Inch Series Callout Structure
Fastener callouts pack a lot of information into a short string. Understanding how to read these specifications is essential for ordering the right part. This guide breaks down both inch and metric callout formats so you can decode any fastener specification.
Inch Series Callout Structure
The standard format for inch-series fasteners:
[Size]-[TPI] x [Length] [Grade/Material] [Finish] [Head Style]
Example: "1/4-20 x 1-1/2 Grade 5 Zinc Hex Cap Screw"
| Element | Value | Meaning |
|---|---|---|
| Size | 1/4 | Major diameter: 0.250 inches |
| TPI | 20 | Threads per inch (coarse thread for 1/4") |
| Length | 1-1/2 | 1.5 inches long |
| Grade | Grade 5 | SAE J429 Grade 5 (120,000 psi tensile) |
| Finish | Zinc | Zinc-plated (electroplated) |
| Head Style | Hex Cap Screw | Hexagonal head, washer face |
Size Designations
Numbered Sizes (Small Screws):
| Number | Major Diameter |
|---|---|
| #2 | 0.086" |
| #4 | 0.112" |
| #6 | 0.138" |
| #8 | 0.164" |
| #10 | 0.190" |
| #12 | 0.216" |
Fractional Sizes:
1/4", 5/16", 3/8", 7/16", 1/2", 9/16", 5/8", 3/4", 7/8", 1", etc.
Thread Pitch (TPI)
Threads per inch—higher number = finer thread
| Size | Coarse (UNC) | Fine (UNF) |
|---|---|---|
| #6 | 32 | 40 |
| #8 | 32 | 36 |
| #10 | 24 | 32 |
| 1/4" | 20 | 28 |
| 5/16" | 18 | 24 |
| 3/8" | 16 | 24 |
| 1/2" | 13 | 20 |
| 3/4" | 10 | 16 |
| 1" | 8 | 12 |
Rule: If no pitch is specified, assume UNC (coarse) thread.
Length Measurement
- Hex bolts/screws: Measured from under the head to tip
- Flat head screws: Measured from top of head to tip (overall)
- Socket head cap screws: Under head to tip
Metric Callout Structure
The standard format for metric fasteners:
M[Diameter]-[Pitch] x [Length] [Class] [Finish] [Style]
Or with coarse pitch implied:
M[Diameter] x [Length] [Class] [Finish] [Style]
Example: "M8-1.25 x 25 Class 10.9 Plain Hex Bolt"
| Element | Value | Meaning |
|---|---|---|
| M | M | Metric designation |
| Diameter | 8 | 8mm nominal diameter |
| Pitch | 1.25 | 1.25mm between threads (coarse for M8) |
| Length | 25 | 25mm long |
| Class | 10.9 | Property class (1040 MPa / ~Grade 8) |
| Finish | Plain | Unplated (black oxide or bare) |
| Style | Hex Bolt | Standard hex head |
Common Metric Sizes
| Diameter | Coarse Pitch | Fine Pitch |
|---|---|---|
| M3 | 0.5 | 0.35 |
| M4 | 0.7 | 0.5 |
| M5 | 0.8 | 0.5 |
| M6 | 1.0 | 0.75 |
| M8 | 1.25 | 1.0 |
| M10 | 1.5 | 1.0 or 1.25 |
| M12 | 1.75 | 1.25 or 1.5 |
| M16 | 2.0 | 1.5 |
| M20 | 2.5 | 1.5 |
Rule: If pitch is omitted (e.g., "M8 x 25"), assume coarse pitch.
Property Classes and Grades
Inch (SAE J429)
| Head Marking | Grade | Tensile Strength |
|---|---|---|
| (none) | Grade 2 | 74,000 psi |
| 3 lines | Grade 5 | 120,000 psi |
| 6 lines | Grade 8 | 150,000 psi |
Metric (ISO 898-1)
| Class | Tensile Strength | US Equivalent |
|---|---|---|
| 4.6 | 400 MPa (58 ksi) | ≈ Grade 2 |
| 8.8 | 800 MPa (116 ksi) | ≈ Grade 5 |
| 10.9 | 1040 MPa (151 ksi) | ≈ Grade 8 |
| 12.9 | 1220 MPa (177 ksi) | > Grade 8 |
Stainless Steel
Inch: F593 Group (C=304, D=316)
Metric: A2-70 (304) or A4-80 (316)
Common Finish Abbreviations
| Abbreviation | Full Name | Notes |
|---|---|---|
| ZP, Zinc | Zinc plated | Clear/blue chromate |
| YZ, Yellow Zinc | Yellow zinc | Yellow chromate, thicker |
| HDG, HG | Hot-dip galvanized | Heavy coating, outdoor use |
| MG | Mechanically galvanized | Tumbled zinc coating |
| BLK, BO | Black oxide | Minimal corrosion protection |
| Plain | Uncoated | Bare steel or mill finish |
| SS, Stainless | Stainless steel | 304 or 316 |
| Brass | Brass | Yellow brass (decorative) |
Head Style Quick Reference
| Abbreviation | Description |
|---|---|
| Hex | Standard hexagonal head |
| HH, Hex Head | Hexagonal head bolt |
| HCS, Hex Cap | Hex cap screw (washer face) |
| HHCS | Heavy hex cap screw |
| SHCS | Socket head cap screw |
| FHCS | Flat head cap screw (countersunk) |
| BHCS | Button head cap screw |
| Pan | Pan head (machine screws) |
| Flat | Flat/countersunk head |
| Oval | Oval countersunk head |
| Truss | Truss head (wide, low profile) |
| Round | Round head (dome top) |
| Lag | Lag bolt (wood screw threads) |
| Carriage | Carriage bolt (round head, square neck) |
Complete Callout Examples
Inch Series
| Callout | Translation |
|---|---|
| 1/4-20 x 1 GR5 ZP HCS | 1/4" coarse, 1" long, Grade 5, zinc, hex cap screw |
| 3/8-16 x 2 GR8 Plain Hex | 3/8" coarse, 2" long, Grade 8, unplated hex bolt |
| #10-24 x 3/4 SS Pan Phil | #10 fine, 3/4" long, stainless, pan head Phillips |
| 1/2-13 x 3 A325 HDG | 1/2" structural, 3" long, A325, hot-dip galvanized |
| 5/16-18 x 1 SHCS | 5/16" coarse, 1" long, socket head cap screw |
Metric Series
| Callout | Translation |
|---|---|
| M8 x 25 10.9 ZP | M8 coarse, 25mm long, Class 10.9, zinc |
| M6-1.0 x 16 A2-70 | M6 coarse, 16mm, 304 stainless |
| M10 x 1.25 x 40 12.9 | M10 fine pitch, 40mm, Class 12.9 |
| M12 x 35 8.8 HDG | M12 coarse, 35mm, Class 8.8, galvanized |
| DIN 912 M5 x 12 10.9 | Socket cap, M5, 12mm, Class 10.9 |
Common Mistakes to Avoid
1. Confusing pitch and TPI
- Inch: 20 TPI = 20 threads per inch
- Metric: 1.25 pitch = 1.25mm between threads
2. Assuming pitch when not specified
- Always assume coarse unless "fine" is noted
3. Mixing Grade and Class
- Grade 5 is inch (SAE)
- Class 8.8 is metric (ISO)
- They're similar but not identical specifications
4. Measuring length wrong
- Flat heads: measure overall length
- All others: measure from under head
5. Forgetting the finish
- "Plain" often means black oxide
- Bare steel will rust quickly
Quick Decode Cheat Sheet
Inch: "[Diameter]-[TPI] x [Length] [Grade] [Finish] [Style]"
Metric: "M[Diameter] x [Length] [Class] [Finish] [Style]"
When in doubt:
- No pitch specified = coarse thread
- No finish specified = check with supplier
- No grade = likely Grade 2 or commercial quality
Still have questions about a fastener callout? Contact us with your specification and we'll help you find exactly what you need.