Complete Metric Thread Size Table and Selection Manual

METRIC THREAD SIZES AND SELECTION MANUAL
Based on ISO 261

INTRODUCTION

Standard: ISO 261:1998 – ISO general purpose metric screw threads – General plan

Purpose: This manual helps you select the correct diameter and pitch combinations for metric screw threads.

Related Standards:

  • ISO 68-1: Basic profile for metric screw threads
  • ISO 724: Basic dimensions
  • ISO 965-1: Tolerances
  • ISO 262: Selected sizes for screws, bolts and nuts

UNDERSTANDING COARSE AND FINE THREADS

Thread TypeDefinitionWhen to Use
COARSE PitchThe largest metric pitch used in current practice for each diameter (default)General purpose, standard fasteners, quick assembly, stronger threads
FINE PitchSmaller pitches available for each diameterPrecision adjustment, thin-walled parts, better vibration resistance, fine tuning

IMPORTANT NOTES:

  • The terms “coarse” and “fine” do NOT indicate quality level
  • “Coarse” simply means the standard/default pitch for that diameter
  • When specifying coarse pitch, the pitch value can be omitted from designation (e.g., M10 means M10 × 1.5)
  • Fine pitches must always be specified in the designation (e.g., M10 × 1.25)

SELECTION PRIORITY

Choose diameters in this order:

When to UseChoice
Always prefer these diameters (most common, best availability)1st Choice
Use if 1st choice diameters don’t meet requirements2nd Choice
Use only when necessary (less common, limited availability)3rd Choice

Pitches shown in parentheses ( ) should be avoided whenever possible.

DIAMETER AND PITCH COMBINATIONS

Complete table of all metric thread sizes (M1 to M300)

DiameterCoarse PitchFine Thread PitchesChoice
M10.250.21st
M1.10.250.23rd
M1.20.250.22nd
M1.40.30.21st
M1.60.350.21st
M1.80.350.23rd
M20.40.251st
M2.20.450.253rd
M2.50.450.351st
M30.50.351st
M3.50.60.352nd
M40.70.51st
M4.50.750.53rd
M50.80.51st
M5.50.5–3rd
M610.75, 0.51st
M710.753rd
M81.251, 0.751st
M91.251, 0.753rd
M101.51.25, 1, 0.751st
M111.51, 0.753rd
M121.751.5, 1.25, 1, 0.751st
M1421.5, 1.25*, 12nd
M151.513rd
M1621.5, 11st
M171.513rd
M182.52, 1.5, 12nd
M202.52, 1.5, 11st
M222.52, 1.5, 12nd
M2432, 1.5, 11st
M2521.5, 13rd
M261.5–3rd
M2732, 1.5, 12nd
M2821.5, 13rd
M303.5(3), 2, 1.5, 11st
M3221.53rd
M333.5(3), 2, 1.5, 12nd
M351.5**–3rd
M3643, 2, 1.51st
M381.5–3rd
M3943, 2, 1.52nd
M4032, 1.53rd
M424.54, 3, 2, 1.51st
M454.54, 3, 2, 1.52nd
M4854, 3, 2, 1.51st
M5032, 1.53rd
M5254, 3, 2, 1.52nd
M5543, 2, 1.53rd
M565.54, 3, 2, 1.51st
M5843, 2, 1.53rd
M605.54, 3, 2, 1.52nd
M6243, 2, 1.53rd
M6464, 3, 2, 1.51st
M6543, 2, 1.53rd
M6864, 3, 2, 1.52nd
M7064, 3, 2, 1.53rd
M7264, 3, 2, 1.52nd
M7543, 2, 1.53rd
M7664, 3, 2, 1.52nd
M7821.53rd
M8064, 3, 2, 1.51st
M8221.53rd
M8564, 3, 23rd
M9064, 3, 22nd
M9564, 3, 23rd
M10064, 3, 21st
M10564, 3, 23rd
M11064, 3, 22nd
M11564, 3, 23rd
M12064, 3, 23rd
M12586, 4, 3, 21st
M13086, 4, 3, 22nd
M13564, 3, 23rd
M14086, 4, 3, 21st
M14564, 3, 23rd
M15086, 4, 3, 21st
M15564, 3, 23rd
M16086, 4, 31st
M16564, 33rd
M17086, 4, 32nd
M17564, 33rd
M18086, 4, 31st
M18564, 33rd
M19086, 4, 32nd
M19564, 33rd
M20086, 4, 31st
M20564, 33rd
M21086, 4, 32nd
M21564, 33rd
M22086, 4, 31st
M22586, 4, 32nd
M23064, 33rd
M23564, 33rd
M24086, 4, 31st
M24564, 33rd
M25086, 4, 31st
M25564, 33rd
M26086, 4, 32nd
M26586, 42nd
M27086, 42nd
M275643rd
M28086, 41st
M28586, 42nd
M290643rd
M295643rd
M30086, 41st

Notes:

  • * M14 × 1.25 is only for spark plugs for engines (special case)
  • ** M35 × 1.5 is only for locking nuts for bearings (special case)
  • Pitches in parentheses ( ) should be avoided whenever possible
  • Choice: 1st = Most common, best availability | 2nd = Use if 1st not suitable | 3rd = Use only when necessary

EXTRA FINE PITCHES

If threads finer than those in the tables are necessary, only use these pitches:

Pitch (mm)Maximum Recommended Diameter (mm)
0.2Not specified
0.25Not specified
0.35Not specified
0.522
0.7533
180
1.5150
2200
3300

Important Warning: As diameter increases for a given pitch, there is increasing difficulty in complying with tolerances. Do not exceed the maximum recommended diameter for each pitch.

QUICK SELECTION GUIDE

STEP-BY-STEP SELECTION PROCESS

STEP 1: Select Diameter

  1. Choose from 1st choice if possible – these are most common
  2. If 1st choice doesn’t meet needs, choose from 2nd choice
  3. Only use 3rd choice when absolutely necessary

STEP 2: Select Pitch

  1. Use COARSE pitch for general purpose applications (this is the default)
  2. Use FINE pitch only when specifically required for:
    • Precision adjustment
    • Thin-walled components
    • Better vibration resistance
    • Fine tuning mechanisms

STEP 3: Designation

  • Coarse pitch: Write as “M[diameter]” (e.g., M10)
  • Fine pitch: Write as “M[diameter] × [pitch]” (e.g., M10 × 1.25)

COMMON APPLICATIONS BY SIZE

Thread SizeTypical Applications
M1 to M3Electronics, watches, precision instruments, micro-mechanics
M4 to M6Small electronics enclosures, consumer products, light assemblies
M8 to M12General machinery, automotive, furniture, standard fasteners (MOST COMMON)
M14 to M20Heavy machinery, automotive suspension, structural connections
M24 to M36Large machinery, construction equipment, heavy structural applications
M42 to M68Very heavy machinery, marine equipment, large structural connections
M80 and largerMassive structures, ship propellers, large hydraulic equipment, special applications

PRACTICAL EXAMPLES

EXAMPLE 1: Standard Bolt Selection

Requirement: General purpose bolt for machinery assembly

Selection:

  • Diameter: M12 (1st choice, very common)
  • Pitch: 1.75 (coarse – default)
  • Designation: M12

EXAMPLE 2: Fine Thread for Adjustment

Requirement: Precision adjustment screw for instrument

Selection:

  • Diameter: M10 (1st choice)
  • Pitch: 1 mm (fine – for precise adjustment)
  • Designation: M10 × 1

EXAMPLE 3: Large Structural Bolt

Requirement: Heavy structural connection

Selection:

  • Diameter: M24 (1st choice for this size)
  • Pitch: 3 (coarse – default)
  • Designation: M24

EXAMPLE 4: Thin-Walled Component

Requirement: Thread in thin-walled aluminum housing

Selection:

  • Diameter: M8 (1st choice)
  • Pitch: 1 mm (fine – more thread engagement in thin wall)
  • Designation: M8 × 1

SPECIAL CASES TO REMEMBER

ThreadSpecial ApplicationNote
M14 × 1.25Spark plugs for enginesThis specific combination reserved for spark plugs only
M35 × 1.5Locking nuts for bearingsThis specific combination reserved for bearing lock nuts only
Pitches in ( )Avoid if possibleUse only when absolutely necessary

SUMMARY CHECKLIST

Before finalizing your thread selection, verify:

☐ Have you selected from 1st choice if possible?
☐ Is coarse pitch adequate, or do you specifically need fine pitch?
☐ If using fine pitch, is it really necessary for your application?
☐ Have you avoided pitches shown in parentheses ( )?
☐ For extra fine pitches, have you checked the maximum diameter limit?
☐ Is the designation correct? (Coarse = M[d], Fine = M[d] × [P])
☐ Have you considered availability and cost? (1st choice = most available)

METRIC THREAD DESIGNATION – QUICK GUIDE FOR DRAWINGS
STANDARD METRIC THREAD SIZE TABLE
ISO 965-1
ISO 724 Metric Screw Threads – Metric Tap Drill Table

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