Choosing the correct drill bit size is essential for drilling accurate holes, creating proper clearances, preparing pilot holes, and achieving a reliable fit for fasteners and other components. Drill bits are commonly labeled using different measurement systems, including decimal inches, fractional inches, and metric millimeters. Because these systems do not always use the same increments, converting between them manually can be time-consuming and prone to rounding errors.
Drill Bit Conversion Calculator
What Is a Drill Bit Conversion Calculator?
A Drill Bit Conversion Calculator is a measurement tool used to convert drill bit diameters between different units. The calculator in this tool supports four input formats:
- Inches
- Millimeters
- Centimeters
- Fractional inches
The result can be displayed in inches, millimeters, centimeters, or fractional inches. In addition, the calculator reports the decimal-inch equivalent, metric measurements, and nearest 1/64-inch size.
For example, if a drill bit measures 1/4 inch, its metric equivalent is:
1/4 inch = 0.25 inch = 6.35 mm = 0.635 cm
Similarly, a 10 mm drill bit is approximately:
10 mm = 0.3937 inch
The ability to view the same drill bit size in several measurement systems can make it easier to select replacement bits or match an existing hole size.
Why Drill Bit Size Conversion Is Important
Drill bits are manufactured in many different sizes. Depending on the tool set, country, application, or manufacturer, a bit may be marked in metric or imperial units.
For example, one drill set might contain:
- 3 mm
- 4 mm
- 5 mm
- 6 mm
- 8 mm
- 10 mm
Another set might use:
- 1/8″
- 5/32″
- 3/16″
- 1/4″
- 5/16″
- 3/8″
These measurements describe the diameter of the drill bit, but they use different systems. A conversion calculator helps you understand how the sizes relate to one another.
This is particularly helpful when a project specification calls for one unit while your available drill bits are labeled using another.
How to Use the Drill Bit Conversion Calculator
Using this calculator is straightforward.
Step 1: Enter the Drill Bit Size
Enter the numerical size of your drill bit into the Drill Bit Size field.
For example, if you have a 10 mm drill bit, enter:
10
If you are working with a decimal-inch measurement, such as 0.25 inches, enter:
0.25
Step 2: Select the Starting Unit
Choose the unit represented by your input.
The calculator supports:
- Inches
- Millimeters
- Centimeters
- Fractional Inch
If you select inches, the number you enter is treated as a decimal-inch measurement.
For example:
0.375 inches
Step 3: Select a Fractional Size When Needed
If you select Fractional Inch, the calculator provides a list of common fractional sizes ranging from 1/64 inch through 1/2 inch.
You can select a fraction such as:
- 1/8″
- 3/16″
- 1/4″
- 5/16″
- 3/8″
- 7/16″
- 1/2″
The calculator then uses the corresponding decimal-inch value for conversion.
Step 4: Select the Conversion Unit
Choose the unit you want to convert to.
Available options include:
- Millimeters
- Inches
- Centimeters
- Fractional Inch
Step 5: Calculate
Click Calculate to display the results.
The calculator provides the converted size as well as several additional measurements.
Drill Bit Conversion Formulas
Understanding the formulas behind drill bit conversion can help you verify measurements and understand how different units relate to each other.
Inches to Millimeters
The fundamental conversion is:
1 inch = 25.4 millimeters
Therefore:
Millimeters = Inches × 25.4
Example
For a 3/8-inch drill bit:
3/8 = 0.375 inch
Then:
0.375 × 25.4 = 9.525 mm
So a 3/8-inch drill bit has a diameter of:
9.525 mm
Millimeters to Inches
To convert millimeters into inches:
Inches = Millimeters ÷ 25.4
Example
Suppose you have a 12 mm drill bit.
12 ÷ 25.4 = 0.47244 inches
Therefore, a 12 mm drill bit is approximately:
0.4724 inch
This can also be compared with common fractional sizes.
Inches to Centimeters
Since:
1 inch = 2.54 centimeters
the formula is:
Centimeters = Inches × 2.54
For example:
0.5 × 2.54 = 1.27 cm
Therefore, a 1/2-inch drill bit has a diameter of:
1.27 cm
Centimeters to Inches
To convert centimeters to inches:
Inches = Centimeters ÷ 2.54
For example:
2.54 ÷ 2.54 = 1 inch
So a drill bit measuring 2.54 cm in diameter is exactly 1 inch.
Fractional Inch to Decimal Inch
Fractional drill bit sizes can be converted to decimal inches by dividing the numerator by the denominator.
The formula is:
Decimal Inches = Numerator ÷ Denominator
For example:
1/4 = 1 ÷ 4 = 0.25 inch
Another example:
5/16 = 5 ÷ 16 = 0.3125 inch
And:
7/32 = 7 ÷ 32 = 0.21875 inch
Once the fractional size has been converted into decimal inches, it can be converted into millimeters using the 25.4 conversion factor.
Nearest 1/64-Inch Conversion
The calculator also identifies the nearest fractional-inch size using increments of 1/64 inch.
The basic process is:
Nearest 1/64 = Round(Decimal Inches × 64) ÷ 64
For example, suppose a drill bit is approximately:
0.3937 inch
Multiply by 64:
0.3937 × 64 ≈ 25.2
Rounding gives:
25/64
Therefore, the nearest 1/64-inch size is approximately:
25/64″
This is an approximation. It does not mean the original drill bit is exactly 25/64 inch.
Drill Bit Conversion Example
Suppose you have a 10 mm drill bit and want to understand its equivalent measurements.
Step 1: Convert millimeters to inches
Use:
Inches = 10 ÷ 25.4
The result is:
0.3937008 inches
Rounded to a practical number of decimal places:
0.3937 inch
Step 2: Convert to centimeters
Use:
Centimeters = 10 ÷ 10
Therefore:
10 mm = 1 cm
Step 3: Find the nearest 1/64-inch size
Multiply the decimal-inch value by 64:
0.3937 × 64 ≈ 25.20
Round to the nearest whole numerator:
25/64
So the calculator can show approximately:
| Measurement | Result |
|---|---|
| Original size | 10 mm |
| Decimal inches | 0.3937 in |
| Millimeters | 10 mm |
| Centimeters | 1 cm |
| Nearest 1/64″ | 25/64″ |
The important point is that 25/64 inch is only the nearest fractional approximation. A 10 mm drill bit is not exactly the same diameter as a 25/64-inch drill bit.
Common Drill Bit Conversion Examples
The following table provides useful reference values for several common fractional drill sizes.
| Fractional Size | Decimal Inches | Approx. Millimeters | Centimeters |
|---|---|---|---|
| 1/16″ | 0.0625 | 1.5875 mm | 0.15875 cm |
| 1/8″ | 0.125 | 3.175 mm | 0.3175 cm |
| 3/16″ | 0.1875 | 4.7625 mm | 0.47625 cm |
| 1/4″ | 0.25 | 6.35 mm | 0.635 cm |
| 5/16″ | 0.3125 | 7.9375 mm | 0.79375 cm |
| 3/8″ | 0.375 | 9.525 mm | 0.9525 cm |
| 7/16″ | 0.4375 | 11.1125 mm | 1.11125 cm |
| 1/2″ | 0.5 | 12.7 mm | 1.27 cm |
These values are useful for quick reference, but the calculator can provide more precise conversions for individual measurements.
Fractional Drill Bit Size Chart
Fractional drill bits are commonly expressed using fractions of an inch. Each fraction represents a specific diameter.
| Drill Bit | Decimal Inch |
|---|---|
| 1/64″ | 0.015625 |
| 1/32″ | 0.03125 |
| 3/64″ | 0.046875 |
| 1/16″ | 0.0625 |
| 5/64″ | 0.078125 |
| 3/32″ | 0.09375 |
| 7/64″ | 0.109375 |
| 1/8″ | 0.125 |
| 9/64″ | 0.140625 |
| 5/32″ | 0.15625 |
| 11/64″ | 0.171875 |
| 3/16″ | 0.1875 |
| 13/64″ | 0.203125 |
| 7/32″ | 0.21875 |
| 15/64″ | 0.234375 |
| 1/4″ | 0.25 |
| 17/64″ | 0.265625 |
| 9/32″ | 0.28125 |
| 19/64″ | 0.296875 |
| 5/16″ | 0.3125 |
| 21/64″ | 0.328125 |
| 11/32″ | 0.34375 |
| 23/64″ | 0.359375 |
| 3/8″ | 0.375 |
| 25/64″ | 0.390625 |
| 13/32″ | 0.40625 |
| 27/64″ | 0.421875 |
| 7/16″ | 0.4375 |
| 29/64″ | 0.453125 |
| 15/32″ | 0.46875 |
| 31/64″ | 0.484375 |
| 1/2″ | 0.5 |
This chart can be particularly helpful when comparing fractional drill bits with metric sizes.
Metric vs. Fractional Drill Bits
Metric and fractional drill bits are both widely used, but they divide measurements differently.
Metric drill bits are generally identified directly in millimeters. For example:
- 4 mm
- 5 mm
- 6 mm
- 8 mm
- 10 mm
Fractional drill bits use fractions of an inch:
- 1/8″
- 3/16″
- 1/4″
- 5/16″
- 3/8″
Neither system is inherently a replacement for the other. The important consideration is matching the required hole diameter to the appropriate drill bit.
When an exact hole size is important, it is better to use the specified measurement rather than automatically selecting the nearest fractional or metric equivalent.
When to Use a Metric Equivalent
A metric equivalent can be useful when:
- Your drill set is metric but the instructions use inches.
- Your project specification is in millimeters.
- You need to compare two drill bit sets.
- You are replacing a missing drill bit.
- You are working with imported equipment.
- You need to estimate the diameter of an existing hole.
- You are comparing imperial and metric fasteners.
However, a close conversion should not automatically be treated as an exact replacement.
For precision machining, the difference between two nearby drill sizes can matter significantly.
Why Rounding Matters
Drill bit diameter is a physical measurement, so precision can be important. A calculator may display a converted value to several decimal places, but the available drill bit may only be manufactured in certain standard increments.
For example:
10 mm = 0.3937008 inch
If this is rounded to:
0.394 inch
the difference may be insignificant for some general-purpose tasks but important for precision applications.
Likewise, the nearest fractional size may not be an exact equivalent.
The calculator therefore provides both the precise decimal conversion and the nearest 1/64-inch representation so that users can distinguish between an exact unit conversion and a convenient fractional approximation.
Choosing the Right Drill Bit Size
Selecting a drill bit involves more than simply converting measurements.
The correct size depends on the purpose of the hole.
Clearance Holes
A clearance hole allows a fastener, such as a bolt, to pass through without engaging the material.
The required drill diameter should be selected based on the fastener and desired clearance.
Pilot Holes
Pilot holes are typically smaller than the final hole size. They help guide larger drill bits and can reduce wandering, especially when drilling into wood or other materials.
Tapped Holes
When preparing a hole for threading, the drill diameter is normally smaller than the nominal thread diameter. The appropriate tap-drill size should come from a reliable thread-size chart or manufacturer specification rather than simply choosing a nearby converted fraction.
Woodworking
In woodworking, drill size can affect screw installation, dowel joints, hardware installation, and other connections. The required size depends on the fastener and material.
Metalworking
Metalworking often requires more precise hole dimensions. Material type, drilling method, cutting conditions, and final hole requirements should all be considered.
Drill Bit Conversion Tips
Keep the following tips in mind when converting drill bit sizes:
- Remember that 1 inch equals exactly 25.4 mm.
- Do not confuse drill diameter with drill length.
- Treat fractional conversions as approximations when the sizes do not match exactly.
- Use the specified drill size for precision work whenever possible.
- Check the fastener manufacturer’s recommendations when drilling holes for bolts or screws.
- Use appropriate measuring tools when identifying an unknown drill bit.
- Do not assume that the nearest fractional size is interchangeable with a metric bit.
- Consider material and application before choosing the final drill size.
- Use a drill chart for specialized applications such as tapping.
- Keep extra decimal precision when working on precision projects.
Benefits of Using This Drill Bit Conversion Calculator
The calculator can save time by bringing several common conversions together in one place.
Quick Unit Conversion
You can move between inches, millimeters, centimeters, and fractional inches without manually calculating each conversion.
Multiple Results
Instead of showing only the requested conversion, the calculator also displays decimal inches, millimeters, centimeters, and the nearest 1/64-inch fraction.
Fractional Size Support
The built-in fractional choices make it easier to work with traditional imperial drill bit sizes.
Useful for Workshop Projects
The tool can be useful for hobbyists, technicians, builders, mechanics, woodworkers, and anyone who regularly works with drill bits.
Easier Metric-Imperial Comparison
The calculator provides a convenient way to compare measurements when project documentation and available tools use different systems.
Drill Bit Conversion vs. Drill Bit Selection
It is important to distinguish between converting a drill bit size and selecting a drill bit for a specific application.
Conversion answers the question:
“What is this drill bit size in another measurement unit?”
Selection answers:
“Which drill bit should I use for this particular hole?”
The calculator is designed primarily for conversion. It does not determine the correct tap drill, clearance-hole size, pilot-hole size, or material-specific drilling procedure.
For those applications, you should use appropriate engineering, fastener, machining, or manufacturer specifications.
Frequently Asked Questions
1. What does a drill bit conversion calculator do?
A drill bit conversion calculator converts drill bit diameter measurements between inches, millimeters, centimeters, and fractional inches. It can also provide the nearest 1/64-inch fractional representation.
2. How do I convert a drill bit from inches to millimeters?
Multiply the inch measurement by 25.4. For example, a 1/4-inch drill bit is 0.25 inch, and 0.25 × 25.4 equals 6.35 mm.
3. How do I convert millimeters to inches?
Divide the millimeter measurement by 25.4. For example, 10 mm divided by 25.4 is approximately 0.3937 inch.
4. What is 1/4 inch in millimeters?
A 1/4-inch drill bit is 0.25 inch in decimal form. Multiplying by 25.4 gives 6.35 mm.
5. What is 3/8 inch in millimeters?
A 3/8-inch drill bit equals 0.375 inch. Multiplying 0.375 by 25.4 gives 9.525 mm.
6. What is 1/2 inch in millimeters?
A 1/2-inch drill bit is 0.5 inch. Therefore, 0.5 × 25.4 equals 12.7 mm.
7. What is the nearest fractional size for a metric drill bit?
The nearest fractional size depends on the metric diameter. The calculator compares the decimal-inch equivalent with 1/64-inch increments and returns the closest fractional representation.
8. Is the nearest fractional drill bit exactly equal to the metric size?
Not necessarily. A nearest fractional size is an approximation unless the two measurements mathematically represent the same diameter. For precision work, use the specified drill diameter rather than relying on a close equivalent.
9. Can I convert centimeters to drill bit inches?
Yes. Divide the centimeter measurement by 2.54 to obtain inches. For example, 1 cm equals approximately 0.3937 inch.
10. What is the formula for converting fractional inches to decimal inches?
Divide the numerator by the denominator. For example, 5/16 inch is 5 ÷ 16 = 0.3125 inch.
11. What does the nearest 1/64-inch result mean?
It represents the fractional inch size closest to the converted decimal-inch measurement when sizes are limited to increments of 1/64 inch.
12. Is 10 mm the same as 3/8 inch?
No. A 10 mm drill bit is approximately 0.3937 inch, while 3/8 inch equals 9.525 mm. They are relatively close but have different diameters.
13. Can this calculator be used for woodworking?
Yes. It can help convert drill bit measurements when working with woodworking plans, hardware, screws, dowels, and other projects. The appropriate final drill size should still be determined according to the specific application.
14. Can this calculator determine the correct tap drill size?
No. It is a unit-conversion tool rather than a tap-drill sizing reference. Tap-drill selection should be based on the thread specification, desired thread percentage, material, and applicable engineering or manufacturer recommendations.
15. Why are drill bit conversions useful?
Conversions are useful when drill bits, project plans, fasteners, machinery, or technical specifications use different measurement systems. Converting the measurements makes it easier to compare sizes and identify suitable tools.
Final Thoughts
The Drill Bit Conversion Calculator provides a convenient way to translate drill bit diameters between common measurement systems. By supporting inches, millimeters, centimeters, and fractional inches, it can simplify many everyday measurement tasks in workshops, garages, construction projects, woodworking, and general maintenance.
The key conversion to remember is 1 inch = 25.4 millimeters. From that relationship, you can convert decimal-inch measurements into millimeters or reverse the calculation to convert metric sizes into inches. Fractional sizes can also be converted by dividing the numerator by the denominator.
For general projects, the nearest fractional measurement can provide a useful approximation. For precision applications, however, even a small difference in diameter may matter. Always distinguish between an exact unit conversion and the nearest available drill bit size.
By entering a drill bit size, selecting the appropriate starting unit, and choosing the desired conversion unit, you can quickly obtain the measurement you need while also seeing the corresponding decimal, metric, and fractional values.