Complete Guide to Bolts
Specifications, sizing charts, measurement guides, and grade information for hex bolts, carriage bolts, lag bolts, and more
Downloadable Reference Guides
Quick access to all bolt specification charts and guides
US Bolt Size Chart
Diameter and thread identification
Metric Bolt Size Chart
Metric diameter and thread pitch
Bolt Torque Chart
Torque specs for all grades
Bolt Measurement Guide
How to measure bolt types
Bolt Grade Chart
Grade markings and strength
Materials & Coatings Guide
Steel grades and finishes
US Bolt Dimensions
Standard imperial bolt sizing follows established specifications for head dimensions, thread pitch, and wrench sizes. Use these charts to identify bolts and select the correct tools.
Bolt Head and Wrench Sizes
The most common measurement you'll need is the wrench size that fits the bolt head. Here are standard sizes:
| Bolt Diameter | Wrench Size | Head Height | Thread Pitch |
|---|---|---|---|
| 1/4" | 7/16" | 11/64" | 20 TPI |
| 5/16" | 1/2" | 13/64" | 18 TPI |
| 3/8" | 9/16" | 1/4" | 16 TPI |
| 7/16" | 5/8" | 19/64" | 14 TPI |
| 1/2" | 3/4" | 11/32" | 13 TPI |
| 9/16" | 13/16" | 3/8" | 12 TPI |
| 5/8" | 15/16" | 27/64" | 11 TPI |
| 3/4" | 1-1/8" | 1/2" | 10 TPI |
| 7/8" | 1-5/16" | 37/64" | 9 TPI |
| 1" | 1-1/2" | 21/32" | 8 TPI |
Download US Bolt Size Chart
Complete 'lay-over' sizing chart for identifying US bolt diameter and thread count
US Bolt Diameter & Thread Chart (PDF)Recommended Bolt Torque Values
Proper torque is critical for bolt performance. Under-torquing allows loosening; over-torquing risks stripping or breaking. These values are in ft-lbs for dry threads:
| Bolt Size | Grade 2 | Grade 5 | Grade 8 |
|---|---|---|---|
| 1/4" | 4 ft-lb | 8 ft-lb | 10 ft-lb |
| 5/16" | 8 ft-lb | 15 ft-lb | 20 ft-lb |
| 3/8" | 15 ft-lb | 30 ft-lb | 40 ft-lb |
| 7/16" | 25 ft-lb | 45 ft-lb | 60 ft-lb |
| 1/2" | 40 ft-lb | 75 ft-lb | 100 ft-lb |
| 9/16" | 60 ft-lb | 110 ft-lb | 150 ft-lb |
| 5/8" | 85 ft-lb | 150 ft-lb | 200 ft-lb |
| 3/4" | 140 ft-lb | 270 ft-lb | 380 ft-lb |
Standard Thread Lengths
Most hex bolts follow these standard thread length specifications:
- Up to 6" long: Threaded 2× diameter + 1/4"
- Over 6" long: Threaded 2× diameter + 1/2"
- Fully threaded: Available in most sizes upon request
Download Torque Specifications
Complete torque charts for all bolt grades and sizes
Bolt Torque Chart (PDF)Metric Bolt Dimensions
Metric bolts are measured in millimeters and use thread pitch (distance between threads) rather than threads per inch. They're common in automotive, machinery, and imported equipment.
Metric Bolt Head and Wrench Sizes
| Bolt Diameter | Wrench Size | Head Height | Thread Pitch |
|---|---|---|---|
| M3 | 5.5mm | 2.0mm | 0.5mm |
| M4 | 7mm | 2.8mm | 0.7mm |
| M5 | 8mm | 3.5mm | 0.8mm |
| M6 | 10mm | 4.0mm | 1.0mm |
| M8 | 13mm | 5.3mm | 1.25mm |
| M10 | 16mm (17mm) | 6.4mm | 1.5mm |
| M12 | 18mm (19mm) | 7.5mm | 1.75mm |
| M14 | 21mm (22mm) | 8.8mm | 2.0mm |
| M16 | 24mm | 10.0mm | 2.0mm |
| M20 | 30mm | 12.5mm | 2.5mm |
Metric Thread Pitch Options
Metric bolts are available in both coarse and fine thread pitches:
- Coarse Thread: Standard pitch (shown above) - most common for general use
- Fine Thread: Available for precision applications requiring tighter tolerances and higher strength
Download Metric Bolt Size Chart
Complete 'lay-over' sizing chart for identifying metric bolt diameter and thread pitch
Metric Bolt Diameter & Thread Chart (PDF)How to Measure Bolts
Accurate measurement is essential for ordering replacement bolts or identifying existing hardware. Different bolt types have different measurement conventions.
Measurement Methods by Bolt Type
Hex Bolts
- Measured from under the head to the end
- Head height is NOT included in length
- Diameter is the major (outer) thread diameter
- Use calipers for accurate measurement
Carriage Bolts
- Measured from under the domed head
- Square neck under head not included
- Dome head sits above the surface
- Common in wood applications
Lag Bolts (Lag Screws)
- Measured from under the hex head
- Point is included in the length
- Wood thread - not machine thread
- Requires pilot hole for installation
Countersunk Bolts
- Measured from top of head to end
- Head sits flush or below surface
- Requires countersunk hole
- Common: flat head, oval head
Understanding Fastener Notation
US Bolt Callout
Example: 1/2"-13 x 3"
- 1/2" = Bolt diameter (major thread diameter)
- 13 = Threads per inch (TPI)
- 3" = Bolt length
Grade markings on head indicate strength (2, 5, or 8)
Metric Bolt Callout
Example: M10-1.5 x 50
- M10 = 10mm bolt diameter
- 1.5 = Thread pitch in mm
- 50 = Length in mm
Property class marking (8.8, 10.9, 12.9) indicates strength
Common Measurement Mistakes
- Including the head: Most bolts are measured from under the head, not including it
- Confusing diameter: Measure the threads, not the shank or head
- Wrong thread count: Use a thread gauge for accurate TPI/pitch identification
- Mixing US and Metric: M10 ≠ 3/8" - they're close but NOT interchangeable
Download Bolt Measurement Guide
Visual guide to measuring different bolt types correctly
Bolt Measurement Guide (PDF)Bolt Materials and Strength Grades
Bolt grade indicates tensile strength - the amount of tension a bolt can withstand before breaking. Always match bolt grade to your application requirements.
US Bolt Grades (SAE)
| Grade | Head Marking | Material | Tensile Strength | Common Uses |
|---|---|---|---|---|
| Grade 2 | No marks | Low carbon steel | 64,000 psi | General purpose, light-duty |
| Grade 5 | 3 radial lines | Medium carbon steel | 120,000 psi | Automotive, construction |
| Grade 8 | 6 radial lines | Medium carbon alloy steel | 150,000 psi | High-stress applications |
| Stainless 18-8 | Stainless or "A2" | 304 stainless steel | 90,000 psi | Corrosion resistance |
| Stainless 316 | "A4" or "316" | 316 stainless steel | 90,000 psi | Marine, chemical resistance |
Grade Selection Quick Guide
Grade 2: Non-critical applications, light loads, temporary fastening
Grade 5: Most common choice - automotive, machinery, structural
Grade 8: High-stress, high-vibration, critical safety applications
Stainless: When corrosion resistance is more important than maximum strength
Download Bolt Grade Chart
Visual guide to identifying bolt grades and understanding tensile strength specifications
Bolt Grade Markings & Strength Chart (PDF)Metric Property Classes
| Property Class | Head Marking | Tensile Strength | US Equivalent |
|---|---|---|---|
| 4.6 | 4.6 | 400 MPa (58,000 psi) | ~Grade 2 |
| 8.8 | 8.8 | 800 MPa (116,000 psi) | ~Grade 5 |
| 10.9 | 10.9 | 1000 MPa (145,000 psi) | ~Grade 8 |
| 12.9 | 12.9 | 1200 MPa (174,000 psi) | Higher than Grade 8 |
Download Materials & Coatings Guide
Comprehensive guide to steel grades, stainless steel types, and protective coatings
Fastener Materials & Coatings Guide (PDF)Common Bolt Types
Different bolt styles are designed for specific applications. Choose the right type based on your connection requirements, load type, and installation method.
Hex Bolts
Features: Six-sided head, machine threads
- Most common general-purpose bolt
- Driven with wrench or socket
- Available partially or fully threaded
- Used with nuts and washers
Best For: Structural connections, machinery, automotive
Shop Hex Bolts →Carriage Bolts
Features: Domed head, square neck under head
- Square neck prevents spinning in wood
- Smooth dome provides finished appearance
- Tamper-resistant from bolt side
- Tightened from nut side only
Best For: Wood connections, decks, playground equipment
Shop Carriage Bolts →Lag Bolts (Lag Screws)
Features: Hex head, coarse wood threads, pointed tip
- Heavy-duty wood fastener
- Requires pilot hole for installation
- No nut required - threads into wood
- High pullout strength in wood
Best For: Heavy timber, ledger boards, structural wood
Shop Lag Bolts →Flange Bolts
Features: Hex head with built-in washer flange
- Integrated washer distributes load
- Serrated flange prevents loosening
- Reduces parts count
- Faster assembly than bolt + washer
Best For: Automotive, frame assembly, vibration resistance
Shop Flange Bolts →Structural Bolts
Features: Heavy hex head, controlled tension
- Meets ASTM A325 or A490 standards
- Designed for steel-to-steel connections
- Larger head than standard hex bolts
- Used with heavy hex nuts
Best For: Steel construction, bridges, heavy structures
Shop Structural Bolts →Elevator Bolts
Features: Large flat head, square neck
- Wide bearing surface under head
- Square neck prevents rotation
- Low-profile installation
- Originally for grain elevators
Best For: Conveyor belts, agricultural equipment, flat surfaces
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