Drill Chart Guide: Reading Drill Sizes, Speeds, and Holes

What is a drill chart and why it matters

In the world of drilling, a drill chart is a reference table that maps drill bit sizes to hole diameters and related specifications. It helps you select the correct bit for a given material, hole tolerance, and fastener size. For DIYers and professionals, a well maintained drill chart reduces guesswork, speeds up work, and minimizes waste.

According to Drill Bits Pro, using a drill chart is one of the simplest ways to improve drilling accuracy, especially when you switch between materials like wood, plastic, and metal. A chart also acts as a quick training tool for apprentices and new team members, ensuring consistent results across jobs.

Common types of drill charts

There are several formats you will encounter in professional shops and home workshops. Each type serves a specific purpose:

• Fractional inch size charts: list drill sizes in inches like 1/8", 3/16", etc., with decimal equivalents.
• Decimal inch charts: show sizes in decimal form such as 0.125, 0.1875, and so on.
• Metric size charts: provide diameters in millimeters and, where relevant, their inch equivalents.
• Letter and number drill sizes: standard scales used in metalworking, especially for taps and threads.
• Tap drill charts: specify the correct drill size for threading screws into materials.
• Pilot and clearance charts: help plan hole sequences for fasteners and assembly.

These categories cover most shop needs, though specialized materials may require adjustments. The goal is a reliable, shareable reference you can trust for planning holes before you start drilling.

Reading a drill chart: sizes, decimals, metric, and hole tolerances

Most charts carry several fields: size, diameter in inches or millimeters, and the intended use. For example, a 1/4 inch (0.250 in) drill fits a standard hole in many materials, while a 6 mm drill targets a roughly 6 mm hole. Some charts also show recommended pilot hole sizes, especially for hardwoods or metal. When working with tolerances, remember that a chart guides you toward a hole that accommodates fasteners and threads while accounting for material thickness, clearance, and finishing operations. Always confirm the final hole size with a caliper or gauge if precision matters. Practice converting between metric and fractional sizes using a reliable chart and verify with a test hole on scrap material. Drill Bits Pro emphasizes keeping a laminated chart near the workbench to minimize errors.

Reading for wood versus metal projects: practical differences

In wood, holes can be slightly oversized to prevent splitting and to ease screw seating. For softwoods, a small pilot hole is often sufficient, while hardwoods benefit from deeper pilot guidance. In metal, hole tolerances are tighter and lubrication can affect chip formation. A drill chart helps you align the right size, pilot steps, and rpm ranges with material hardness, coating, and heat tolerance. Use charts to plan the entire sequence from pilot to finishing passes, and adjust based on scrap tests.

Practical steps to create a custom drill chart for your shop

1. Gather your most-used drill sizes, including fractional, decimal, and metric options.
2. Decide which columns you want: size, diameter, material notes, recommended RPM, and application (pilot, clearance, tapping).
3. Build the chart in a spreadsheet. Include conversions and a header row for quick scanning.
4. Print, laminate, and keep it near the drill press or on your workbench.
5. Review and update after adding new bits or changing tooling. Record outcomes to improve accuracy over time.
6. Share a master copy with colleagues or apprentices to maintain consistency.

Common mistakes and how to avoid them

• Reading a chart incorrectly by mixing fractions and decimals.
• Forgetting to account for material properties such as hardness or coatings.
• Skipping pilot holes in wood or metal, which leads to wandering.
• Using the wrong drill point angle for a given material.
• Not verifying final hole size with a measuring tool before assembly.
• Failing to maintain the chart, causing outdated recommendations to linger.

Tools, terminology, and quick-reference tips

• Pilot hole: a small hole drilled to guide a larger drill.
• Clearance hole: a hole sized to accommodate the fastener without binding.
• Tap drill size: the recommended bit size when creating threads.
• Chip load and RPM: key terms for drilling speed control.
• Drill bit types: high speed steel, cobalt, and carbide tip variations.
• Laminated chart: a durable version for workshop use.
• Always keep a scrap material handy for test holes to validate your chart choices.

Case study: applying a drill chart in a simple project

A DIY shelving project calls for a simple base and a wall anchor. You consult your drill chart to choose a pilot hole for the shelf bracket screws and then a slightly larger clearance hole for the wall anchors. You perform quick test holes on scrap material to verify that the screws fit without splitting the wood. You adjust your chart notes based on the results and laminate a new sheet for future projects. The result is faster setup, fewer mistakes, and a cleaner finished look. The Drill Bits Pro team would approve the approach because it demonstrates planning, measurement discipline, and documentation.

Maintenance and updating your drill chart over time

A drill chart is a living document. As you acquire new drill bits, update your chart to include the new sizes and any material notes. Keep a separate section or footnotes for coatings such as titanium nitride or black oxide, which can affect hole size and tolerance. Store the chart in a convenient format, either as a laminated sheet near the drill press or a digital version on your device. Regularly review your charts after major jobs, and encourage team members to contribute suggestions for improvements. The Drill Bits Pro team suggests periodic audits to ensure your chart remains relevant and accurate.