Thread Sizes & Thread Chart for Machined Parts
Metric (M) and Unified (UNC/UNF) designations, common sizes, internal vs external threads, tolerance classes, and thread milling vs tapping — decoded for machined parts.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 8 min read
On this page
Quick Answer
Thread sizes come in two families: metric (M8 × 1.25 = 8 mm diameter, 1.25 mm pitch) and Unified (1/4-20 UNC = quarter-inch, 20 threads per inch). The tolerance class tells you the fit — 6g/6H for metric external/internal, 2A/2B for Unified. Common sizes run M3–M12 and #10-32 to 3/8-24. Tapping is the standard for internal threads; thread milling is for flexibility and hard materials.
Two Families, One Idea
Every thread on a machined part belongs to one of two families — metric ISO or Unified inch — and each encodes the same three facts: the diameter, the pitch, and the fit. Metric uses the M prefix with millimeters; Unified uses inches with threads-per-inch. Read those three facts correctly and you know exactly what to machine and how tight it has to be. This guide decodes both systems, the common sizes, and the tolerance classes, so a thread callout on a drawing is never a guess.
Metric Threads: M, Diameter, Pitch
A metric thread is written as M8 × 1.25 — M prefix, 8 mm major diameter, 1.25 mm pitch. Coarse pitch is implied by diameter, so M10 alone means the standard coarse pitch; you only write the pitch when it is a fine or special thread (M10 × 1.25, M12 × 1.5). The fit is appended as a tolerance class: 6g for external threads and 6H for internal are the defaults, with 6h a tighter zero-allowance external variant.
Unified Threads: Inches and TPI
A Unified thread is written as 1/4-20 UNC — quarter-inch major diameter, 20 threads per inch, Unified Coarse series. Fine pitch swaps the series: 1/4-28 UNF. Because the series implies the TPI (1/4 UNC is always 20), the number is sometimes omitted, but writing it out removes ambiguity. Fits use 2A for external and 2B for internal as the standard medium classes.
Common Thread Sizes Chart
The sizes that show up on CNC drawings, by family:
| Standard | Common sizes | Designation | Default fit |
|---|---|---|---|
| Metric coarse | M3, M4, M5, M6, M8, M10, M12 | M8 × 1.25 | 6H internal / 6g external |
| Metric fine | M8 × 1, M10 × 1.25, M12 × 1.5 | M10 × 1.25 | 6H / 6g |
| Unified coarse (UNC) | #10-32, 1/4-20, 5/16-18, 3/8-16 | 1/4-20 UNC | 2B internal / 2A external |
| Unified fine (UNF) | 1/4-28, 3/8-24, 1/2-20 | 1/4-28 UNF | 2B / 2A |
This is a shop-reference chart, not an exhaustive standard — a full tap-drill table would list every size and drill. But these are the threads that cover the overwhelming majority of machined parts, and the designation rules apply to any size.
Internal vs External, and the Fit Classes
External threads are on the outside of a cylinder (screws, bolts, shafts); internal threads are inside a hole (nuts, tapped bores). The notation encodes the distinction by letter case: metric uses lowercase for external (6g) and uppercase for internal (6H); Unified uses A for external and B for internal (2A vs 2B). The default medium fit is assumed when no class is written, which is why most drawings just say M6 or 1/4-20 and leave the fit implied. For the tolerance language behind those classes, see our tolerance guide. For how those threads pair with heads, drives, and body styles, see our guide to types of fasteners. When the part is a small threaded shaft or pin and the quantity is high, Swiss machining usually wins the quote — the guide bushing keeps long, slender threads from deflecting.
For the hole modification that seats a flat-head screw, see our countersink holes guide.
Tapping vs Thread Milling, and Depth
Internal threads are made two ways. Tapping drives a tap into a drilled hole — fast and economical for standard holes in production-friendly materials. Thread milling cuts the thread with a helical toolpath — more flexible (one tool for multiple diameters and pitches), better for large holes and hard materials, and lower tap-break risk. On depth, the profile follows the pitch: external thread depth is about 0.613 × pitch, internal about 0.541 × pitch, and blind holes are tapped slightly deeper than the required full thread to account for the chamfer and incomplete threads. For the hole that precedes the thread, see our reaming guide.
FAQ
How are metric thread sizes designated?
Metric threads use an M prefix, the nominal major diameter in millimeters, and the pitch in millimeters. So M8 × 1.25 means an 8 mm major diameter with a 1.25 mm pitch. Coarse pitch is often implied — M10 written alone means the standard coarse pitch for a 10 mm thread, so you only need to write the pitch when it is a fine or special thread.
How are Unified (inch) thread sizes designated?
Unified threads use the nominal diameter in inches plus threads per inch (TPI). 1/4-20 UNC means a quarter-inch diameter with 20 threads per inch, in the Unified Coarse series; 1/4-28 UNF is the fine-pitch version. When the series implies the pitch — 1/4 UNC is always 20 TPI — the TPI is sometimes omitted, but writing it out avoids confusion.
What does 6H / 6g or 2A / 2B mean on a thread?
Those are the tolerance classes — the fit. In metric, 6g is the standard external (bolt/screw) class and 6H the standard internal (nut/tapped hole) class; 6h is a tighter zero-allowance external variant. In Unified threads, 2A is the standard external class and 2B the standard internal class. If no class is written, the medium default (6g/6H or 2A/2B) is generally implied.
What is the difference between internal and external threads?
External threads are on the outside of a cylinder — screws, bolts, shafts. Internal threads are on the inside of a hole — nuts and tapped bores. The notation tells them apart by case: metric uses lowercase letters for external (6g) and uppercase for internal (6H); Unified uses A for external and B for internal (2A vs 2B).
Should I tap or thread mill?
Tap for fast, economical internal threads in production-friendly materials — tapping is the workhorse for standard holes. Thread mill when you need flexibility: one tool for multiple diameters and pitches, larger holes, hard materials, or reduced tap-break risk. The choice is a shop-process decision, not something you usually need to specify on the drawing — but knowing the difference helps you ask the right questions.
How deep should a threaded hole be?
The thread profile depth follows the pitch — for ISO metric threads, external thread depth is about 0.613 × pitch and internal about 0.541 × pitch. For blind holes, the practical rule is to tap slightly deeper than the required full thread to account for the chamfer and incomplete threads at the bottom, so the fastener has full engagement. The exact engagement you need depends on the load and material.
Related Resources
CNC Machining Surface Finish: The Ra Chart, Explained
The CNC machining surface finish chart: Ra values in μm and μin, N1–N12 grades, what each process achieves, what…
Passivation of Stainless Steel: Process, Specs, and When Parts Need It
Passivation of stainless steel explained for machined parts: the chemistry, ASTM A967 and AMS 2700 methods, nitric…
GD&T Basics for CNC Machined Parts: The Symbols That Matter
GD&T basics for machined parts: the 14 symbols explained, feature control frames, datums, MMC bonus tolerance, true…
How to Outsource CNC Machining to China, Step by Step
How to outsource CNC machining to China, step by step: vet suppliers, compare landed cost, control quality remotely,…
Sources
Standards and references behind the numbers on this page.
- ISO 724:2023, General purpose metric screw threads — basic dimensions — major, pitch and minor diameters behind the metric thread chart
- ASME B1.1-2024, Unified Inch Screw Threads (UN, UNR and UNJ thread forms) — UNC and UNF dimensions and the 2A/2B class system
- ISO metric screw thread — Wikipedia — coarse-pitch series and the 6H/6g tolerance-class conventions
Threads Tapped and Milled, to Class
IsoCNC single-point turns, taps, and thread-mills metric and Unified threads every day — power-transmission square profiles included, see the square thread guide — external threads mostly on the lathe, covered in our CNC turning guide, to 6H/6g and 2A/2B fit classes, verified with thread gauges. Send the drawing; an engineer reviews every RFQ, typically within 1–2 business days.