Fractional vs Metric Drill Bits: How to Choose and Convert Between Systems

Most US hardware stores sell drill bits in fractional inch sizes (1/4″, 3/8″, 1/2″), but imported hardware, metric fasteners, and European equipment use millimeter sizes (6mm, 8mm, 10mm). Knowing which system to use — and how to convert when you need a close match — saves trips to the store and prevents the frustration of drilling a hole that’s slightly wrong. This guide covers both systems and gives you a practical conversion reference to use on the job.

Understanding Drill Bit Sizing Systems

There are actually four drill bit sizing systems used in the US:

• Fractional inch: The most common in US hardware stores — 1/16″, 1/8″, 3/16″, 1/4″, 5/16″, 3/8″, 7/16″, 1/2″, and so on. Sets typically go in 1/64″ increments for precision.
• Metric (mm): Used for metric fasteners (M4, M6, M8 bolts), European appliances, automotive work, and any hardware specified in millimeters. 3mm, 4mm, 5mm, 6mm, 8mm, 10mm, 12mm are common.
• Wire gauge / number drill: A numbered system (#1–#80) used in machining, primarily for small precision drilling and tapping pilot holes. #60 = 0.040 inch, #30 = 0.1285 inch.
• Letter drill: An alphabetical system (A–Z) used for precision machining, filling size gaps between fractional and wire gauge sizes. Letter drills range from A (0.234″) to Z (0.413″).

For most DIY and home construction work in the US, you’ll only encounter fractional and metric. Wire gauge and letter drills are primarily machinist tools.

Safety Precautions

• Always verify bit diameter before drilling for tapping or anchoring. A hole 0.5mm too large can cause an anchor to spin freely or a tapped thread to strip. When precise fit matters, test on scrap material first.
• Wear safety glasses regardless of bit type or size.
• When in doubt between two close sizes, choose the smaller one — you can always enlarge a hole but you can’t shrink it.

Fractional to Metric Conversion Chart

Common Metric Fastener Pilot Hole Sizes

If you’re drilling pilot holes for metric screws or tapping holes for metric threads, use these standard sizes:

When to Use Fractional Bits

• All standard US construction work — framing, decking, drywall, cabinetry
• US-standard hardware: plastic expansion anchors, lag bolts, wood screws
• Any application where the instruction or spec sheet lists the hole size in inches
• Matching hole saws, Forstner bits, and other large boring tools (almost all sold in fractional sizes in the US)

When to Use Metric Bits

• European-made appliances, IKEA furniture, and imported hardware with metric bolt specifications
• Automotive work — nearly all modern vehicles use metric hardware
• Machinery and industrial equipment (most modern machinery uses metric fasteners)
• Any application where instructions specify a hole size in millimeters
• Tapping holes for metric threads (M4, M6, M8, etc.)

What If You Don’t Have the Exact Size?

When you need a metric size and only have fractional bits (or vice versa), look up the decimal equivalent of both and find the closest available size. For most non-precision applications (anchor holes, cable pass-throughs, rough framing), 0.5mm or 1/64″ difference is irrelevant. For precision applications (tapping threads, fitting dowels, press-fit tolerances), get the exact size.

Formula: inches × 25.4 = millimeters. Millimeters ÷ 25.4 = inches. A calculator and this formula converts any size instantly.

Pro Tips

• Buy a combination fractional/metric set — most major brands sell indexed sets that include both systems in a single case. This covers 90% of US home project needs without hunting for specific sizes.
• Check bit markings before use. Some international bit brands stamp sizes in mm even when the bit is nominally near a fractional size — verify before drilling a critical hole.
• For anchor holes specifically, the anchor package always lists the drill bit size in both fractional and metric — check there first before consulting a conversion chart. See our guide on drill bit sizes for wall anchors for a complete anchor sizing reference.
• A set of number drills (#1–#60) fills the precision size gaps between standard fractional and metric sets — worth adding if you do any metalwork or tapping.

Frequently Asked Questions

Are metric and fractional drill bits interchangeable?

They fit the same 3/8″ or 1/2″ drill chuck (assuming standard shank sizes), but the cutting diameters are different measurement systems. Use the conversion chart above to find the closest equivalent when you need to substitute. For non-critical applications, the closest available size usually works fine.

What is the metric equivalent of 1/4 inch?

1/4″ = 0.250″ = 6.35mm. The closest standard metric drill bit is 6mm (slightly smaller) or 6.5mm (slightly larger). For most applications, 6mm is used when a slightly snug fit is needed and 6.5mm when a loose clearance fit is preferred.

Which system should I use for IKEA furniture?

IKEA uses metric hardware throughout. When drilling for IKEA connector bolts, cam locks, and shelf pins, use metric bits. IKEA instruction booklets specify the exact sizes needed in millimeters — follow those exactly for correct fit.

Why do US drill bit sets have letter and number sizes?

Letter (A–Z) and wire gauge (1–80) systems come from precision machining — they fill size gaps between standard fractional sizes that are needed when drilling pilot holes for specific tap sizes. For general home DIY, you rarely need letter or number drills unless you’re tapping metric threads where exact tap drill sizes matter. See our complete drill bit sizes guide for the full system breakdown.

Can I use fractional bits for metric fasteners?

Yes, by using a close fractional equivalent. For non-critical holes (clearance holes, cable runs, rough anchoring), a fractional bit within 0.5mm of the metric requirement works fine. For precision applications like tapping threads, use the exact metric size — the thread quality depends on being within 0.1–0.2mm of the correct diameter.

Conclusion

For US home projects: fractional inch bits cover 80% of your work. Keep a metric set for imported hardware, automotive work, and appliance installations. The conversion formula (inches × 25.4 = mm) handles any situation where you need to cross between systems. And when in doubt about which size fits a specific anchor or fastener, check the hardware package — it almost always lists both systems.

Related guides on Power Tools Today:

• Drill Bit Sizes Explained
• What Size Drill Bit for Wall Anchor
• How to Store Drill Bits Properly
• Drill Bit Wobble Causes and Fix
• Spade Bit vs Forstner Bit Guide