3/8” Bolt & Screw Dimensions: Diameter & Head Size

3/8″ bolt dimensions and screw sizes are utilized extensively in heavy machinery, automotive suspension, and structural mounting applications. This size is standardized with a 0.375″ nominal major diameter and is available in 3/8″-16 coarse (UNC), 3/8″-24 fine (UNF), or 3/8″-32 extra-fine (UNEF) thread pitches, representing 16, 24, and 32 threads per inch (TPI) respectively. This reference chart follows ANSI/ASME B18.6.3 and B18.3 standards for the head size and ANSI/ASME B1.1 standards for the thread dimensions to ensure compatibility across imperial fastening applications.

Disclaimer: Dimensions and tolerances may vary slightly depending on head style, manufacturer, or material — always verify with the official product datasheet before use.

Thread diagram fot bolt and screw depicting the major diameter, minor diameter, pitch diameter, and threads per inch
Hex head diagram
Cap head diagram
Low-cap head diagram
Button head diagram
Flat head diagram
Pan head diagram
3/8-16 UNC Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.3595 0.3737
Pitch Diameter 0.3266 0.3331
Minor Diameter Not specified 0.2993
Threads Per Inch (TPI) 16
Major Diameter 0.3643 0.3737
Pitch Diameter 0.3287 0.3331
Minor Diameter Not specified 0.2993
Threads Per Inch (TPI) 16
Major Diameter 0.3656 0.375
Pitch Diameter 0.3311 0.3344
Minor Diameter Not specified 0.3006
Threads Per Inch (TPI) 16
3/8-20 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.3657 0.3738
Pitch Diameter 0.3372 0.3413
Minor Diameter Not specified 0.3143
Threads Per Inch (TPI) 20
Major Diameter 0.3669 0.375
Pitch Diameter 0.3394 0.3425
Minor Diameter Not specified 0.3155
Threads Per Inch (TPI) 20
3/8-24 UNF Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.3631 0.3739
Pitch Diameter 0.3411 0.3468
Minor Diameter Not specified 0.3243
Threads Per Inch (TPI) 24
Major Diameter 0.3667 0.3739
Pitch Diameter 0.343 0.3468
Minor Diameter Not specified 0.3243
Threads Per Inch (TPI) 24
Major Diameter 0.3678 0.375
Pitch Diameter 0.345 0.3479
Minor Diameter Not specified 0.3254
Threads Per Inch (TPI) 24
3/8-28 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.3674 0.3739
Pitch Diameter 0.3471 0.3507
Minor Diameter Not specified 0.3314
Threads Per Inch (TPI) 28
Major Diameter 0.3685 0.375
Pitch Diameter 0.3491 0.3518
Minor Diameter Not specified 0.3325
Threads Per Inch (TPI) 28
3/8-32 UNEF Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.368 0.374
Pitch Diameter 0.3503 0.3537
Minor Diameter Not specified 0.3368
Threads Per Inch (TPI) 32
Major Diameter 0.369 0.375
Pitch Diameter 0.3522 0.3547
Minor Diameter Not specified 0.3378
Threads Per Inch (TPI) 32
  • Major Diameter – The largest diameter of the external thread, measured crest to crest. This defines the nominal size of the screw or bolt.
  • Pitch Diameter – The diameter of an imaginary cylinder where the thread thickness equals the space between threads. This dimension primarily controls thread fit and tolerance class of 2A and 3A.
  • Minor Diameter – The smallest diameter of the external thread, measured root to root. This affects thread strength and clearance.
  • Threads Per Inch (TPI) – The number of complete threads within one inch of thread length. Higher TPI values indicate a finer thread pitch.
Hex Head – ANSI/ASME B18.2.1
Dimension Size (in)
MinMax
Width Across Flats 0.544 0.562
Width Across Corners 0.62 0.65
Head Height 0.226 0.268
Hex Socket Cap Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 0.54 0.562
Head Height 0.368 0.375
Hex Socket Size 5/16 (0.312 in)
Hex Socket Low Cap Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 0.55 0.562
Head Height 0.182 0.192
Hex Socket Size 3/16 (0.188 in)
Hex Socket Button Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 0.636 0.656
Head Height 0.185 0.199
Hex Socket Size 7/32 (0.219 in)
Hex Socket Flat Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 0.72 0.781
Head Height 0.234 Not specified
Hex Socket Size 7/32 (0.219 in)
Slotted Flat Head – ANSI/ASME B18.6.3
Dimension Size (in)
MinMax
Head Diameter 0.67 0.717
Head Height 0.23 Not specified
Slot Width 0.081 0.094
Slot Depth 0.07 0.106
Phillips Flat Head – ANSI/ASME B18.6.3
Dimension Size (in)
MinMax
Head Diameter 0.67 0.717
Head Height 0.23 Not specified
Drive Size 4
Phillips Pan Head – ANSI/ASME B18.6.3
Dimension Size (in)
MinMax
Head Diameter 0.716 0.74
Head Height 0.244 0.261
Drive Size 4
Inch to Metric Converter
Enter inches to convert to mm
Previous & Next Size Chart
← 5/16
→ 7/16

← Choose Another Size

Frequently Asked Questions

What is the tap drill size for 3/8″-16, 3/8″-24, and 3/8″-32 screws?

Tap drill selection is based on the material and the required thread engagement. For aluminum, brass, or plastic (75% thread), a 5/16″ (0.313″) drill bit is specified for 3/8″-16, a Q (0.332″) for 3/8″-24, and an 11/32″ (0.344″) for 3/8″-32. For steel, stainless, or iron (50% thread), a Q (0.332″) drill bit is recommended for 3/8″-16, an S (0.348″) for 3/8″-24, and a T (0.358″) for 3/8″-32.

What clearance hole size is needed for a 3/8″ screw?

For a Close Fit, a W (0.386″) drill bit is utilized. For a Normal/Free Fit (standard clearance), an X (0.397″) drill bit is specified to allow the screw body to pass through the material without binding.

What is the difference between a bolt and a screw?

The difference is based on the intent of the fastener’s use; If the primary intent is for the fastener to pass through an unthreaded holes and be assembled with a nut, it is a Bolt. In this application, you usually hold the head and turn the nut to apply tension. But if the primary intent is for the fastener to be installed into a threaded (tapped) hole, it is a Screw. In this application, the connection is tightened by turning the head of the fastener itself.