Metric Drill Bit Conversion Chart: mm to Inches Quick Reference

Metric Drill Bit Conversion Chart: mm to Inches Quick Reference

To convert metric drill bit sizes to inches, divide the millimeter value by 25.4. A 6mm bit = 0.2362 inches; a 10mm bit = 0.3937 inches. Most shops keep a printed conversion chart handy to quickly match metric bits to the nearest fractional, number, or letter drill equivalent without doing math on every job.

What You’ll Need

• Metric drill bits (or a combined metric/fractional set)
• Calipers to verify bit diameter when precision matters
• This conversion chart (print it out and keep it in your shop)
• Drill or drill press
• Safety glasses

Safety Precautions

• Verify your bit size with calipers for critical work. Some imported metric bit sets are 0.05–0.1mm under nominal diameter. When drilling a tap hole for metric threads, an undersized bit reduces thread engagement.
• Match speed to size and material. Larger metric bits require slower RPM. See the drill press speed chart for guidance.
• Use cutting fluid in metal regardless of bit size or coating.
• Secure your workpiece before drilling — especially with larger metric bits that generate significant torque.

The Conversion Formula

The math is simple:

• mm to inches: Divide by 25.4 — Example: 8mm ÷ 25.4 = 0.3150″
• Inches to mm: Multiply by 25.4 — Example: 3/8″ × 25.4 = 9.525mm

In practice, most metric bits don’t match a fractional size exactly — that’s the whole point of having both systems. Use the chart below to find the nearest equivalent or the closest available size in your bit set.

Metric Drill Bit Conversion Chart: 1mm–13mm

Common Metric Tap Drill Sizes

When drilling tap holes for metric threads, the tap drill size is calculated as: Tap Drill = Thread Diameter − Thread Pitch. Here are the most common metric thread tap drill sizes:

Metric vs Fractional: Which System Is Better?

In the US, fractional inch drill bits are standard for woodworking and general construction. Metric bits are more common in automotive, electronics, and any application where metric fasteners are involved. Many professional shops keep both systems — a full fractional set for general work and a metric set for imported machinery, metric fasteners, and precision metalwork.

For a detailed comparison, see: Fractional vs Metric Drill Bits

Finding the Nearest Available Drill Bit

When you need a specific metric size and don’t have it, here’s how to find the closest substitute:

1. Convert the metric size to inches using the formula (mm ÷ 25.4).
2. Look up the decimal in the conversion chart to find the nearest fractional, number, or letter size.
3. Decide whether to go slightly smaller (under-size) or slightly larger (over-size) based on your application: tap drill holes should be under-size; clearance holes can be over-size.
4. For precision work, always use calipers to verify the actual diameter of the substitute bit before drilling.

Pro Tips

• Print this chart and laminate it. A laminated drill size chart on your workbench saves time on every job involving metric and inch conversions.
• A 13-piece metric set (1mm–13mm in 1mm steps) covers 90% of metric work. Add half-sizes (1.5, 2.5, 3.5mm etc.) for the most complete coverage.
• For automotive work, metric is almost always the right choice. Japanese and European vehicles are almost entirely metric-fastened; using inch bits for tap holes in metric threads will produce undersized or oversized results.
• Drill bit size charts and conversions are available as free phone apps. iPhone and Android users can download machinist reference apps that include full conversion tables, tap drill charts, and speed calculators.

Frequently Asked Questions

What metric drill bit is closest to 1/4 inch?

1/4 inch = 6.35mm. The closest common metric size is 6.5mm (0.256″). If you need a slight undersize, 6mm (0.236″) works. For a very close match, 6.35mm bits are available from machinist supply stores, though not as commonly stocked as 6mm or 6.5mm.

What is the metric equivalent of a 3/8 inch drill bit?

3/8 inch = 9.525mm. The closest standard metric size is 9.5mm (0.374″). For an exact match, 9.5mm is the standard metric size that falls closest to 3/8 inch without exceeding it.

Can I use a metric drill bit in an American drill?

Yes — metric drill bits fit in any standard drill chuck as long as the shank diameter is within the chuck’s capacity. Most metric bits have a round shank compatible with standard chucks. The only difference is the cutting diameter — just verify your target size using the conversion chart.

What is the most common metric drill bit size for general woodworking?

3mm, 4mm, 5mm, 6mm, and 8mm cover the majority of woodworking applications — shelf pin holes, pilot holes, dowel holes, and hardware mounting. A basic 19-piece metric set from 1mm to 10mm handles almost all residential woodworking tasks.

Why are there gaps in metric drill bit sets?

Standard metric sets typically offer whole-number and half-millimeter increments. For very precise sizing between these steps, you’d need to use number drill bits, letter drill bits, or special-order metric bits. This is why machinists keep multiple drill size systems — each fills gaps left by the others.

Conclusion

The metric-to-inch conversion formula (divide by 25.4) and this reference chart are the two tools that solve 99% of metric drill bit conversion questions. Print the chart, keep it in your shop, and you’ll never have to guess which bit is closest to the metric size you need.

Related drill bit guides on Power Tools Today:

• Drill Bit Sizes Explained: Complete Reference
• Number and Letter Drill Bit Sizes Chart
• Fractional vs Metric Drill Bits
• Drill Bit Coating Types Guide
• Drill Press Speed Chart