Thread Size Calculator

Look up the major diameter, minor diameter, pitch, and recommended tap-drill size for a UN (inch) or metric screw thread

Thread Specification

Thread Series

Thread Size

Thread Engagement (%)

75% is standard for most materials; drop to 50% for hard metals or deep blind holes.

Recommended Tap Drill #7 0.2013" calculated

Major Ø 0.2500"

Pitch / TPI 20 TPI

Minor Ø 0.1887"

Recommended drill for 1/4"-20 at 75% thread engagement: #7.

Tap Drill Diameter Tap Drill = Major − (Pitch × 1.299 × Engagement%) 60°-thread approximation (Machinery's Handbook constant 1.299038)

1/4"-20 — Basic Thread Dimensions

Basic (nominal) dimensions for a 60° unified or metric thread, computed from major diameter and pitch. These are reference dimensions, not Class 2A/2B fit tolerances — check a mechanical engineering handbook for tolerance-critical work.

Major Diameter 0.2500"

Pitch Diameter 0.2175"

Minor Diameter 0.1887"

Pitch 20 TPI (0.0500" pitch)

Pitch Diameter D₂ = Major − (0.6495 × Pitch)

Minor Diameter D₁ = Major − (1.2269 × Pitch)

Full Thread & Tap Drill Chart

Standard major diameter, pitch, minor diameter, pitch diameter, and recommended tap drill for every size in a series.

Size	Major Ø	Pitch / TPI	Minor Ø	Pitch Ø	Tap Drill

Tap drill column computed at 75% thread engagement — UNC (Unified National Coarse).

4 min read 5 steps 9 terms 3 examples 6 FAQs Tap Drill = Major Diameter − (Pitch × 1.299038 × Engagement% …

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Walk-through

How to Use This Calculator

5 steps

Choose a thread series

Pick UNC, UNF, Metric Coarse, or Metric Fine depending on the standard your fastener, tap, or existing hole follows. UNC and UNF are the two Unified inch standards used across North America; Metric Coarse and Metric Fine follow the ISO 68-1 standard used almost everywhere else.

Select the thread size

Choose the specific designation, e.g. 1/4"-20 for a quarter-inch UNC bolt or M6 x 1.0 for a 6mm coarse-pitch metric screw. The dropdown only lists standard sizes for the series you picked.

Set the thread engagement percentage

75% is the default and the standard recommendation for most materials. Drop to 50% for hard metals, deep blind holes, or brittle materials where full 75% thread engagement makes tapping difficult without adding meaningful strength.

Read the recommended tap drill

The hero result on the Tap Drill tab shows the nearest standard drill designation — a number, letter, or fractional drill size for inch threads, or a millimeter size for metric threads — along with the exact calculated decimal.

Check dimensions or browse the full chart

Switch to Thread Dimensions to see the major, pitch, and minor diameter for the size you selected, or open Full Chart to browse the complete lookup table for every standard size in a series.

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Reference

Formula & Methodology

3 formulas

Tap drill diameter (75%-thread approximation)

Tap Drill = Major Diameter − (Pitch × 1.299038 × Engagement% / 100)

The standard 60°-thread tap drill formula from Machinery's Handbook. Pitch is 1/TPI for inch (UNC/UNF) threads or the stated pitch in mm for metric threads. At 75% engagement this reproduces the well-known reference values — e.g. 1/4"-20 UNC gives 0.2013" (a #7 drill), and M6 x 1.0 gives 5.03mm, which rounds to the standard 5.0mm drill.

Pitch diameter (basic)

Pitch Diameter = Major Diameter − (0.649519 × Pitch)

The basic (nominal) pitch diameter for a 60° Unified or ISO metric thread — the diameter at which the thread groove width equals the thread ridge width.

Minor diameter (basic, external)

Minor Diameter = Major Diameter − (1.226869 × Pitch)

The basic (nominal) minor diameter — the smallest diameter of the thread, measured at the root. This calculator reports basic dimensions, not the tighter Class 2A/2B fit tolerances used for precision fasteners.

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Glossary

Key Terms Explained

9 terms

Major diameter The largest diameter of a screw thread, measured across the crest of an external thread (or the root of an internal thread). This is the number in a thread designation — e.g. the 0.25" in 1/4"-20, or the 6 in M6.

Minor diameter The smallest diameter of a screw thread, measured at the root of an external thread (or crest of an internal thread). It's roughly the diameter of the metal core left after tapping a hole.

Pitch diameter The diameter at which the thread ridge and thread groove are the same width — halfway between major and minor diameter. It's the dimension used to check thread fit with go/no-go gauges.

Pitch The axial distance between two adjacent thread crests, in inches or millimeters. For inch threads, pitch equals 1 divided by threads per inch (TPI).

TPI (Threads Per Inch) The number of complete threads in one inch of an inch-series (UNC/UNF) screw, measured along the axis. Higher TPI means a finer, more closely spaced thread.

Thread engagement The percentage of full thread depth that is actually cut when tapping a hole. 75% engagement is the standard target — it provides most of a fastener's holding strength while leaving the tap easier to turn than a 100% full thread.

UNC (Unified National Coarse) The coarse-pitch inch thread standard used for most general-purpose bolts, machine screws, and nuts in North America. Fewer, deeper threads per inch than UNF at the same diameter.

UNF (Unified National Fine) The fine-pitch inch thread standard. More threads per inch than UNC at the same diameter, giving a larger minor diameter (more shear strength on thin-walled parts), finer adjustment, and better resistance to vibration loosening.

Tap drill The drill bit size used to bore a hole before cutting internal threads with a tap. Sizing it correctly controls how much thread engagement the finished hole ends up with.

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Scenarios

Real-World Examples

3 worked examples

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Home mechanic

Tapping a 1/4"-20 UNC hole in steel at the standard 75% engagement

Thread series UNC Thread size 1/4"-20 Thread engagement 75%

The calculator recommends a #7 drill (0.201"), matching the standard machinist's reference chart. Major diameter is 0.250", minor diameter is 0.189", and pitch is 0.050" (1/20").

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Machinist

Tapping M6 x 1.0 metric coarse in aluminum at 75% engagement

Thread series Metric Coarse Thread size M6 x 1.0 Thread engagement 75%

The calculator recommends a 5.0mm drill, the standard published tap drill size for M6 x 1.0. Major diameter is 6.00mm and minor diameter is 4.77mm.

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Toolmaker in hardened steel

Reducing thread engagement to 50% for a deep blind hole in hard tool steel

Thread series UNC Thread size 3/8"-16 Thread engagement 50%

Dropping engagement from 75% to 50% enlarges the tap drill (easier tapping, lower torque) at the cost of some thread strength — an accepted trade-off in hard materials where a 75% thread is difficult to tap without breaking the tap.

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Deep Dive

Understanding the Thread Size Calculator

3 chapters · 4 min read

Picking the wrong tap drill is one of the most common — and most expensive — mistakes in a home shop or machine shop: too small and you snap the tap in the hole, too large and the threads never grip. This calculator applies the standard 60°-thread tap drill formula to any UNC, UNF, metric coarse, or metric fine size, and shows the full basic thread geometry alongside it.

How the tap drill formula works

Every 60° Unified or ISO metric thread follows the same basic-dimension relationships, published in Machinery's Handbook and ASME B1.1 / ISO 68-1. The tap drill diameter is the major diameter minus the pitch scaled by a constant (1.299038) and by the desired thread engagement percentage. At the standard 75% engagement, this formula reproduces the tap drill sizes found on every standard machinist's chart — a 1/4"-20 UNC gives 0.201" (a #7 drill), and an M6 x 1.0 gives 5.0mm.

Minor diameter and pitch diameter follow the same pattern with different constants (1.226869 and 0.649519 respectively). These are basic (nominal) dimensions — the numbers a drawing would call out before adding Class 2A/2B fit tolerances for precision fasteners.

Inputs and what they mean

Thread series picks the family of standard sizes: UNC and UNF (inch, North America) or Metric Coarse and Metric Fine (ISO, most of the rest of the world). Thread size selects the specific designation within that series — for example 1/2"-13 UNC or M12 x 1.75 metric coarse. Thread engagement is the one input worth experimenting with: 75% is the default and suits most materials, but many machinists drop to 50% engagement in hard or brittle materials, or in deep blind holes, where a full 75% thread is difficult to tap and doesn't add much real-world strength.

Limits and edge cases

This calculator reports basic (nominal) thread dimensions, not Class 2A/2B fit tolerances — for tolerance-critical or safety-critical fastening, verify against ASME B1.1 (inch) or ISO 965 (metric) tolerance tables. The tap drill designation shown for inch sizes is the nearest standard jobber drill by decimal size; very unusual engagement percentages may land between two standard drills, in which case the exact calculated decimal (also shown) is the more precise reference. For blind (non-through) holes, drill 1-2 pitch lengths deeper than the thread depth needed, since a tap cannot fully form threads all the way to the bottom of a drilled hole.

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Questions

Frequently Asked Questions

6 questions

What drill size do I use to tap a 1/4-20 hole?+

Use a #7 drill (0.201"). At the standard 75% thread engagement, the tap drill formula gives 0.2013", which rounds to the standard #7 jobber drill — the size found on every machinist's tap drill chart.

What size drill bit do I need for an M6 tap?+

Use a 5.0mm drill for M6 x 1.0 at 75% thread engagement. The quick rule of thumb for metric threads is drill size ≈ major diameter minus pitch, then adjusted for the target engagement percentage.

What thread engagement percentage should I use?+

75% is the standard for most materials and general-purpose fastening — it gives most of a fastener's holding strength while keeping the tap easy to turn. Drop to 50% engagement in hard metals or deep blind holes, where a full 75% thread makes tapping difficult without adding meaningful strength.

What is the difference between UNC and UNF threads?+

UNC (Unified National Coarse) has fewer, deeper threads per inch and is the general-purpose US standard for most bolts and machine screws. UNF (Unified National Fine) packs more threads per inch into the same diameter, giving a larger minor diameter, finer adjustment, and better resistance to vibration loosening — at the cost of being easier to cross-thread and more sensitive to over-torquing.

Where can I see the full tap drill chart?+

The Full Chart tab lists major diameter, pitch or TPI, minor diameter, pitch diameter, and the recommended tap drill for every standard UNC, UNF, metric coarse, and metric fine size covered by this calculator.

Do I need a different drill size for a blind hole?+

The drill diameter itself doesn't change for a blind (non-through) hole, but drill 1-2 pitch lengths deeper than the thread depth you actually need, since a standard tap can't fully form threads all the way to the bottom of the hole.

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