A 10mm drill bit is a metric tool, but many drill presses, chucks, and project plans still speak in inches. If you are holding a 10mm bit and need the inch equivalent, the conversion is straightforward: 10mm equals approximately 0.3937 inches. The practical question is not just the decimal answer, but which fractional drill bit comes closest and when that substitute is safe to use.
The difference between a 10mm bit and its nearest inch sizes can decide whether a dowel slides in smoothly or a bolt binds in metal. A 10mm drill bit is slightly larger than 3/8 inch and slightly smaller than 13/32 inch. The closest common fractional size is 25/64 inch, though it is not an exact match. Understanding the tolerances behind those numbers helps you choose the right bit without guesswork.
Understanding the Exact 10mm to Inches Conversion
The exact conversion relies on the fixed relationship between millimeters and inches: one inch equals exactly 25.4 millimeters. To convert 10mm to inches, divide 10 by 25.4. The result is 0.3937007874 inches, which is normally rounded to 0.3937 inches. That decimal is the true inch equivalent of a 10mm drill bit, regardless of what the label on the bit says.
When you move from decimal inches to fractions, the number becomes less precise because fractional drill bits only come in specific increments. Multiplying 0.3937 by 64 gives about 25.2/64, so the nearest 64th is 25/64 inch. That bit measures 0.390625 inches, or 9.92mm. In other words, a 25/64 inch drill bit is only about 0.0031 inches smaller than a true 10mm bit, a difference thinner than a sheet of paper.
Decimal Inches and Fractional Equivalents
In decimal form, 10mm is 0.3937 inches. If you compare it to common fractional sizes, it sits between 3/8 inch (0.3750 inches) and 13/32 inch (0.40625 inches). It is closer to 13/32 than to 3/8 in a 32nd-inch set, but in a 64th-inch set the closest match is 25/64 inch. Letter-sized drill bits also offer an option: a letter X bit measures 0.3970 inches, which is slightly larger than 10mm but very close.
For everyday work, many machinists and woodworkers treat 25/64 inch as the go-to substitute for 10mm. If the hole must pass a 10mm shaft with a little extra room, a 13/32 inch bit may be the better choice because it is 0.0126 inches larger than 10mm. That extra clearance is about 0.32mm, which can be helpful in painted or slightly uneven materials but too loose for precision fits.
When to Use a 25/64 vs 13/32 Inch Bit
Use 25/64 inch when you need a tight fit and the material can tolerate a hole slightly under 10mm. This is common for dowels, pins, and situations where you can enlarge the hole with a round file or reamer if needed. Because 25/64 is only 0.0031 inches smaller than 10mm, it often works well in wood, plastic, and soft metals where drill bits tend to cut slightly oversize anyway.
Choose 13/32 inch when clearance matters more than exact size. If you are drilling a hole for a 10mm bolt, cable, or rod that needs to slide freely, the larger bit reduces the risk of binding. The trade-off is a visibly looser fit. For metalworking or mechanical assemblies, check the tolerance requirements first; a 13/32 inch hole may be too loose if the 10mm shaft is meant to be press-fit or precisely located.
Practical Drill Bit Conversion and Selection
Converting 10mm to inches is only half the job. You also need to consider chuck size, material, and the type of hole. A standard 10mm drill bit usually has a 10mm shank, which will not fit a 3/8 inch chuck because 3/8 inch equals 9.525mm. You need a 1/2 inch chuck, a reduced-shank bit, or a metric chuck that can handle the 10mm shank. This is a common surprise for anyone mixing metric bits with imperial tools.
Material behavior also matters. A twist drill can wander, especially in hand drilling, so the finished hole may be larger than the bit's nominal size. Hard metals demand the correct speed, feed, and cutting fluid, while wood may benefit from a brad-point bit for clean edges. If the hole is critical, drill a test hole in scrap and measure it with calipers before committing to the final workpiece.
Conversion Chart for Common Metric and Imperial Drill Bits
The following chart gives quick reference values for 10mm and nearby imperial sizes. It is useful when you are standing at a drill press with a fractional set and need the closest match. Keep in mind that drill bit manufacturing tolerances can vary, so the decimal values are nominal.
For a 10mm drill bit, the exact inch value is 0.3937. The closest common fractional bit is 25/64 inch, while 13/32 inch provides more clearance. Letter X is another close option if your set includes letter drills. If you need the hole to be truly 10mm, use a 10mm bit or ream the hole after drilling.
| Drill Bit Size | Decimal Inches | Millimeters | Relationship to 10mm |
|---|---|---|---|
| 25/64" | 0.3906" | 9.92mm | Closest common fractional; 0.078mm smaller |
| 10mm | 0.3937" | 10.00mm | Exact metric size |
| Letter X | 0.3970" | 10.08mm | Slightly larger; close imperial alternative |
| 13/32" | 0.4062" | 10.32mm | Larger clearance substitute |
| 3/8" | 0.3750" | 9.53mm | Noticeably smaller |
Tips for Choosing Between Metric and Imperial Drill Bits
Start with the fastener or shaft that will go into the hole. If it is a 10mm dowel, a 10mm bit is ideal, but a 25/64 inch bit may work if the dowel can be sanded or compressed slightly. If it is a 10mm bolt, remember that bolts need clearance, so a 10mm hole is usually too tight. In that case, step up to a 10.5mm, 11mm, or 13/32 inch bit depending on the fit you need.
Also check your drill's chuck capacity, bit coating, and point angle. A 135-degree split-point bit is better for hard metal, while a brad-point bit is better for wood. Use lower speeds for metal and higher speeds for wood, and always secure the workpiece. When in doubt, drill a pilot hole first and enlarge gradually; this gives you more control than jumping straight to a 10mm or 13/32 inch bit.
A 10mm drill bit converts to 0.3937 inches, and 25/64 inch is the closest common fractional substitute. If you need a little more room, 13/32 inch or a letter X bit can bridge the gap. Keep a conversion chart near your drill press, measure critical holes with calipers, and let the fit you need—not just the label—guide your final choice.