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Convert any drill bit size
Type a size in any one format — a fraction like 3/8, a decimal like 0.375, or millimeters like 9.525 — and get the other two plus the nearest standard fraction.
FREE REFERENCE CHART · CONSTRUCTION
Drill Bit Size Chart: Fraction to Decimal to Millimeters
Every fractional drill bit size from 1/64″ to 1″ in decimal inches and millimeters — a printable chart, a live converter, and the clearance-hole rule. On the default — 1/4″ — the bit is 0.2500 in, or 6.35 mm.
How to read a drill bit size
A bit stamped 3/8 drills a hole three-eighths of an inch across — the fraction is the diameter in inches, always reduced, so you will see 1/8 rather than 4/32. A bit stamped 6.5 is metric: the number is the diameter in millimeters, no conversion needed. The decimal column is the universal translator: every size compares cleanly in decimals, which is why machinists think in thousandths. The chart below lists every 64th from 1/64″ to 1/2″ plus the common larger fractions, computed exactly (decimal = fraction, millimeters = inches × 25.4).
Fractional drill bit size chart
| Fraction | Decimal (in) | Millimeters |
|---|---|---|
| 1/64″ | 0.0156 | 0.3969 |
| 1/32″ | 0.0312 | 0.7937 |
| 3/64″ | 0.0469 | 1.1906 |
| 1/16″ | 0.0625 | 1.5875 |
| 5/64″ | 0.0781 | 1.9844 |
| 3/32″ | 0.0938 | 2.3812 |
| 7/64″ | 0.1094 | 2.7781 |
| 1/8″ | 0.1250 | 3.1750 |
| 9/64″ | 0.1406 | 3.5719 |
| 5/32″ | 0.1562 | 3.9688 |
| 11/64″ | 0.1719 | 4.3656 |
| 3/16″ | 0.1875 | 4.7625 |
| 13/64″ | 0.2031 | 5.1594 |
| 7/32″ | 0.2188 | 5.5562 |
| 15/64″ | 0.2344 | 5.9531 |
| 1/4″ | 0.2500 | 6.3500 |
| 17/64″ | 0.2656 | 6.7469 |
| 9/32″ | 0.2812 | 7.1437 |
| 19/64″ | 0.2969 | 7.5406 |
| 5/16″ | 0.3125 | 7.9375 |
| 21/64″ | 0.3281 | 8.3344 |
| 11/32″ | 0.3438 | 8.7312 |
| 23/64″ | 0.3594 | 9.1281 |
| 3/8″ | 0.3750 | 9.5250 |
| 25/64″ | 0.3906 | 9.9219 |
| 13/32″ | 0.4062 | 10.3187 |
| 27/64″ | 0.4219 | 10.7156 |
| 7/16″ | 0.4375 | 11.1125 |
| 29/64″ | 0.4531 | 11.5094 |
| 15/32″ | 0.4688 | 11.9062 |
| 31/64″ | 0.4844 | 12.3031 |
| 1/2″ | 0.5000 | 12.7000 |
| 9/16″ | 0.5625 | 14.2875 |
| 5/8″ | 0.6250 | 15.8750 |
| 3/4″ | 0.7500 | 19.0500 |
| 7/8″ | 0.8750 | 22.2250 |
| 1″ | 1.0000 | 25.4000 |
Tip: use your browser's print function for a clean one-page chart — the table is styled to print without the site chrome.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Decimal inches | numerator ÷ denominator | 3/8 → 0.375 in. Exact for fractions; this is the value machinists compare. |
| Millimeters | inches × 25.4 | 0.375 in → 9.525 mm. Exact by definition of the inch. |
| Nearest standard fraction | round(decimal × 64) / 64, reduced | The closest 64th to a decimal or metric entry — handy when the store only stocks fractions. |
The clearance-hole rule
The bit should be slightly larger than the fastener when the fastener must pass through freely — pick the next standard size up from the fastener's diameter. Common pairings: a #6 screw (0.138″) takes a 9/64″ bit, a #8 screw (0.164″) takes 11/64″, a #10 screw (0.190″) takes 13/64″, a 1/4″ bolt takes 17/64″, a 5/16″ bolt takes 21/64″, and a 3/8″ bolt takes 25/64″. For threaded holes you tap yourself, use a proper tap-drill table instead — clearance sizes make weak threads.
Who uses this
- DIYers staring at a bit stamped 0.3438 and a plan that calls for 11/32″ — the converter confirms they are the same bit.
- Woodworkers picking pilot and clearance holes for screws, where one size up or down decides whether the joint holds or splits.
- Anyone mixing metric and imperial hardware — a 6.5 mm bit is 0.2559″, just over 1/4″, close enough for a clearance hole but not for a press fit.
- Students and apprentices learning to read the fractional, decimal, and metric markings stamped on bit shanks.
Accuracy & limitations
The conversions on this page are mathematically exact — the inch is defined as 25.4 mm. Real drill bits have manufacturing tolerances, and a worn or resharpened bit drills slightly oversize, so treat the fourth decimal as information, not a promise. Numbered and letter-gauge drill sizes (like #7 or F) follow their own historical tables and are not covered here; fractional, decimal, and metric cover the hardware-store sets.
Cite this reference chartBright Side Kit. “Drill Bit Size Chart: Fraction to Decimal to Millimeters.” brightoolbox.com, 2026.
Bright Side Kit. "Drill Bit Size Chart: Fraction to Decimal to Millimeters." https://brightoolbox.com/drill-bit-size-chart/ Accessed October 22, 2026.Frequently asked questions
What size is a 1/4 inch drill bit in mm?
Exactly 6.35 mm (0.25 × 25.4). A 6.5 mm bit is the closest common metric size, at 0.2559 inches — fine for clearance holes, not for precision fits.
What drill bit do I use for a 1/4 inch bolt?
A 17/64″ bit (0.2656″) is the standard clearance size — slightly larger than the 0.25″ bolt so it passes through freely. A 9/32″ bit (0.2812″) also works where the fit can be looser.
How do I read the size stamped on a drill bit?
A fraction like 3/8 is the diameter in inches. A plain decimal like 0.375 is the diameter in decimal inches. A number like 6.5 with no fraction is millimeters. Fractional sets progress in 1/64″ steps; the chart above translates between all three.
What is the decimal equivalent of 3/8 inch?
0.375 inches, or 9.525 mm. In general, divide the numerator by the denominator (3 ÷ 8 = 0.375), then multiply by 25.4 for millimeters.
Do I need an exact size match between metric and imperial bits?
For clearance holes in wood or plastic, the closest size is fine — a 10 mm bit (0.3937″) works where a 3/8″ (0.375″) hole was planned. For threaded holes, press fits, or metalwork, convert exactly and pick the right size; a few thousandths matter there.
Why are some drill bit sizes skipped in sets?
Cheap sets stock the common fractions and skip the odd 64ths most people never reach for. The chart lists every 64th so you can identify the odd sizes when a plan or an old bit calls for one.