Fasteners & Dimensions
Complete dimension reference for metric bolts, nuts, washers, and screws. ISO standard dimensions for M3-M24 fasteners.
Understanding Metric Fasteners
Metric fasteners are standardized mechanical components used globally for joining materials. The ISO metric system uses millimeters for all dimensions and follows international standards ensuring interchangeability across manufacturers.
Our fastener reference covers standard dimensions for bolts, nuts, washers, and screws from M3 to M24 - the most commonly used sizes in engineering and manufacturing.
Metric Fastener Naming Convention
Format: M[Diameter] × [Length] - [Grade]
Example: M8 × 25 - 8.8
= 8mm diameter, 25mm length, Grade 8.8 steel
- M = Metric system
- Number after M = Nominal diameter in mm
- After × = Length in mm (for bolts/screws)
- After - = Material grade/class
Standard Hex Bolt Dimensions
| Size | Thread Pitch | Head Width | Head Height | Spanner |
|---|---|---|---|---|
| M4 | 0.7mm | 7mm | 2.8mm | 7mm |
| M5 | 0.8mm | 8mm | 3.5mm | 8mm |
| M6 | 1.0mm | 10mm | 4.0mm | 10mm |
| M8 | 1.25mm | 13mm | 5.3mm | 13mm |
| M10 | 1.5mm | 17mm | 6.4mm | 17mm |
| M12 | 1.75mm | 19mm | 7.5mm | 19mm |
| M14 | 2.0mm | 22mm | 8.8mm | 22mm |
| M16 | 2.0mm | 24mm | 10mm | 24mm |
| M20 | 2.5mm | 30mm | 12.5mm | 30mm |
| M24 | 3.0mm | 36mm | 15mm | 36mm |
Thread Pitch - Coarse vs Fine
| Size | Coarse Pitch | Fine Pitch | When to Use Fine |
|---|---|---|---|
| M6 | 1.0mm | 0.75mm | Precision adjustment Vibration resistance Thin materials Finer control |
| M8 | 1.25mm | 1.0mm | |
| M10 | 1.5mm | 1.25mm, 1.0mm | |
| M12 | 1.75mm | 1.5mm, 1.25mm | |
| M16 | 2.0mm | 1.5mm |
Hex Nut Dimensions
| Size | Width Across Flats | Height (Standard) | Height (Thin/Jam) |
|---|---|---|---|
| M4 | 7mm | 3.2mm | 2.2mm |
| M6 | 10mm | 5mm | 3.2mm |
| M8 | 13mm | 6.5mm | 4mm |
| M10 | 17mm | 8mm | 5mm |
| M12 | 19mm | 10mm | 6mm |
| M16 | 24mm | 13mm | 8mm |
| M20 | 30mm | 16mm | 10mm |
Washer Dimensions
| Bolt Size | ID (mm) | OD Standard | OD Large | Thickness |
|---|---|---|---|---|
| M4 | 4.3 | 9mm | 12mm | 0.8mm |
| M6 | 6.4 | 12mm | 18mm | 1.6mm |
| M8 | 8.4 | 16mm | 24mm | 1.6mm |
| M10 | 10.5 | 20mm | 30mm | 2.0mm |
| M12 | 13 | 24mm | 37mm | 2.5mm |
| M16 | 17 | 30mm | 50mm | 3.0mm |
Bolt Grades and Strength
| Grade | Tensile (MPa) | Yield (MPa) | Application |
|---|---|---|---|
| 4.6 | 400 | 240 | Non-critical, low strength |
| 8.8 | 800 | 640 | General engineering (most common) |
| 10.9 | 1000 | 900 | High strength, automotive |
| 12.9 | 1200 | 1080 | Very high strength, critical joints |
| A2-70 | 700 | 450 | Stainless steel, corrosion resistant |
| A4-80 | 800 | 600 | Marine grade stainless |
Grade marking: First number × 100 = Tensile strength (MPa). Second number × 10 = Yield/Tensile ratio (%). Grade 8.8 = 800 MPa tensile, 80% yield ratio.
Hole Size for Clearance Fit
| Bolt Size | Close Fit | Normal Fit | Loose Fit |
|---|---|---|---|
| M4 | 4.3mm | 4.5mm | 5.0mm |
| M6 | 6.4mm | 6.6mm | 7.0mm |
| M8 | 8.4mm | 9.0mm | 10mm |
| M10 | 10.5mm | 11mm | 12mm |
| M12 | 13mm | 14mm | 15mm |
| M16 | 17mm | 18mm | 19mm |
Frequently Asked Questions
What's the minimum thread engagement for a bolt?
Rule of thumb: Minimum engagement = 1.5× bolt diameter for steel into steel, 2× for aluminum or cast iron. M10 bolt should engage at least 15mm of thread in steel, 20mm in aluminum.
How do I measure a bolt correctly?
For hex bolts: Length is measured from under the head to the end (threads + shank). For countersunk screws: Length includes the head. Thread diameter is measured at the outer thread peaks. Use a thread pitch gauge to confirm pitch.
What torque should I use?
Torque depends on bolt grade, diameter, and friction. For Grade 8.8 dry steel: M6 ≈ 10 Nm, M8 ≈ 25 Nm, M10 ≈ 50 Nm, M12 ≈ 85 Nm. Reduce by 20-30% for lubricated threads. Always refer to manufacturer specifications for critical joints.
When should I use lock washers or thread lock?
Use vibration-resistant methods when: joint experiences vibration, thermal cycling, or periodic loosening is observed. Options: Nord-lock washers (best), nyloc nuts, thread locker (Loctite), spring washers (least effective). Critical joints may need multiple methods.