Bolt Size Chart — Metric (mm) & SAE Sizes, Free Printable PDF
Every standard bolt size in one place: metric M2 through M48 and SAE 1/4” through 1-1/2”, with head dimensions, thread pitch, and the exact wrench or socket size for each. Use this chart before any hardware-store run, parts order, or fastener identification job. Printable PDF version available below.
Printable bolt size chart (PDF) Metric, SAE and JIS tables, free to download and print.
How to read this chart
Each row is a bolt diameter. Metric bolts are identified by M-number (the shank diameter in millimeters); SAE bolts by fractional inches. Thread pitch is the distance between threads — metric pitches are in mm, SAE in threads-per-inch (TPI). The wrench/socket column is the across-flats measurement of the hex head, which determines which tool you need.
Metric bolt sizes (M2–M48)
Metric hex bolts follow ISO 898 (property classes 4.6 through 12.9). Common automotive sizes are M6, M8, M10, M12, and M14. Thread pitch comes in coarse (standard) and fine; coarse is default unless the spec says otherwise. Head size depends on the standard the bolt was made to: M8 = 13 mm, M10 = 16 mm (ISO 4014) or 17 mm (older DIN 931), M12 = 18 or 19 mm, M14 = 21 or 22 mm.
SAE bolt sizes (1/4”–1-1/2”)
SAE hex bolts follow ASTM/SAE grade standards (Grade 2, 5, 8). Common sizes are 1/4”, 5/16”, 3/8”, 7/16”, and 1/2”. Thread counts are UNC (coarse) or UNF (fine). Head markings identify grade: no marks = Grade 2, three radial lines = Grade 5, six radial lines = Grade 8.
When to use metric vs SAE
Virtually every passenger vehicle sold in the US after 1992 uses metric fasteners, including domestic makes. SAE bolts are still common on older domestic trucks, lawn equipment, some appliances, and ham-radio hardware. If your project is post-1995 and not lawn equipment, assume metric.
Metric Bolt Size Chart (M2–M48)
| Size | Diameter | Coarse Pitch | Fine Pitch | Wrench / Socket |
|---|---|---|---|---|
| M2 | 2 mm | 0.4 mm | 0.25 mm | 4 mm |
| M2.5 | 2.5 mm | 0.45 mm | 0.35 mm | 5 mm |
| M3 | 3 mm | 0.5 mm | 0.35 mm | 5.5 mm |
| M4 | 4 mm | 0.7 mm | 0.5 mm | 7 mm |
| M5 | 5 mm | 0.8 mm | 0.5 mm | 8 mm |
| M6 | 6 mm | 1.0 mm | 0.75 mm | 10 mm |
| M7 | 7 mm | 1.0 mm | 0.75 mm | 11 mm |
| M8 | 8 mm | 1.25 mm | 1.0 mm | 13 mm |
| M10 | 10 mm | 1.5 mm | 1.25 mm | 16 mm (DIN: 17 mm) |
| M12 | 12 mm | 1.75 mm | 1.25 mm | 18 mm (DIN: 19 mm) |
| M14 | 14 mm | 2.0 mm | 1.5 mm | 21 mm (DIN: 22 mm) |
| M16 | 16 mm | 2.0 mm | 1.5 mm | 24 mm |
| M18 | 18 mm | 2.5 mm | 1.5 mm | 27 mm |
| M20 | 20 mm | 2.5 mm | 1.5 mm | 30 mm |
| M22 | 22 mm | 2.5 mm | 1.5 mm | 34 mm (DIN: 32 mm) |
| M24 | 24 mm | 3.0 mm | 2.0 mm | 36 mm |
| M27 | 27 mm | 3.0 mm | 2.0 mm | 41 mm |
| M30 | 30 mm | 3.5 mm | 2.0 mm | 46 mm |
| M33 | 33 mm | 3.5 mm | 2.0 mm | 50 mm |
| M36 | 36 mm | 4.0 mm | 3.0 mm | 55 mm |
| M39 | 39 mm | 4.0 mm | 3.0 mm | 60 mm |
| M42 | 42 mm | 4.5 mm | 3.0 mm | 65 mm |
| M45 | 45 mm | 4.5 mm | 3.0 mm | 70 mm |
| M48 | 48 mm | 5.0 mm | 3.0 mm | 75 mm |
Per ISO 261/262. Coarse (standard) and fine thread pitch in mm; wrench is the across-flats hex size per ISO 4014/4017, with the older DIN 931/933 size in brackets where it differs (M10, M12, M14, M22).
SAE (Inch) Bolt Size Chart (1/4"–1-1/2")
| Size | TPI Coarse (UNC) | TPI Fine (UNF) | Wrench / Socket |
|---|---|---|---|
| 1/4" | 20 | 28 | 7/16" |
| 5/16" | 18 | 24 | 1/2" |
| 3/8" | 16 | 24 | 9/16" |
| 7/16" | 14 | 20 | 5/8" |
| 1/2" | 13 | 20 | 3/4" |
| 9/16" | 12 | 18 | 13/16" |
| 5/8" | 11 | 18 | 15/16" |
| 3/4" | 10 | 16 | 1-1/8" |
| 7/8" | 9 | 14 | 1-5/16" |
| 1" | 8 | 12 | 1-1/2" |
| 1-1/8" | 7 | 12 | 1-11/16" |
| 1-1/4" | 7 | 12 | 1-7/8" |
| 1-3/8" | 6 | 12 | 2-1/16" |
| 1-1/2" | 6 | 12 | 2-1/4" |
Per ASME B18.2.1 / B1.1. Diameter is the nominal size in inches; TPI is threads per inch (coarse vs fine); wrench is the across-flats hex size.
SAE Wrench Size → Bolt Size (Reverse Lookup)
| Wrench Size | Bolt Size | UNC TPI | UNF TPI |
|---|---|---|---|
| 7/16" | 1/4" | 20 | 28 |
| 1/2" | 5/16" | 18 | 24 |
| 9/16" | 3/8" | 16 | 24 |
| 5/8" | 7/16" | 14 | 20 |
| 3/4" | 1/2" | 13 | 20 |
| 13/16" | 9/16" | 12 | 18 |
| 15/16" | 5/8" | 11 | 18 |
| 1-1/8" | 3/4" | 10 | 16 |
| 1-5/16" | 7/8" | 9 | 14 |
| 1-1/2" | 1" | 8 | 12 |
| 1-11/16" | 1-1/8" | 7 | 12 |
| 1-7/8" | 1-1/4" | 7 | 12 |
| 2-1/16" | 1-3/8" | 6 | 12 |
| 2-1/4" | 1-1/2" | 6 | 12 |
Bolt heads only — nuts differ at 7/16" (11/16" wrench) and 9/16" (7/8"); heavy-hex bolts and nuts (1/2 in and up) are 1/8 in larger across flats than the values above (1/2 in heavy hex = 7/8 in, not 3/4 in).
Bolt Size Chart FAQ
Is there a printable bolt size chart PDF?
Yes. The printable bolt size chart (PDF), linked at the top of this page, has these tables from this page: the socket size quick reference, metric hex bolts (ISO 4014/4017 with the DIN 931/933 differences), JIS B 1180 reduced heads, metric hex nuts (ISO 4032 vs DIN 934) and SAE inch bolts. It is free to download and print.
What are the standard metric bolt sizes in mm?
This chart lists the ISO metric bolt sizes M2, M2.5, M3, M4, M5, M6, M7, M8, M10, M12, M14, M16, M18, M20, M22, M24, M27, M30, M33, M36, M39, M42, M45 and M48. The number after the M is the nominal diameter in millimeters, so an M8 bolt is 8 mm across the threads. Hex head width across flats (the wrench or socket size) runs from 4 mm at M2 to 75 mm at M48 under ISO 4014/4017.
What is the head size of each metric bolt?
Hex head width across flats under ISO 4014/4017, with the older DIN 931/933 size in brackets where it differs: M5 8 mm, M6 10 mm, M8 13 mm, M10 16 mm (DIN 17 mm), M12 18 mm (DIN 19 mm), M14 21 mm (DIN 22 mm), M16 24 mm, M18 27 mm, M20 30 mm. Some Japanese hardware uses smaller JIS B 1180 reduced heads: M8 12 mm, M10 14 mm and M12 17 mm.
What is the M8 bolt head size and length?
An M8 hex bolt head is 13 mm across flats (a 13 mm wrench or socket) under both ISO 4014/4017 and DIN 931/933; the JIS B 1180 reduced head is 12 mm. M8 coarse pitch is 1.25 mm (fine 1.0 mm). This chart gives head width across flats, not head height. Bolt length is measured separately, under the head to the tip; M8 bolts are commonly sold in 12, 16, 20, 25, 30, 35, 40, 50, 60 mm lengths.
What is bolt pitch, and what is the coarse pitch for each size?
Bolt pitch is the distance between adjacent threads. Metric pitch is measured in millimeters; SAE pitch is given as threads per inch (TPI). Standard coarse metric pitch (ISO 261/262): M4 0.7 mm, M5 0.8 mm, M6 1.0 mm, M8 1.25 mm, M10 1.5 mm, M12 1.75 mm, M14 2.0 mm, M16 2.0 mm, M18 2.5 mm, M20 2.5 mm, M22 2.5 mm, M24 3.0 mm. Fine pitches are listed in the ISO table. SAE coarse (UNC) threads per inch: 1/4" 20, 5/16" 18, 3/8" 16, 1/2" 13.
Tools for measuring and replacing bolts
As an Amazon Associate WhatSizeBolt earns from qualifying purchases. Tool picks are general options, not a fit guarantee for your vehicle or fastener.
- Digital CaliperMeasure shank diameter and head across-flats, then read the row off this chart.Digital Calipers — view on Amazon
- Thread Pitch Gauge (Metric + SAE)Settles coarse vs fine and metric vs SAE in seconds.Thread Pitch Gauge Set (Metric + SAE) — view on Amazon
- Metric Hex Bolt Assortment (M4–M12, Class 8.8)The common automotive sizes from this chart, with matching nuts and washers.Metric Grade 8.8 Hex Bolt & Nut Assortment, M4–M12 (1,075 pc) — view on Amazon
Related lookups
- Measure the bolt
- Check thread pitch
- Identify the grade
- Find a wrench/socket size
- Compare metric torque estimates
- Check vehicle-specific specs
- Standard Torque Specs by Grade
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