How to Measure a Screw or Bolt: Diameter, Pitch and Length
Measure an unknown screw correctly: major diameter, metric pitch or inch TPI, length by head style, drive, material and a final fit check.
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Fastener Resource Center
Practical charts, standards references and selection guides for identifying metric and inch fasteners with confidence.
UNC & UNFUnified inch threads
ISO MetricDiameters and pitches
ASME · ASTM · DINStandards cross-references
Start with the data
Fastener charts and reference tools
Use these evergreen references to identify threads, calculate drilling sizes and compare specifications.
01Most used
Thread Pitch Chart
UNC, UNF and metric thread sizes with TPI, pitch and measurement instructions.
Open thread chart → 02Tap Drill Size Chart
Metric, UNC and UNF tap drills plus clearance-hole sizes.
View drill sizes → 03Standards Cross-Reference
Compare ASME, ASTM, ISO and DIN fastener designations.
Compare standards → 04Metric–Inch Conversion
Nearest nominal diameters, wrench sizes and length conversions.
Convert sizes → 05Bolt Grades & Markings
Understand SAE grades, metric classes and stainless markings.
Identify markings →
Featured technical guide
How to Measure a Screw or Bolt: Diameter, Pitch and Length
Measure an unknown screw correctly: major diameter, metric pitch or inch TPI, length by head style, drive, material and a final fit check.
Read the guide
Browse by task
What are you trying to do?
Move from the problem you have to the specification or product family you need.
Identify a thread
Measure diameter and determine TPI or metric pitch.
Prepare a tapped hole
Find the drill diameter for the required internal thread.
Compare metric and inch
Find nearby sizes without assuming interchangeability.
Check strength markings
Interpret head marks and property-class designations.
From the knowledge base
Latest fastener guides
Screw Thread Pitch Chart: Metric, UNC & UNF Sizes
Identify common Unified inch and ISO metric screw threads with UNC, UNF, TPI, and metric-pitch charts, plus practical measurement guidance.
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M3 or #4-40: why they are not interchangeable
Compare M3 × 0.5 and #4-40 UNC dimensions, learn why the threads are not interchangeable, and identify them without damaging the mating part.
Read guide →From reference to hardware
Shop by fastener family
Once you have confirmed the thread, material and standard, continue to the appropriate product category.
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