The Challenge
Metric drill bits and imperial drill bits don't line up. A 6mm bit is not the same as 1/4 inch (6.35mm) — it's 0.35mm smaller, which matters when you're tapping threads or drilling clearance holes for bolts. European tool sets are sized in millimeters; US hardware stores sell fractional inch and wire gauge bits. A DIN 338 metric set goes 1, 1.5, 2, 2.5, 3mm — none of those are exact inch equivalents. The formula is straightforward: inches = mm ÷ 25.4. A 3mm bit is 0.1181 inches — closest fractional size is 1/8 inch (3.175mm), which is 0.175mm larger. For precision work like tapping M4 threads, that difference matters. For rough woodworking, the nearest fractional bit works fine.
Metric Drill Bit Sizes: mm to Inches Conversion Chart
| Metric (mm) | Decimal (in) | Nearest Fractional (in) | Fractional Exact (mm) | Difference (mm) |
|---|---|---|---|---|
| 0.5 mm | 0.0197 in | 1/64 in | 0.397 mm | −0.103 |
| 1.0 mm | 0.0394 in | 3/64 in | 1.191 mm | +0.191 |
| 1.5 mm | 0.0591 in | 1/16 in | 1.588 mm | +0.088 |
| 2.0 mm | 0.0787 in | 5/64 in | 1.984 mm | −0.016 |
| 2.5 mm | 0.0984 in | 3/32 in | 2.381 mm | −0.119 |
| 3.0 mm | 0.1181 in | 1/8 in | 3.175 mm | +0.175 |
| 3.5 mm | 0.1378 in | 9/64 in | 3.572 mm | +0.072 |
| 4.0 mm | 0.1575 in | 5/32 in | 3.969 mm | −0.031 |
| 4.5 mm | 0.1772 in | 11/64 in | 4.366 mm | −0.134 |
| 5.0 mm | 0.1969 in | 13/64 in | 5.159 mm | +0.159 |
| 5.5 mm | 0.2165 in | 7/32 in | 5.556 mm | +0.056 |
| 6.0 mm | 0.2362 in | 15/64 in | 5.953 mm | −0.047 |
| 6.5 mm | 0.2559 in | 17/64 in | 6.747 mm | +0.247 |
| 7.0 mm | 0.2756 in | 9/32 in | 7.144 mm | +0.144 |
| 7.5 mm | 0.2953 in | 19/64 in | 7.541 mm | +0.041 |
| 8.0 mm | 0.3150 in | 5/16 in | 7.938 mm | −0.062 |
| 8.5 mm | 0.3346 in | 21/64 in | 8.334 mm | −0.166 |
| 9.0 mm | 0.3543 in | 23/64 in | 9.128 mm | +0.128 |
| 9.5 mm | 0.3740 in | 3/8 in | 9.525 mm | +0.025 |
| 10.0 mm | 0.3937 in | 25/64 in | 9.922 mm | −0.078 |
| 10.5 mm | 0.4134 in | 27/64 in | 10.716 mm | +0.216 |
| 11.0 mm | 0.4331 in | 7/16 in | 11.113 mm | +0.113 |
| 11.5 mm | 0.4528 in | 29/64 in | 11.509 mm | +0.009 |
| 12.0 mm | 0.4724 in | 15/32 in | 11.906 mm | −0.094 |
| 12.5 mm | 0.4921 in | 31/64 in | 12.303 mm | −0.197 |
| 13.0 mm | 0.5118 in | 33/64 in | 13.097 mm | +0.097 |
Metric Tapping Drill Sizes for Common M-Threads
| Thread | Pitch (mm) | Tapping Drill (mm) | Decimal (in) | Nearest Fractional |
|---|---|---|---|---|
| M2 | 0.40 | 1.6 mm | 0.0630 in | 1/16 in (1.588mm) |
| M2.5 | 0.45 | 2.05 mm | 0.0807 in | 5/64 in (1.984mm) |
| M3 | 0.50 | 2.5 mm | 0.0984 in | 3/32 in (2.381mm) |
| M4 | 0.70 | 3.3 mm | 0.1299 in | 21/64 in — use metric |
| M5 | 0.80 | 4.2 mm | 0.1654 in | 11/64 in (4.366mm) |
| M6 | 1.00 | 5.0 mm | 0.1969 in | 13/64 in (5.159mm) — use metric |
| M8 | 1.25 | 6.8 mm | 0.2677 in | 17/64 in (6.747mm) |
| M10 | 1.50 | 8.5 mm | 0.3346 in | 21/64 in (8.334mm) |
| M12 | 1.75 | 10.2 mm | 0.4016 in | 13/32 in (10.319mm) |
| M16 | 2.00 | 14.0 mm | 0.5512 in | 35/64 in (13.891mm) |
Why Metric and Fractional Drill Bits Rarely Align
Fractional inch drill bits are defined in multiples of 1/64 inch (0.397mm). Metric drill bits step in 0.5mm increments for common sizes. Because 0.5mm and 1/64 inch (0.397mm) don't share a common factor, the two systems only coincide at multiples of 25.4mm — meaning 1/4 inch (6.35mm), 1/2 inch (12.7mm), and 1 inch (25.4mm) are the only clean crossover points in the practical drilling range. Every other metric size falls between two fractional sizes. Wire gauge drill sizes were developed in the US specifically to address this gap, offering finer increments than fractional bits for precision metalwork where the nearest 1/64 inch is still too coarse.
Choosing the Right Substitute When Your Exact Size Is Unavailable
- Calculate the exact inch equivalent: mm ÷ 25.4 = decimal inches
- Identify the nearest fractional bit smaller and larger using the chart above
- For tapping holes: always go smaller — an undersize hole can be enlarged, a stripped thread cannot be fixed
- For clearance holes: go to the next size larger so the fastener passes freely
- For woodworking pilot holes: nearest size in either direction is acceptable — wood compresses slightly
Metric vs Fractional Drill Bits: When Each System Wins
Thread Tapping: Never Substitute a Larger Drill
- M3 tapping drill is 2.5mm — using 3/32 inch (2.381mm) is safe, it's 0.119mm smaller
- Using 1/8 inch (3.175mm) instead of 2.5mm leaves 0.675mm oversize — the tap will cut almost no thread
- On small screws (M2–M4) a 0.1mm oversize tapping drill can cut thread engagement in half
- When in doubt: drill smaller, tap, test fit — you can always enlarge a hole, never shrink it
Step-by-Step Workflow
Enter drill bit diameter in millimeters
Get exact decimal inch equivalent instantly
See nearest fractional inch size and the gap in mm — decide if substitution is acceptable
Specifications
- Formula
- inches = mm ÷ 25.4
- 3mm
- 0.1181 in (nearest: 1/8 in = 3.175mm)
- 5mm
- 0.1969 in (nearest: 13/64 in = 5.159mm)
- 6mm
- 0.2362 in (nearest: 15/64 in = 5.953mm)
- 8mm
- 0.3150 in (nearest: 5/16 in = 7.938mm)
- 10mm
- 0.3937 in (nearest: 25/64 in = 9.922mm)
- 12mm
- 0.4724 in (nearest: 15/32 in = 11.906mm)
Best Practices
- For M-thread tapping: drill size = thread diameter − pitch. M6×1.0 needs a 5.0mm bit — not the 'nearest' 13/64
- Clearance holes need the next size up: M6 bolt clearance = 6.4mm, not 6mm
- Wire gauge drill sizes (like #10 = 2.184mm) fill gaps between fractional inch sizes — useful for precision metalwork
- Pilot holes in hardwood: use 90% of screw shank diameter in mm for clean entry without splitting
- When a metric bit isn't available, go slightly smaller rather than larger for tapping — oversize holes strip threads
Frequently Asked Questions
What is a 6mm drill bit in inches?
6mm ÷ 25.4 = 0.2362 inches. The nearest fractional size is 15/64 inch (5.953mm), which is 0.047mm smaller than 6mm. For clearance holes this substitution is usually fine. For metric thread tapping or press-fit work, use the actual 6mm bit — the fractional substitute introduces real dimensional error.
What is a 3mm drill bit in inches?
3mm = 0.1181 inches. Nearest fractional equivalent is 1/8 inch (3.175mm), which is 0.175mm larger. That gap is significant for tapping M3 threads (which need a 2.5mm pilot hole) but fine for most woodworking pilot holes.
What is a 10mm drill bit in inches?
10mm = 0.3937 inches. The nearest fractional size is 25/64 inch (9.922mm) — 0.078mm under 10mm. The nearest over is 13/32 inch (10.319mm). For bolt clearance holes where a little slop is acceptable, 25/64 works. For anything requiring a true 10mm hole, use metric.
Is there an exact mm to fractional inch drill bit match?
Very few exact matches exist. 1/4 inch = 6.35mm, 1/2 inch = 12.7mm, 1 inch = 25.4mm — these are exact. Most other fractional sizes fall between metric increments. Wire gauge and letter-size drill bits were developed specifically to fill these gaps for precision metalworking.
What drill bit do I need for M6 threads?
M6×1.0 thread requires a 5.0mm tapping drill. In fractional inches that's 0.1969 inches — closest is 13/64 inch (5.159mm), which is 0.159mm too large and will produce weak or loose threads. Use the 5.0mm metric bit. For M6 clearance (bolt passes through freely), use 6.4mm or 17/64 inch (6.747mm).
How do wire gauge drill sizes relate to mm?
Wire gauge sizes (numbered 1–80) are an American system that fills fractional inch gaps. #80 is the smallest at 0.343mm (0.0135in). #60 = 1.016mm, #40 = 2.184mm, #30 = 3.264mm, #10 = 2.184mm... wait — #10 = 2.184mm, #1 = 5.791mm. They're used in metalworking and electronics where fractional bits are too coarse.
Can I use a slightly larger drill bit instead of the exact metric size?
Depends on the application. For rough woodworking pilot holes: yes, nearest size is fine. For tapping threads: no — even 0.1mm oversize will reduce thread engagement and weaken the joint. For clearance holes (bolt passing through): go slightly larger, not smaller, to ensure the fastener passes freely. For press-fit or interference fits: exact size is mandatory.