When it comes to working with drills, precision matters. Whether you’re a professional machinist, a carpenter, an engineer, or a weekend DIY enthusiast, choosing the correct drill bit size is essential. Using the wrong drill size can ruin a project, damage materials, or create unsafe conditions. That’s why a drill size chart is one of the most valuable references you can have in your workshop.
In this guide, we’ll cover what a drill size chart is, why it’s important, different drill size systems (imperial, metric, wire gauge, letter sizes), and include a comprehensive chart you can use anytime.
What Is a Drill Size Chart?
A drill size chart is a reference table that lists the standard sizes of drill bits and their corresponding measurements. It helps you:
Match a drill bit to the correct screw size.
Avoid errors in drilling holes too big or too small.
Work with both metric (mm) and imperial (inch) systems.
Understand fractional, number, and letter drill bit designations.
Why Is Drill Size Important?
Choosing the right drill size impacts:
Accuracy – The hole should fit the fastener snugly.
Strength – Too large a hole weakens the grip of screws or bolts.
Material Protection – Prevents cracking, splitting, or excess wear.
Safety – Reduces chances of drill slips or broken bits.
For example, if you are drilling a pilot hole for a wood screw, using the correct bit prevents the wood from splitting while ensuring the screw sits tightly.
Different Drill Bit Size Systems
Drill bits are classified in several systems depending on country and application.
1. Fractional Inch Sizes (Imperial)
Common in the US.
Measured in inches (e.g., 1/16”, 3/8”, 1/2”).
Useful in woodworking, metalworking, and construction.
2. Metric Sizes
Standard in most of the world.
Measured in millimeters (e.g., 3mm, 10mm, 12.5mm).
Increasingly common in engineering and manufacturing.
3. Numbered Drill Sizes (Wire Gauge)
Ranges from very small (#80 = 0.0135 in) to larger sizes (#1 = 0.228 in).
Widely used in precision engineering and electronics.
4. Letter Drill Sizes
Uses letters A to Z (A = 0.234 in, Z = 0.413 in).
Fills gaps between fractional and numbered sizes.
Standard Drill Size Chart (Fractional, Metric, Number & Letter)
Here’s a reference drill size chart combining the most used systems:
| Fraction (inch) | Decimal (inch) | Metric (mm) | Number Size | Letter Size |
|---|---|---|---|---|
| 1/16 | 0.0625 | 1.59 | #52 (0.0635) | – |
| 5/64 | 0.0781 | 1.98 | #47 (0.0785) | – |
| 3/32 | 0.0938 | 2.38 | #42 (0.0935) | – |
| 7/64 | 0.1094 | 2.78 | #36 (0.1065) | – |
| 1/8 | 0.1250 | 3.18 | #30 (0.1285) | – |
| 9/64 | 0.1406 | 3.57 | #28 (0.1405) | – |
| 5/32 | 0.1563 | 3.97 | #25 (0.1495) | – |
| 11/64 | 0.1719 | 4.37 | #21 (0.1590) | – |
| 3/16 | 0.1875 | 4.76 | #12 (0.1890) | – |
| 7/32 | 0.2188 | 5.56 | #1 (0.2280) | A (0.2340) |
| 1/4 | 0.2500 | 6.35 | – | C (0.2420) |
| 5/16 | 0.3125 | 7.94 | – | O (0.3160) |
| 3/8 | 0.3750 | 9.53 | – | U (0.3680) |
| 7/16 | 0.4375 | 11.11 | – | – |
| 1/2 | 0.5000 | 12.70 | – | – |
| 9/16 | 0.5625 | 14.29 | – | – |
| 5/8 | 0.6250 | 15.88 | – | – |
| 3/4 | 0.7500 | 19.05 | – | – |
| 1 | 1.0000 | 25.40 | – | – |
👉 This chart shows equivalents between different systems so you can cross-reference quickly.
Also Read:
How to Use the Drill Size Chart
Identify your screw/bolt size – For example, a #10 screw requires a 3/16” pilot hole.
Check the material – Softer materials (wood, plastic) may need slightly smaller bits than harder materials (steel).
Cross-reference systems – If you only have metric bits but the blueprint specifies imperial, use the chart to find the closest equivalent.
Drill Size for Pilot Holes
Pilot holes are smaller guide holes that make driving screws easier. General rule:
Softwood → drill bit = screw’s minor diameter.
Hardwood → drill bit = screw’s full diameter.
Example: For a #8 wood screw (0.164” diameter), use a 7/64” bit in softwood and a 1/8” bit in hardwood.
Drill Size for Tapping (Thread Cutting)
When creating threads with a tap, using the correct drill size ensures proper thread depth.
Formula:
Tap Drill Size=Major Diameter–(1/Pitch)Tap Drill Size = Major Diameter – (1 / Pitch)
Example: For M6 × 1.0 tap → 6 – 1 = 5 mm drill.
Tips for Accurate Drilling
Mark the hole with a center punch to prevent bit wandering.
Use the right speed – smaller bits = higher RPM, larger bits = slower RPM.
Lubricate metal drilling with cutting oil to reduce friction and heat.
Clamp materials securely to avoid movement.
Check sharpness – dull bits cause oversized holes.
Common Drill Bit Materials
High-Speed Steel (HSS) – General purpose, affordable.
Cobalt – Handles hard metals like stainless steel.
Carbide – Extremely hard, used in precision industries.
Diamond-coated – For glass, ceramic, and stone.
Safety Precautions
Always wear safety goggles.
Never hold material with your hands while drilling.
Remove chips frequently to avoid clogging.
Use appropriate drill speed and pressure.
Fun Fact About Drill Sizes
The smallest commercial drill bit is about 0.002 inches (50 microns), thinner than a human hair.
NASA engineers use specialized micro-bits to drill circuit boards for spacecraft electronics.
Final Thoughts
A drill size chart is more than just a table—it’s a practical tool that ensures accuracy, safety, and efficiency in every drilling task. Whether you’re working in metal, wood, or plastic, referencing the chart helps you avoid costly mistakes.
Keep this guide handy in your workshop or save a printable version for quick access. Remember: measure twice, drill once!