What Size Drill Bit for the #8 Screw?

Welcome to our comprehensive guide on determining the correct drill bit size for a #8 screw. If you’ve ever found yourself pondering, “What size drill bit for the #8 screw?” you’re in the right place. This article aims to demystify the process, providing you with the knowledge necessary to ensure your #8 screws fit snugly and securely into your materials.

 Whether you’re a seasoned DIY enthusiast or a novice just dipping your toes into the world of drilling and fastening, understanding the appropriate drill bit size for your #8 screw is essential for successful and reliable projects. So, let’s jump in and unravel the mystery of selecting the perfect drill bit size for your #8 screw.

Screw Sizing Standards

Screw sizing standards are established systems that categorize and specify screws based on their size and characteristics. Two widely used screw sizing standards are the Unified Thread Standard (UTS) and the Metric Thread Standard.

Unified Thread Standard (UTS): This standard, commonly used in the United States, combines the Unified National Thread (UN) and the Unified National Thread Form (UNF). It employs both imperial and metric measurements, providing a wide range of thread sizes for screws.

Metric Thread Standard: The metric thread standard is prevalent in countries following the metric system. It uses millimeters to measure screw dimensions and thread pitch. Screws under this standard are designated by their primary diameter and thread pitch, such as M6x1.0, where M6 represents the primary diameter, and 1.0 is the thread pitch in millimeters.

How to Read and Interpret Screw Size Labels?

Screw size labels typically consist of two essential components:

  1. Primary Diameter: This is the nominal or outer diameter of the screw’s threads. The Unified Thread Standard often expresses it in inches (e.g., 1/4 inch), while the Metric Thread Standard represents it in millimeters (e.g., M6 for 6 millimeters).
  2. Thread Pitch: The thread pitch indicates the distance between adjacent threads on the screw. In the Unified Thread Standard, it is given in lines per inch (TPI), while in the Metric Thread Standard, it is stated in millimeters (e.g., 1.0 for a 1.0 mm pitch).

To interpret a screw size label, refer to the primary diameter and thread pitch. For example, a screw labeled 1/4-20 (UTS) has a primary diameter of 1/4 inch and 20 threads per inch, while an M6x1.0 (Metric) screw has a primary diameter of 6 millimeters and a 1.0 mm thread pitch.

Why Drill Bit Size Matters When Working with Screws?

Drill bit size plays a crucial role when working with screws because it determines the size of the hole or pilot hole that must be created to accommodate the screw. 

Matching the drill bit size to the screw size ensures:

  • Proper Thread Engagement: The drill bit creates a hole in the material, allowing the screw to pass through. The hole should be appropriately sized to provide the correct thread engagement, ensuring a secure connection.
  • Preventing Splitting: In materials prone to splitting, such as wood, a correctly sized pilot hole can help prevent the material from splitting when the screw is inserted.
  • Ease of Installation: An appropriately sized pilot hole makes it easier to drive the screw into the material without excessive force, reducing the risk of damaging the screw or workpiece.
  • Thread Forming: Sometimes, the screw may need to tap its threads as it enters the material. The correct drill bit size allows the screw to create threads that match its design, ensuring a tight fit.

To select the appropriate drill bit size for a specific screw, refer to drill and tap charts or use the formula mentioned earlier (Drill Bit Size = Major Diameter of Thread / Number of Threads Per Inch). Matching the drill bit size to the screw size is fundamental for achieving a secure and reliable connection while minimizing the risk of issues during installation.

Drill Bit Anatomy

Drill bits are cutting tools used to create holes in various materials. They consist of several key components:

  • Shank: The shank is the round and hollow part of the bit that fits into the chuck of a drill or drill press. It’s the part you grip to insert the bit into the tool.
  • Flute: Flutes are spiral grooves that run along the length of the drill bit. They serve two primary purposes: to remove chips (swarf) from the hole and to provide a channel for cutting fluid or air to cool and lubricate the bit.
  • Body: The body of the drill bit comprises the flute, extending from the shank to the tip. The diameter of the body corresponds to the drill bit’s size.
  • Point or Tip: The tip, also known as the point, is the cutting edge of the drill bit. It varies in shape depending on the bit type and its intended application.

Types of Drill Bits for Different Materials

Various drill bits are designed to work with specific materials and applications. Here are some common types:

Twist Drill Bits

  • Suitable for: Wood, metal, plastic, and more.
  • Characteristics: Spiral flute, pointed tip.

Masonry Drill Bits

  • Designed for: Concrete, brick, stone, and other masonry materials.
  • Key Features: Often equipped with a carbide tip for durability.

Wood Drill Bits

  • Optimized for: Wood.
  • Notable Features: Sharp, pointed tip with spurs for clean hole entry and chip removal.

Metal Drill Bits

  • Engineered for: Metal.
  • Distinctive Traits: Robust construction to withstand the hardness of metal materials.

Tile and Glass Drill Bits

  • Specialized for: Ceramic tiles, glass, porcelain.
  • Typical Characteristics: Spear-pointed tip with a carbide or diamond-coated tip.

Forstner Bits

  • Ideal for: Flat-bottomed holes in wood.
  • Notable Advantage: Creates clean, precise holes with minimal tear-out.

Step Drill Bits

  • Comprises: Multiple steps with increasing diameters.
  • Purpose: Allows drilling holes of various sizes without changing bits.

Countersink Bits

  • Used to: Create a conical recess around a drilled hole for screw heads to flush with the surface.

Hole Saw Bits

  • Designed for: Cutting large-diameter holes in wood, metal, or plastic, consisting of a circular saw-like blade.

Relationship Between Drill Bit Size and Screw Size

The relationship between drill bit size and screw size is significant for achieving a secure and proper fit when fastening with screws. When selecting a drill bit for a screw, choosing a bit size that matches or slightly exceeds the screw’s primary diameter (the widest part of the screw’s threads, excluding the head) is crucial.

The general rule of thumb is to use a drill bit slightly smaller than the screw’s primary diameter. This ensures a secure grip while still allowing the screw to bite into the material. If the drill bit is too small, it may not provide enough room for the screw threads, resulting in a weaker connection.

For example, if you’re working with a 1/4-inch screw, you would typically use a drill bit that matches the screw’s size or is slightly smaller, such as a 7/64-inch or 3/16-inch drill bit, depending on the material and application. Using the correct drill bit size helps prevent issues like stripped threads or poor fastener grip while ensuring a robust and reliable connection.

Guide on Calculating the Right Drill Bit Size for #8 Screws

Measuring the Screw Diameter

To determine the correct drill bit size for #8 screws, begin by measuring the screw’s diameter. #8 screws have a primary diameter of approximately 0.164 inches or 4.16 millimeters. This measurement represents the widest part of the screw’s threads, excluding the head.

Selecting an Appropriate Drill Bit Size

To ensure a proper fit and secure fastening, choose a drill bit size that matches or slightly exceeds the screw’s primary diameter. For #8 screws with a primary diameter of 0.164 inches:

  • Select a drill bit slightly smaller than 0.164 inches to allow the screw threads to grip the material securely.
  • A 7/64-inch drill bit, equivalent to approximately 0.109 inches or 2.77 millimeters, is suitable for #8 screws. It provides enough clearance for the screw threads while ensuring a secure fit.

Tips for Accuracy in Determining the Correct Size

  • Use Precision Instruments: When measuring the screw diameter, use a set of calipers or a micrometer for precise results. Avoid relying solely on visual estimation.
  • Refer to Screw Charts: Consult screw charts or manufacturer specifications to verify the primary diameter of #8 screws. This ensures accuracy in selecting the appropriate drill bit size.
  • Test Fit: For critical applications, perform a test fit by drilling a pilot hole with the selected drill bit size and inserting the #8 screw. The screw should thread easily and securely into the hole. Adjust the pilot hole size if necessary.

Common Mistakes to Avoid When Choosing Drill Bit Size

  • Using the Wrong Measurement: Ensure you’re measuring the primary diameter of the screw threads and not the overall diameter, including the screw head.
  • Choosing an Oversized Drill Bit: Selecting a drill bit that is too large can result in a loose fit, reduced thread engagement, and weaker fastening.
  • Neglecting Material Considerations: The type of material you’re working with may influence the choice of drill bit size. Softer materials may require a slightly smaller drill bit, while stiffer materials may need a slightly larger one.
  • Not Using Test Fits: Avoid skipping the step of test fitting the screw in the drilled hole to ensure the chosen drill bit size provides a secure and snug fit.

Using a Drill Bit Size Chart

Introduction to Drill Bit Size Charts

Drill bit size charts are invaluable references for individuals involved in drilling and fastening tasks. These charts offer a range of drill bit sizes tailored to accommodate various screw sizes, materials, and applications.

They play a pivotal role in simplifying the selection of the correct drill bit size, ensuring secure and precise screw fitting. Whether in print or online, these charts are indispensable tools for anyone engaged in construction, woodworking, metalworking, or DIY projects.

Where to Find Reliable Drill Bit Size Charts Online

Reliable drill bit size charts are readily available on various online platforms. These include:

  • Manufacturer Websites: Many reputable drill bit manufacturers, such as Bosch, DeWalt, or Irwin Tools, provide downloadable charts and guides on their websites.
  • Online Hardware Stores: Popular online hardware stores like Home Depot, Lowe’s, or Amazon often incorporate drill bit size charts within their product information sections.
  • Specialized Websites: Websites catering to machining, woodworking, or metalworking enthusiasts frequently offer comprehensive sizing references.
  • Educational Websites: DIY forums and instructional blogs often feature drill bit size charts as part of their tutorials.
  • PDF Downloads: PDF versions of drill bit size charts can be easily found online for those who prefer printed references.

How to Use a Chart to Select the Right Drill Bit Size for #8 Screws

Utilizing a drill bit size chart to pinpoint the appropriate drill bit size for #8 screws involves a straightforward process:

  1. Locate the #8 Screw Section: Find the section of the chart dedicated to #8 screws.
  2. Identify the Primary Diameter: For #8 screws, the primary diameter is approximately 0.164 inches or 4.16 millimeters.
  3. Choose the Recommended Drill Bit Size: Look for the recommended drill bit size that aligns with or slightly exceeds this primary diameter in the chart. Typically, for #8 screws, a drill bit size around 7/64 inches (0.109 inches or 2.77 millimeters) is suggested.
  4. Confirm Material-Specific Recommendations: Some charts may provide material-specific recommendations to consider.
  5. Perform a Test Fit: After selecting the drill bit size, conduct a test fit by drilling a pilot hole with the chosen bit and inserting a #8 screw. The screw should thread smoothly and securely. Adjust the hole size if needed.

Safety Gear Recommendations

Ensuring safety during drilling and fastening operations is paramount. Here are some safety gear recommendations:

  • Safety Glasses or Goggles: Always wear safety glasses or goggles to protect your eyes from flying debris, chips, and potential hazards.
  • Hearing Protection: If you are using power drills extensively or in noisy environments, wear ear protection such as earmuffs or earplugs to prevent hearing damage.
  • Dust Mask or Respirator: While working with materials that generate dust or fumes, such as wood, wear a dust mask or respirator to safeguard your respiratory system.
  • Gloves: Use appropriate gloves to protect your hands from sharp edges, splinters, or heat, especially when handling materials or hot workpieces.
  • Work Clothing: Wear clothing that covers your arms and legs to minimize exposure to sharp objects and hot metal shavings.
  • Closed-Toe Footwear: Use closed-toe shoes or boots with slip-resistant soles to protect your feet from falling objects or accidental slips.

Proper Drilling Techniques for Different Materials


  • Start with a Pilot Hole: Use a center punch to create a pilot hole to prevent the drill bit from wandering.
  • Moderate Speed: Drill at a moderate speed to avoid splintering or overheating.
  • Use Wood-Specific Bits: For clean holes in wood, consider using wood-specific drill bits or Brad Point bits.


  • Lubrication: Lubricate the drill bit with cutting oil or a suitable lubricant to reduce friction and heat buildup.
  • Pilot Hole First: Begin with a smaller pilot hole before moving to the final size to prevent work hardening of the metal.
  • Steady Pressure: Apply slow and steady pressure to prevent overheating and extend drill bit life.

Concrete and Masonry

  • Use Masonry Bits: Use masonry drill bits designed for drilling into concrete, brick, stone, and similar materials.
  • Hammer Drill Mode: If available, use a hammer drill mode, as it aids in drilling through hard materials like concrete.
  • Clear Debris: Periodically withdraw the bit to clear debris and maintain efficient drilling.
  • Plastic
  • Use Appropriate Bits: Select a sharp, high-speed steel (HSS) or plastic-specific drill bit.
  • Slow Drilling: Drill slowly to a moderate speed to avoid melting or deforming the plastic.
  • Secure Workpiece: Ensure the plastic workpiece is secured to prevent vibration and cracking.

Pre-Drilling Pilot Holes for #8 Screws

Pre-drilling pilot holes is often advisable when working with #8 screws, especially in more rigid materials. Follow these steps:

  1. Mark Hole Locations: Use a pencil or marking tool to indicate where to insert the screws.
  2. Select the Drill Bit: Choose a drill bit that matches the recommended size for #8 screws based on your drill bit size chart (e.g., 7/64 inches).
  3. Center Punch: Before drilling, use a center punch to create small indentations at the marked locations. This prevents the drill bit from wandering and ensures accurate hole placement.
  4. Drill Pilot Holes: Insert the selected drill bit into your drill and drill the pilot holes at the center-punched locations. Apply moderate pressure, keep the drill perpendicular to the workpiece, and use a slow to medium speed to prevent splitting or cracking.
  5. Test Fit: After drilling the pilot holes, insert the #8 screws to ensure they thread smoothly and securely. Adjust the depth of the pilot holes if necessary.

Troubleshooting Common Issues

How to Fix Over-Drilled or Under-Drilled Holes?

For over-drilled holes, where the hole is too large, solutions include inserting a wooden dowel or plug secured with wood glue and then drilling a new pilot hole of the correct size. In under-drilled spots, where the hole is too small, carefully widen it with a slightly larger drill bit to accommodate the screw.

Dealing with Stripped or Damaged Screws

When dealing with stripped or damaged screws, you have several options. For stripped screw heads with intact threads, try using pliers or a screwdriver to back out the screw. If that fails, employ a screw extractor, drilling a small hole and using the extractor to grip and remove the screw.

Consider creating new threads with a tap and die set or a self-tapping screw when the threads are damaged in the material. In severe cases, you may need to fill the damaged hole with epoxy or wood filler, drill a new pilot hole, and insert a new screw. Ensure the new screw is appropriate for the load and material to maintain the integrity of your project.


In conclusion, determining the right drill bit size for a #8 screw is crucial in any DIY or construction project. The correct size ensures a secure and stable connection while preventing damage to the materials. By referring to the manufacturer’s recommendations or using a drill bit size chart, you can quickly identify the appropriate drill bit diameter for your #8 screw.

Remember that precision and accuracy are crucial to achieving the best results, so take the time to measure and choose the right drill bit size to make your projects safer and more successful. Whether you’re an experienced pro or a dedicated DIY enthusiast, selecting the correct drill bit size cannot be overstated, as it ultimately contributes to the strength and longevity of your fastened components.

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