Selecting the correct hex head screw size is essential for creating strong, secure, and long-lasting connections. Whether you’re working on construction, automotive repairs, machinery, woodworking, or a DIY project, using the right screw dimensions improves both safety and performance.
This guide explains how to read a hex head screw size chart, understand screw dimensions, choose the correct grade, and select the right fastener for your application.
Hex Head Screw Size Chart
| Nominal or Basic Product Diameter Click on the size to shop products | Body Diameter (E) | Width Across Flats (F) | Width Across Corners (G) | Head Height (H) | Thread Length (LT) | ||||||||
| Max | Min | Basic | Max | Min | Max | Min | Basic | Max | Min | <or=6″ | >6″ | ||
| 1/4 | .2500 | .2500 | .2450 | 7/16 | .438 | .428 | .505 | .488 | 5/32 | .163 | .150 | .750 | 1.000 |
| 5/16 | .3125 | .3125 | .3065 | 1/2 | .500 | .489 | .577 | .557 | 13/64 | .211 | .195 | .875 | 1.125 |
| 3/8 | .3750 | .3750 | .3690 | 9/16 | .562 | .551 | .650 | .628 | 15/64 | .243 | .226 | 1.000 | 1.250 |
| 7/16 | .4375 | .4375 | .4305 | 5/8 | .625 | .612 | .722 | .698 | 9/32 | .291 | .272 | 1.125 | 1.375 |
| 1/2 | .5000 | .5000 | .4930 | 3/4 | .750 | .736 | .866 | .840 | 5/16 | .323 | .302 | 1.250 | 1.500 |
| 9/16 | .5625 | .5625 | .5545 | 13/16 | .812 | .798 | .938 | .910 | 23/64 | .371 | .348 | 1.375 | 1.625 |
| 5/8 | .6250 | .6250 | .6170 | 15/16 | .938 | .922 | 1.083 | 1.051 | 25/64 | .403 | .378 | 1.500 | 1.750 |
| 3/4 | .7500 | .7500 | .7410 | 1-1/8 | 1.125 | 1.100 | 1.299 | 1.254 | 15/32 | .483 | .455 | 1.750 | 2.000 |
| 7/8 | .8750 | .8750 | .8660 | 1-5/16 | 1.312 | 1.285 | 1.516 | 1.465 | 35/64 | .563 | .531 | 2.000 | 2.250 |
| 1 | 1.0000 | 1.0000 | .9900 | 1-1/2 | 1.500 | 1.469 | 1.732 | 1.675 | 39/64 | .627 | .591 | 2.250 | 2.500 |
| 1-1/8 | 1.1250 | 1.1250 | 1.1140 | 1-11/16 | 1.688 | 1.631 | 1.949 | 1.859 | 11/16 | .718 | .658 | 2.500 | 2.750 |
| 1-1/4 | 1.2500 | 1.2500 | 1.2390 | 1-7/8 | 1.875 | 1.812 | 2.162 | 2.066 | 25/32 | .813 | .749 | 2.750 | 3.000 |
| 1-1/2 | 1.5000 | 1.5000 | 1.4880 | 2-1/4 | 2.250 | 2.175 | 2.598 | 2.480 | 1-5/16 | .974 | .902 | 3.250 | 3.500 |
| Hex Cap Screws: Tolerance on Length | |||||||
|---|---|---|---|---|---|---|---|
| Nominal Screw Size | Nominal Screw Length | ||||||
| Up to 1″ | Over 1″ – 2-1/2″ | Over 2-1/2″ – 4″ | Over 4″ – 6″ | Longer than 6″ | |||
| 1/4 – 3/8 | -.03 | -.04 | -.06 | -.10 | -.18 | ||
| 7/16 & 1/2 | -.03 | -.06 | -.08 | -.10 | -.18 | ||
| 9/16 – 3/4 | -.03 | -.08 | -.10 | -.10 | -.18 | ||
| 7/8 and 1 | – | -.10 | -.14 | -.16 | -.20 | ||
| 1-1/8 – 1-1/2 | – | -.12 | -.16 | -.18 | -.22 | ||
| Hex Cap Screws: Performance & Mechanical Stats | ||||||
|---|---|---|---|---|---|---|
Grade 2 | Grade 5 | Grade 8 | Stainless Steel 18-8 | Stainless Steel 316 | ||
| Applications | Non-critical applications, economical | Greater tensile strength than Grade 2; yield-to-tensile ratio is the lowest of all heat-treated steels. | High stress applications | General corrosion resistance such as chemical or food-processing equipment | Greater corrosion resistance, strength and temperature | |
| Material | Low or medium carbon steel. AISI – 1006-1050 | Medium carbon steel (or alloy), heat treated | Medium carbon alloy steel, heat treated | Austenitic alloys: 303, 303Se, 304, XM7 (chromium 17-19% and nickel content 8-10%) | 316 Stainless Steel (Molybdenum content 2-3% and nickel content 10-14%) | |
| Heat Treatment | N/A | Heat treated, oil or water quenched, tempered at 800 Deg (F) min. | Heat treated, oil quenched, tempered at 800 Deg (F) min. | |||
| Hardness | 1/4″-3/4″ D, Up to 6″ L: Rockwell B80-B1001/4″-3/4″ D, Over 6″ L: Rockwell B70-B1007/8″-1-1/2″ D, all lengths: Rockwell B70-B100 | See below | See below | 1/4″-5/8″ D: Rockwell B95-C323/4″-1″ D: Rockwell B80-C32 | 1/4″-5/8″ D: Rockwell B95-C3233/4″-1″ D: Rockwell B80-C32 | |
| Core Hardness | See above | 1/4″-1″ D, all lengths: Rockwell C25-C341-1/8″-1-1/2″ D, all lengths: Rockwell C19-C30 | 1/4″-1-1/2″, all lengths: Rockwell C33-C39 | See above | See above | |
| Surface Hardness | See above | 1/4″-1″ D, all lengths: Rockwell 30N54 max1-1/8″-1-1/2″ D, all lengths: Rockwell 30N50 max | 1/4″-1-1/2″ D, all lengths: Rockwell 30N58.6 max | See above | See above | |
| Proof Load | 1/4″-3/4″ D, Up to 6″ L: 55,000 psi1/4″-3/4″ D, Over 6″ L: 33,000 psi7/8″-1-1/2″ D, all lengths: 33,000 psi | 1/4″-1″ D, all lengths: 85,000 psi1-1/8″-1-1/2″ D, all lengths: 74,000 psi | 1/4″-1-1/2″ D, all lengths: 120,000 psi | |||
| Yield Strength | 1/4″-3/4″ D, Up to 6″ L: 57,000 psi min.1/4″-3/4″ D, Over 6″ L: 36,000 psi min7/8″-1-1/2″ D, all lengths: 36,000 psi min | 1/4″-1″ D, all lengths: 92,000 psi min1-1/8″-1-1/2″ D, all lengths: 81,000 psi min | 1/4″-1-1/2″ D, all lengths: 130,000 psi min | 1/4″-5/8″ D, 2.25 D and longer: 65,000 psi min3/4″ (2.25D and longer) & 7/8″-1″ D (3D and longer): 45,000 psi min | 1/4″-5/8″ D, 2.25 D and longer: 65,000 psi min3/4″ (2.25D and longer) & 7/8″-1″ D (3D and longer): 45,000 psi min | |
| Tensile Strength | 1/4″-3/4″ D, Up to 6″ L: 74,000 psi min.1/4″-3/4″ D, Over 6″ L: 60,000 psi min7/8″-1-1/2″ D, all lengths: 60,000 psi min | 1/4″-1″ D, all lengths: 120,000 psi min1-1/8″-1-1/2″ D, all lengths: 105,000 psi min | 1/4″-1-1/2″ D, all lengths: 150,000 psi min | 1/4″-5/8″ D, 2.25 D and longer: 100,000-150,000 psi3/4″ (2.25D and longer) & 7/8″-1″ D (3D and longer): 85,000-140,000 psi | 1/4″-5/8″ D, 2.25 D and longer: 100,000-150,000 psi3/4″ (2.25D and longer) & 7/8″-1″ D (3D and longer): 85,000-140,000 psi | |
| Elongation | 18% min | 14% min | 12% min | 4D – 1/4″-5/8″ D: 20% min, 3/4″-1″ D: 25% min | 4D – 1/4″-5/8″ D: 20% min, 3/4″-1″ D: 25% min | |
| Reduction of Area | 35% min | 35% min | 35% min | |||
What is a hex head screw?
A hex head screw (also called a hex bolt or hex cap screw) is a fastener with a six-sided head designed to be tightened or loosened using a wrench or socket.
Hex head screws are widely used because they provide excellent torque transfer and can withstand heavy loads. They’re commonly found in:
- Construction
- Automotive repairs
- Heavy machinery
- Industrial equipment
- Furniture assembly
- Agricultural equipment
They are manufactured in various materials, grades, thread types, and sizes to suit different environments and load requirements.
Why Choosing the Correct Screw Size Matters
Using the proper screw size isn’t simply about making parts fit together—it directly affects strength, durability, and safety.
Choosing the wrong size can lead to:
- Loose connections
- Stripped threads
- Material damage
- Reduced load capacity
- Premature fastener failure
When selecting a hex head screw, consider all of the following:
- Diameter
- Length
- Thread pitch
- Material
- Strength grade
- Environmental conditions
Matching these factors to your project helps create a secure and reliable connection.
Understanding Hex Head Screw Dimensions
A hex head screw consists of several important measurements.
Diameter
The diameter refers to the thickness of the screw shaft. Larger diameters generally provide greater load-bearing capacity.
Length
Length is measured from the underside of the head to the end of the screw for standard hex-head screws.
Selecting the proper length ensures enough thread engagement without damaging the opposite side of the material.
Thread Pitch
Thread pitch is the distance between adjacent threads.
- Coarse threads are ideal for softer materials and quick assembly.
- Fine threads provide better holding power and are commonly used where vibration resistance is important.
Head Size
The hex head dimensions determine which wrench or socket size is required for installation.
Types of Hex Head Screws
Different applications require different types of hex screws.
Standard Hex Head Screws
These are the most common fasteners used in general construction and machinery.
Hex Cap Screws
Manufactured with tighter tolerances and higher precision, hex cap screws are commonly used in industrial applications.
Hex Tap Bolts
These are fully threaded and often used where maximum thread engagement is required.
Stainless Steel Hex Head Screws
Stainless steel screws offer excellent corrosion resistance, making them ideal for:
- Marine environments
- Outdoor projects
- Food-processing equipment
- Chemical plants
Metric Hex Head Screws
Metric hex screws follow ISO standards and are commonly used in imported vehicles, machinery, and industrial equipment.
Understanding Screw Grades
Hex head screws are available in different strength grades.
Grade 2
- General-purpose applications
- Lower strength
- Economical option
Grade 5
- Medium-strength fastener
- Heat-treated steel
- Common in automotive applications
Grade 8
- High-strength alloy steel
- Designed for heavy-duty machinery
- Excellent for high-stress applications
Stainless Steel (18-8 & 316)
These grades prioritize corrosion resistance over maximum tensile strength and are ideal for outdoor or wet environments.
How to Measure a Hex Head Screw
Measuring a hex head screw correctly helps ensure compatibility.
Follow these simple steps:
- Measure the shaft diameter.
- Measure the screw length from beneath the head to the tip.
- Identify the thread pitch.
- Measure the width across the flats if selecting the proper wrench size.
Using calipers provides the most accurate measurements.
How to Read a Hex Head Screw Size Chart
Although the chart may appear technical, it’s easy once you understand the columns.
The chart typically includes:
- Nominal diameter
- Body diameter
- Width across flats
- Width across corners
- Head height
- Recommended thread length
Always compare your existing fastener with the chart before purchasing replacements.
Tips for Choosing the Right Hex Head Screw
Before selecting a fastener, ask yourself:
- What material am I fastening?
- Will the connection support heavy loads?
- Will the fastener be exposed to moisture?
- Do I need corrosion resistance?
- Is SAE or metric hardware required?
Answering these questions helps narrow down the correct size and material.
Common Mistakes to Avoid
Many fastening problems come from using the wrong screw.
Avoid these common mistakes:
- Using the wrong thread pitch
- Selecting an incorrect screw length
- Mixing metric and imperial fasteners
- Ignoring strength grades
- Over-tightening the screw
Taking a few extra minutes to verify measurements can prevent costly repairs later.
Conclusion
Choosing the correct hex head screw size is essential for creating strong, secure, and reliable connections. By understanding screw dimensions, thread pitch, strength grades, and the information provided in a hex head screw size chart, you can confidently select the right fastener for almost any project.
Whether you’re replacing a single bolt or assembling industrial equipment, using the correct hex head screw improves safety, performance, and long-term durability.
FAQs
What is a hex head screw?
A hex head screw is a fastener with a six-sided head that is tightened or loosened using a wrench or socket.
How do I measure a hex head screw?
Measure the shaft diameter, screw length (under the head to the tip), and thread pitch. You can also measure the width across the flats to determine the correct wrench size.
What’s the difference between a hex bolt and a hex cap screw?
Hex cap screws are manufactured with tighter tolerances and are designed for precision applications, while hex bolts are generally used with nuts in less critical fastening applications.
Which hex head screw grade is strongest?
Grade 8 hex head screws provide the highest strength among common carbon steel fasteners, making them suitable for heavy-duty and high-stress applications.
Can I use stainless steel hex head screws outdoors?
Yes. Stainless steel, especially 316 stainless steel, offers excellent corrosion resistance and is ideal for outdoor, marine, and humid environments.
Are metric and SAE hex head screws interchangeable?
No. Metric and SAE fasteners have different thread standards and dimensions. Always use matching screws, nuts, and tools for the best fit and safety.