Drill Size Chart in mm: Metric Drill Bit Sizes and Tap Drill Guide for Fasteners

Drilling the correct hole is one of the most fundamental steps in any precision assembly. Whether you are installing a simple screw, preparing a threaded hole, or mounting a rivet nut, the diameter of the drill bit determines how well the fastener performs. A reliable drill size chart in mm removes guesswork and helps engineers, machinists, and maintenance teams deliver consistent results. At Kunshan Hong Yong Sheng Precision Hardware, we work with standard fasteners and custom-machined components every day, so we understand how much difference the right pilot hole can make.

Drill Size Basics: Understanding Metric Measurements

Metric drill sizes are expressed in millimetres, and the number refers to the diameter of the bit. The metric system offers a logical, decimal-based approach that makes it easy to scale from tiny instrument screws up to heavy structural bolts. Unlike imperial sizes, which use fractions or number and letter designations, the metric drill size chart mm is straightforward: a 5.0 mm drill bit cuts a hole that is exactly five millimetres wide.

In industrial practice, metric drill bits are available in a wide range of diameters. Small sizes, such as 0.5 mm and 0.8 mm, are used for precision electronics and fine instrument work. Medium sizes, from 2 mm to 10 mm, cover most general engineering applications and fastener installations. Larger sizes, from 12 mm upward, serve structural and heavy-equipment needs.

However, choosing the right drill size is not simply about picking a bit that matches the fastener diameter. The type of joint, the material, and the threading method all influence the ideal hole size. This is why a comprehensive drill size chart in mm is such a valuable reference for anyone who works with mechanical assemblies.

How to Read a Drill Size Chart mm

A well-organised metric drill size chart typically includes several key pieces of information. First is the nominal drill diameter, which is the actual width of the hole that the bit will produce. Second is the recommended application, such as clearance drilling, tapping, or rivet nut installation. Third, and most importantly, the chart usually pairs each thread size with the corresponding tap drill diameter.

Understanding the relationship between these values is essential. A clearance hole, for example, allows a bolt to pass through without engaging the threads in that plate. A tap hole, on the other hand, is slightly smaller than the nominal thread diameter so that the tap can cut threads into the wall of the hole. Tap drill sizes are calculated based on the thread pitch, and standard charts provide recommended values for each metric thread size.

When you read a drill size chart in mm, look for the thread designation first. Next, verify the pitch, because fine threads require a larger tap drill than coarse threads of the same nominal diameter. Finally, check the recommended hole diameter and select the closest standard drill bit.

Metric Tap Drill Size Chart for Common Threads

The table below provides a quick reference for commonly used metric coarse threads. The tap drill diameters listed are based on approximately 75 to 80 percent thread engagement, which is a practical standard for general engineering applications. For softer materials, a slightly larger hole may be used, while harder materials may need a slightly smaller one to ensure adequate thread strength.

These values assume standard metric coarse threads as defined by ISO 261 and ISO 262. For fine thread series, such as M8 x 1 or M10 x 1.25, the tap drill diameter would be slightly larger because the pitch is smaller and the thread profile is shallower.

Standard Metric Drill Bit Sizes from 0.5 mm to 25 mm

Standard metric drill bits follow a conventional progression of diameters. The table below lists the most commonly stocked sizes, which are readily available in most industrial tooling suppliers. It is worth noting that intermediate sizes are frequently selected for specific applications, particularly when a calculated tap drill diameter does not match a standard size exactly.

For applications where the calculated hole size falls between two standard drill diameters, it is generally safer to choose the larger option when working with ductile materials such as aluminium or mild steel. For harder materials, the smaller option may provide better thread engagement, but the risk of tap breakage also increases.

Choosing the Right Drill Size for Fastener Installation

Installing fasteners correctly requires more than just picking a drill bit that looks about the right size. Different fastener types have different requirements for hole diameter, and selecting the wrong size can lead to weak joints, damaged threads, or components that fail under load.

For standard bolts and screws, the clearance hole should be slightly larger than the fastener diameter so that the bolt slides through freely. A clearance chart is often included in the same drill size chart in mm reference. For tapping operations, the hole must match the tap drill diameter for the specific thread size, as shown in the earlier table. For special fastening systems such as rivet nuts and captive screws, the requirements are even more specific.

Rivet nuts, for example, rely on the hole diameter to achieve proper deformation when the setting tool pulls the threaded portion. If the hole is too small, the rivet nut cannot collapse correctly and may bind before installation is complete. If the hole is too large, the rivet nut may spin or fail to form a secure bulge on the blind side. Our expansion rivet nut range demonstrates how carefully controlled dimensions affect the final fastening quality. A precise pilot hole ensures the flanged portion seats cleanly and the body expands symmetrically to create a strong, vibration-resistant joint.

Captive screws are another example where hole accuracy directly affects function. These screws are designed to remain attached to a panel or enclosure, so the mounting hole must be machined to tight tolerances. If the hole is undersized, the retainer ring will not seat correctly. If it is oversized, the screw may wobble or detach over time. The captive screws we manufacture at Hongyongsheng are produced with consistent head styles and thread forms, and they rely on properly prepared panel holes for smooth operation.

Self-clinching and press-in fasteners follow the same principle. The displacement of material during installation depends on the hole size being within a specific tolerance band. This is why experienced engineers always consult a drill size chart in mm before setting up a process for a new fastener type.

Drilling Tips for Precision Hardware Work

A drill size chart in mm gives you the right diameter, but achieving a clean, accurate hole also depends on technique. Here are several practical points that we emphasise in our own machining environment:

• Use a centre punch or a spotting drill to create an accurate starting point. This prevents the drill bit from wandering, especially on curved or uneven surfaces.
• Match the drill speed to the material. Harder metals such as stainless steel require slower spindle speeds, while aluminium and plastics can be drilled at higher speeds without generating excessive heat.
• Apply cutting fluid or a suitable lubricant for steel and stainless steel. This extends tool life and produces a smoother hole surface, which is particularly important for tapped holes.
• Peck drilling, or withdrawing the bit periodically to clear chips, is essential for deep holes and for materials that produce long, stringy swarf, such as aluminium.
• Deburr the hole after drilling, especially on the entry side. Burrs can interfere with flat seating of screw heads and washers, and they can also affect the measurement of hole diameter.

In addition to these general tips, it is important to verify the hole diameter with a plug gauge or calibrated pin, particularly for holes that will receive press-fit components or precision bushings. This level of verification is standard practice in our own production of shaft components and automotive hardware, where the difference of a few hundredths of a millimetre can affect fit and function.

Why Precision Drilling Matters in Industrial Manufacturing

In high-volume manufacturing, the cost of a drilling error goes far beyond a scrapped part. A poorly drilled hole can result in a stripped thread, a leaking fluid connection, or a loose fastener that leads to premature equipment failure. For industries such as automotive, medical devices, and industrial machinery, the consequences can include warranty claims, safety incidents, and production downtime.

This is why dimensional accuracy is such a critical focus for fastener manufacturers and component suppliers. At Kunshan Hong Yong Sheng, every step of our process, from material selection to final inspection, is oriented toward achieving consistent dimensional results. The same discipline applies to the products we supply, whether they are standard fasteners, custom-machined shafts, or injection-moulded parts. Our approach to quality is described in further detail in our article on dimensional accuracy, which explains how we control tolerances throughout the manufacturing process.

For engineers who are designing new assemblies, taking the time to specify the correct drill size on a drawing saves time and money downstream. It eliminates confusion on the shop floor, reduces the need for rework, and ensures that the final product matches the design intent. A drill size chart in mm is one of the simplest and most effective tools for achieving this clarity.

Whether you are drilling a single prototype in a workshop or setting up a mass-production line, the principles are the same: choose the right bit, follow recommended hole sizes, and verify your work. The tables in this guide cover the most common metric thread sizes and standard drill diameters, giving you a practical starting point for most applications. When in doubt, test on a scrap piece first and confirm the fit before committing to production parts. Taking this extra step ensures that the final assembly performs as intended, and that the connection between fastener and workpiece is as strong as the engineering behind it.