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BYBOLT / FASTENER THREAD SIZE CHART METRIC UNC UNF 8UN

Fastener Thread Size Chart: Metric, UNC, UNF and 8UN

Look up common metric coarse and fine, UNC, UNF and 8UN thread sizes, including nominal diameters, pitch and threads per inch. Learn how to read thread callouts and distinguish similar-looking sizes before ordering.

Comparison of M12 × 1.75 and 1/2-13 UNC external threads, showing nominal diameters and the different thread pitches.

How to Read a Thread Size

A replacement bolt needs more than a similar diameter. Its thread pitch must match the mating part, and the drawing may also specify a thread tolerance class. The diameters in this guide are nominal thread sizes. They are not wrench sizes, clearance-hole diameters or finished-thread inspection limits. For metric threads, M12 × 1.75 means a metric thread with a nominal diameter of 12 mm and a pitch of 1.75 mm. Pitch is measured along the axis between corresponding points on adjacent threads. For Unified inch threads, 1/2-13 UNC means a nominal diameter of half an inch, 13 threads per inch (TPI), and the Unified coarse thread series. Metric pitch and TPI describe spacing differently. A smaller metric pitch means finer spacing; a higher TPI means finer spacing.

Metric Thread Size Chart: Coarse and Selected Fine Pitches

All dimensions below are in millimetres. The fine pitches are selected examples; other diameter–pitch combinations exist. For example, M12 × 1.75, M12 × 1.50 and M12 × 1.25 share the same nominal diameter but have different pitches. “M12 fine thread” leaves more than one possible interpretation. Include the pitch when specifying a replacement or requesting a quotation.

  • M3 — nominal diameter: 3 mm; coarse pitch: 0.50 mm; selected fine pitches: 0.35 mm.
  • M4 — nominal diameter: 4 mm; coarse pitch: 0.70 mm; selected fine pitches: 0.50 mm.
  • M5 — nominal diameter: 5 mm; coarse pitch: 0.80 mm; selected fine pitches: 0.50 mm.
  • M6 — nominal diameter: 6 mm; coarse pitch: 1.00 mm; selected fine pitches: 0.75 mm.
  • M8 — nominal diameter: 8 mm; coarse pitch: 1.25 mm; selected fine pitches: 1.00 mm.
  • M10 — nominal diameter: 10 mm; coarse pitch: 1.50 mm; selected fine pitches: 1.25, 1.00 mm.
  • M12 — nominal diameter: 12 mm; coarse pitch: 1.75 mm; selected fine pitches: 1.50, 1.25 mm.
  • M14 — nominal diameter: 14 mm; coarse pitch: 2.00 mm; selected fine pitches: 1.50 mm.
  • M16 — nominal diameter: 16 mm; coarse pitch: 2.00 mm; selected fine pitches: 1.50 mm.
  • M18 — nominal diameter: 18 mm; coarse pitch: 2.50 mm; selected fine pitches: 2.00, 1.50 mm.
  • M20 — nominal diameter: 20 mm; coarse pitch: 2.50 mm; selected fine pitches: 2.00, 1.50 mm.
  • M22 — nominal diameter: 22 mm; coarse pitch: 2.50 mm; selected fine pitches: 2.00, 1.50 mm.
  • M24 — nominal diameter: 24 mm; coarse pitch: 3.00 mm; selected fine pitches: 2.00 mm.
  • M27 — nominal diameter: 27 mm; coarse pitch: 3.00 mm; selected fine pitches: 2.00 mm.
  • M30 — nominal diameter: 30 mm; coarse pitch: 3.50 mm; selected fine pitches: 2.00 mm.
  • M33 — nominal diameter: 33 mm; coarse pitch: 3.50 mm; selected fine pitches: 2.00 mm.
  • M36 — nominal diameter: 36 mm; coarse pitch: 4.00 mm; selected fine pitches: 3.00 mm.
  • M39 — nominal diameter: 39 mm; coarse pitch: 4.00 mm; selected fine pitches: 3.00 mm.
  • M42 — nominal diameter: 42 mm; coarse pitch: 4.50 mm; selected fine pitches: 3.00 mm.
  • M45 — nominal diameter: 45 mm; coarse pitch: 4.50 mm; selected fine pitches: 3.00 mm.
  • M48 — nominal diameter: 48 mm; coarse pitch: 5.00 mm; selected fine pitches: 3.00 mm.

UNC and UNF Thread Size Chart

Fractional sizes are in inches. Sizes beginning with # are numbered screw sizes: #10 does not mean 10 mm. The metric values below convert the nominal diameter for reference and are rounded to three decimal places. A dash means no UNF combination is listed for that diameter in this guide. Other thread series may exist. To convert TPI into pitch: pitch in mm = 25.4 ÷ TPI. A 1/2-13 UNC thread therefore has a nominal diameter of 12.7 mm and a pitch of approximately 1.954 mm. An M12 × 1.75 thread differs in both diameter and pitch. The inch-to-millimetre conversion helps compare dimensions; it does not make these threads interchangeable.

  • #6 — nominal diameter: 3.505 mm; UNC: 32 TPI; UNF: 40 TPI.
  • #8 — nominal diameter: 4.166 mm; UNC: 32 TPI; UNF: 36 TPI.
  • #10 — nominal diameter: 4.826 mm; UNC: 24 TPI; UNF: 32 TPI.
  • #12 — nominal diameter: 5.486 mm; UNC: 24 TPI; UNF: 28 TPI.
  • 1/4 in — nominal diameter: 6.350 mm; UNC: 20 TPI; UNF: 28 TPI.
  • 5/16 in — nominal diameter: 7.938 mm; UNC: 18 TPI; UNF: 24 TPI.
  • 3/8 in — nominal diameter: 9.525 mm; UNC: 16 TPI; UNF: 24 TPI.
  • 7/16 in — nominal diameter: 11.113 mm; UNC: 14 TPI; UNF: 20 TPI.
  • 1/2 in — nominal diameter: 12.700 mm; UNC: 13 TPI; UNF: 20 TPI.
  • 9/16 in — nominal diameter: 14.288 mm; UNC: 12 TPI; UNF: 18 TPI.
  • 5/8 in — nominal diameter: 15.875 mm; UNC: 11 TPI; UNF: 18 TPI.
  • 3/4 in — nominal diameter: 19.050 mm; UNC: 10 TPI; UNF: 16 TPI.
  • 7/8 in — nominal diameter: 22.225 mm; UNC: 9 TPI; UNF: 14 TPI.
  • 1 in — nominal diameter: 25.400 mm; UNC: 8 TPI; UNF: 12 TPI.
  • 1-1/8 in — nominal diameter: 28.575 mm; UNC: 7 TPI; UNF: 12 TPI.
  • 1-1/4 in — nominal diameter: 31.750 mm; UNC: 7 TPI; UNF: 12 TPI.
  • 1-3/8 in — nominal diameter: 34.925 mm; UNC: 6 TPI; UNF: 12 TPI.
  • 1-1/2 in — nominal diameter: 38.100 mm; UNC: 6 TPI; UNF: 12 TPI.
  • 1-3/4 in — nominal diameter: 44.450 mm; UNC: 5 TPI; UNF: —.
  • 2 in — nominal diameter: 50.800 mm; UNC: 4.5 TPI; UNF: —.

8UN: The Pitch Stays Constant

The 8UN series uses 8 threads per inch, equivalent to 3.175 mm pitch, across its listed diameters. For example, 1-1/2-6 UNC and 1-1/2-8 UN have the same nominal diameter but different pitches. There is an important overlap at 1 inch, where UNC already has 8 TPI. A different series label alone does not establish a dimensional mismatch; check the complete designation and applicable requirements.

  • 1-1/8-8 UN — nominal diameter: 28.575 mm; 8 TPI; pitch: 3.175 mm.
  • 1-1/4-8 UN — nominal diameter: 31.750 mm; 8 TPI; pitch: 3.175 mm.
  • 1-3/8-8 UN — nominal diameter: 34.925 mm; 8 TPI; pitch: 3.175 mm.
  • 1-1/2-8 UN — nominal diameter: 38.100 mm; 8 TPI; pitch: 3.175 mm.
  • 1-3/4-8 UN — nominal diameter: 44.450 mm; 8 TPI; pitch: 3.175 mm.
  • 2-8 UN — nominal diameter: 50.800 mm; 8 TPI; pitch: 3.175 mm.

What the Size Chart Does Not Tell You

A nominal size does not mean every finished external thread will measure exactly that diameter. Manufacturing allowances and tolerances determine the permitted dimensions. Metric callouts may include 6g for an external thread or 6H for an internal thread. Unified callouts may include 2A or 2B, where A identifies an external thread and B an internal thread. These suffixes describe the thread class, not the material or bolt strength grade. The required class and thread direction should follow the drawing. Overall fastener length and threaded length remain separate dimensions. For a BYBOLT enquiry, send the drawing together with the material grade and quantity. If no drawing is available, include the thread designation, fastener type, length and any specified tolerance class or left-hand thread requirement. Material, size and product availability are reviewed for the individual enquiry.

  • M12 × 1.75-6g: metric external thread.
  • 1/2-13 UNC-2A: Unified coarse external thread.
  • 1-1/2-8 UN-2A: external thread in the 8-thread series.

Sources and Scope

This guide covers selected metric M and Unified inch fastener threads. It does not cover NPT, BSP or other pipe-thread systems. The dimensions are reference information, not a manufacturing or inspection specification. Metric coarse pitches: Bossard, Metric ISO threads, January 2025. Selected metric fine pitches: TR Fastenings, Metric fine thread geometry. UNC and UNF combinations: Kennametal, UN standard thread data. 8UN combinations and thread-class explanations: Portland Bolt, Thread Pitch Chart and Bolt Thread Pitch Series.

  • Bossard metric reference: https://assets.eu.ctfassets.net/0vp0u5uh75zd/2tYqENAuufvdsudjM8Qbrc/b3461eaa59c2203d3a8b9509ac24dacf/096_098_Metric_ISOthreads_Fastening_EN_01_2025.pdf
  • TR Fastenings metric fine threads: https://www.trfastenings.com/knowledge-base/thread-geometries/metric-fine
  • Kennametal Unified threads: https://kthreadmilling.kennametal.com/KThreadmillingWeb/help/UN%20standard.htm
  • Portland Bolt thread pitch chart: https://www.portlandbolt.com/technical/thread-pitch-chart/
  • Portland Bolt thread classes: https://www.portlandbolt.com/technical/faqs/bolt-thread-series/
  • Bolt Depot pitch and TPI: https://boltdepot.com/fastener-information/measuring/tpiandpitch
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