, tolerance and specific tap manufacturer recommendations.

Thread System ISO Metric 60°
Primary Use Internal Cutting-Tap Threads
Quick Reference Rule Major Diameter − Pitch

Metric Coarse Threads

Most common general-purpose metric thread series.

Thread Size Major Diameter Pitch Common Tap Drill Inch Equivalent
M2 × 0.40 2.00 mm 0.40 mm 1.60 mm 0.0630 in
M2.5 × 0.45 2.50 mm 0.45 mm 2.05 mm 0.0807 in
M3 × 0.50 3.00 mm 0.50 mm 2.50 mm 0.0984 in
M4 × 0.70 4.00 mm 0.70 mm 3.30 mm 0.1299 in
M5 × 0.80 5.00 mm 0.80 mm 4.20 mm 0.1654 in
M6 × 1.00 6.00 mm 1.00 mm 5.00 mm 0.1969 in
M8 × 1.25 8.00 mm 1.25 mm 6.80 mm 0.2677 in
M10 × 1.50 10.00 mm 1.50 mm 8.50 mm 0.3346 in
M12 × 1.75 12.00 mm 1.75 mm 10.20 mm 0.4016 in
M14 × 2.00 14.00 mm 2.00 mm 12.00 mm 0.4724 in
M16 × 2.00 16.00 mm 2.00 mm 14.00 mm 0.5512 in
M18 × 2.50 18.00 mm 2.50 mm 15.50 mm 0.6102 in
M20 × 2.50 20.00 mm 2.50 mm 17.50 mm 0.6890 in
M24 × 3.00 24.00 mm 3.00 mm 21.00 mm 0.8268 in
M30 × 3.50 30.00 mm 3.50 mm 26.50 mm 1.0433 in

Common Metric Fine Threads

Fine-pitch threads use a smaller pitch and therefore a larger tap drill.

Thread Size Major Diameter Pitch Common Tap Drill Inch Equivalent
M3 × 0.35 Fine 3.00 mm 0.35 mm 2.65 mm 0.1043 in
M4 × 0.50 Fine 4.00 mm 0.50 mm 3.50 mm 0.1378 in
M5 × 0.50 Fine 5.00 mm 0.50 mm 4.50 mm 0.1772 in
M6 × 0.75 Fine 6.00 mm 0.75 mm 5.25 mm 0.2067 in
M8 × 1.00 Fine 8.00 mm 1.00 mm 7.00 mm 0.2756 in
M10 × 1.25 Fine 10.00 mm 1.25 mm 8.80 mm 0.3465 in
M10 × 1.00 Fine 10.00 mm 1.00 mm 9.00 mm 0.3543 in
M12 × 1.50 Fine 12.00 mm 1.50 mm 10.50 mm 0.4134 in
M12 × 1.25 Fine 12.00 mm 1.25 mm 10.80 mm 0.4252 in
M14 × 1.50 Fine 14.00 mm 1.50 mm 12.50 mm 0.4921 in
M16 × 1.50 Fine 16.00 mm 1.50 mm 14.50 mm 0.5709 in
M18 × 1.50 Fine 18.00 mm 1.50 mm 16.50 mm 0.6496 in
M20 × 1.50 Fine 20.00 mm 1.50 mm 18.50 mm 0.7283 in
M24 × 2.00 Fine 24.00 mm 2.00 mm 22.00 mm 0.8661 in
M30 × 2.00 Fine 30.00 mm 2.00 mm 28.00 mm 1.1024 in
Important: These are common reference drill sizes for cutting taps, not universal production requirements. A slightly larger or smaller pre-drill may be selected depending on the required thread percentage, workpiece material, thread tolerance, tap geometry and tooling supplier recommendations.

Common UNC & UNF Tap Drill Sizes

The Unified Thread Standard uses thread size and threads per inch (TPI) instead of metric pitch. UNC uses a coarser pitch, while UNF uses a finer pitch for the same nominal diameter. The tables below list common tap drill sizes for internal threads made with cutting taps.

Thread Form Unified 60°
UNC Unified National Coarse
UNF Unified National Fine

UNC Tap Drill Chart Coarse

Common general-purpose Unified coarse threads.

Thread Size Major Diameter TPI Pitch Common Tap Drill Drill Diameter Metric Equivalent
#4-40 UNC 0.1120 in 40 0.0250 in #43 0.0890 in 2.26 mm
#6-32 UNC 0.1380 in 32 0.0313 in #36 0.1065 in 2.71 mm
#8-32 UNC 0.1640 in 32 0.0313 in #29 0.1360 in 3.45 mm
#10-24 UNC 0.1900 in 24 0.0417 in #25 0.1495 in 3.80 mm
1/4-20 UNC 0.2500 in 20 0.0500 in #7 0.2010 in 5.11 mm
5/16-18 UNC 0.3125 in 18 0.0556 in F 0.2570 in 6.53 mm
3/8-16 UNC 0.3750 in 16 0.0625 in 5/16" 0.3125 in 7.94 mm
7/16-14 UNC 0.4375 in 14 0.0714 in U 0.3680 in 9.35 mm
1/2-13 UNC 0.5000 in 13 0.0769 in 27/64" 0.4219 in 10.72 mm
9/16-12 UNC 0.5625 in 12 0.0833 in 31/64" 0.4844 in 12.30 mm
5/8-11 UNC 0.6250 in 11 0.0909 in 17/32" 0.5313 in 13.49 mm
3/4-10 UNC 0.7500 in 10 0.1000 in 21/32" 0.6563 in 16.67 mm
7/8-9 UNC 0.8750 in 9 0.1111 in 49/64" 0.7656 in 19.45 mm
1-8 UNC 1.0000 in 8 0.1250 in 7/8" 0.8750 in 22.23 mm

UNF Tap Drill Chart Fine

Fine threads use more threads per inch at the same nominal diameter.

Thread Size Major Diameter TPI Pitch Common Tap Drill Drill Diameter Metric Equivalent
#4-48 UNF 0.1120 in 48 0.0208 in #42 0.0935 in 2.37 mm
#6-40 UNF 0.1380 in 40 0.0250 in #33 0.1130 in 2.87 mm
#8-36 UNF 0.1640 in 36 0.0278 in #29 0.1360 in 3.45 mm
#10-32 UNF 0.1900 in 32 0.0313 in #21 0.1590 in 4.04 mm
1/4-28 UNF 0.2500 in 28 0.0357 in #3 0.2130 in 5.41 mm
5/16-24 UNF 0.3125 in 24 0.0417 in I 0.2720 in 6.91 mm
3/8-24 UNF 0.3750 in 24 0.0417 in Q 0.3320 in 8.43 mm
7/16-20 UNF 0.4375 in 20 0.0500 in 25/64" 0.3906 in 9.92 mm
1/2-20 UNF 0.5000 in 20 0.0500 in 29/64" 0.4531 in 11.51 mm
9/16-18 UNF 0.5625 in 18 0.0556 in 33/64" 0.5156 in 13.10 mm
5/8-18 UNF 0.6250 in 18 0.0556 in 37/64" 0.5781 in 14.68 mm
3/4-16 UNF 0.7500 in 16 0.0625 in 11/16" 0.6875 in 17.46 mm
7/8-14 UNF 0.8750 in 14 0.0714 in 51/64" 0.7969 in 20.24 mm
1-12 UNF 1.0000 in 12 0.0833 in 59/64" 0.9219 in 23.42 mm
Common UNC Example 1/4-20 UNC → #7 Drill

A #7 drill measures approximately 0.2010 in (5.11 mm) and is a common cutting-tap reference for 1/4-20 UNC.

Common UNF Example 1/4-28 UNF → #3 Drill

A #3 drill measures approximately 0.2130 in (5.41 mm) and is a common cutting-tap reference for 1/4-28 UNF.

Important: These values are practical reference sizes for cutting taps. The final pre-drill diameter can change with thread class, required thread percentage, workpiece material, tap geometry and tooling manufacturer recommendations.

Understanding Thread Engagement

Common Thread Engagement Ranges

Cutting Tap vs. Forming Tap

Cutting taps and forming taps create internal threads in different ways. Because the material removal process is different, they generally require different pre-drill diameters. The calculator above is intended primarily for conventional cutting taps.

Cutting Tap

Material Is Cut Away

A cutting tap forms the internal thread by removing material and producing chips. The tap drill hole must leave enough material for the cutting edges to generate the required thread profile.

  • Produces chips during tapping
  • Common for general CNC machining
  • Suitable for a wide range of materials
  • Tap drill charts commonly refer to cutting taps
  • Works for through holes and blind holes with suitable tap geometry
Forming / Roll Tap

Material Is Displaced

A forming tap does not cut chips. Instead, it plastically displaces the workpiece material to create the thread form. Because material flows into the thread profile, the starting hole normally needs to be larger than the hole used for a cutting tap.

  • Chipless threading process
  • Also called roll tapping or thread forming
  • Requires ductile workpiece material
  • Usually needs a larger pre-drilled hole
  • Pre-drill tolerance is often more critical
Cutting Tap Smaller pre-drill leaves material to be removed by the tap.
≠
Forming Tap Larger pre-drill allows material to flow into the thread shape.

Key Differences

Feature Cutting Tap Forming / Roll Tap
Thread Creation Material is cut away Material is plastically displaced
Chips Yes No chips from the threading process
Typical Pre-Drill Smaller Larger
Suitable Materials Broad range of machinable metals and plastics Best suited to sufficiently ductile materials
Hole Size Sensitivity Important Often more critical
Chip Control Important, especially in blind holes No cutting chips to evacuate
Calculator on This Page Suitable as a general cutting-tap reference Do not use directly without forming-tap data

Important: Do not use a conventional cutting-tap drill size automatically for a forming tap.

Forming taps generally require a larger and more tightly controlled pre-drill diameter. When a forming tap is specified, use the pre-drill range recommended by the actual tap manufacturer and verify that the workpiece material is suitable for thread forming.

Often Suitable for Forming Ductile Materials

Aluminum alloys, low-carbon steels, copper alloys and some stainless steels are common candidates when material ductility and tooling conditions are appropriate.

Use Caution Brittle or Low-Ductility Materials

Brittle materials or materials with insufficient ductility may not respond well to roll tapping. In these cases, cutting taps or another threading method may be more appropriate.

Thread & Tap Drill FAQ

What size drill is commonly used for M8 × 1.25?

A 6.8 mm drill is a common workshop reference for an M8 × 1.25 internal thread made with a cutting tap.

What tap drill is commonly used for 1/4-20 UNC?

A #7 drill, approximately 0.2010 in or 5.11 mm, is commonly used for a 1/4-20 UNC cutting-tap thread.

Does 75% thread mean 75% of the threaded hole length?

No. It refers to the approximate thread-form engagement, not the axial length of the threaded section.

Can I use the same drill size for a forming tap?

Usually not. Forming taps normally require a larger pre-drilled hole. Always follow the forming-tap manufacturer's recommended pre-drill size.

Can this calculator be used for NPT threads?

No. NPT and NPTF are tapered pipe threads and should use the appropriate pipe-thread tap drill data instead.

This calculator and the tables above are intended as general machining references. For critical production parts, always verify the drawing, thread standard, tolerance class and cutting-tool manufacturer's data.

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