Choosing the correct drill bit size is essential for accurate drilling, tapping, fastening, woodworking, metalworking, machining, and DIY projects. Drill bits are commonly identified using several different measurement systems, including decimal inches, fractional inches, millimeters, and wire gauge numbers. Because these systems do not always use the same naming conventions, converting between them manually can be inconvenient and can lead to mistakes.
Drill Bit Conversion Calculator
What Is a Drill Bit Conversion Calculator?
A Drill Bit Conversion Calculator is a measurement tool designed to translate drill bit sizes between commonly used sizing systems. Instead of searching through conversion charts or performing calculations manually, you can enter a known size and receive several equivalent measurements.
The calculator supports five input methods:
- Inches
- Millimeters
- Fractional inches
- Decimal inches
- Wire gauge
After calculating, the tool provides five useful results:
- Decimal Inches
- Millimeters
- Nearest Fraction
- Nearest 1/64 Fraction
- Nearest Standard Gauge
This is particularly helpful because drill bits can be labeled differently depending on the manufacturer, application, industry, or country.
For example, a drill bit might be identified as 1/4 inch, 0.2500 inch, or approximately 6.35 mm. These refer to essentially the same nominal diameter, even though the labeling systems are different.
Why Drill Bit Size Conversion Matters
Drill bit diameter directly affects the size of the hole produced. Using a bit that is too large or too small can affect the fit of a screw, bolt, fastener, dowel, or threaded component.
For example, when drilling a pilot hole, the required diameter may be specified in millimeters while your drill set is marked in fractional inches. Without a quick conversion method, selecting the appropriate bit can take additional time.
Accurate drill bit conversion is useful for:
- Metalworking
- Woodworking
- Construction
- Automotive repairs
- Mechanical work
- CNC machining
- Manufacturing
- Electrical installation
- Plumbing projects
- Home improvement
- DIY repairs
- Pilot-hole drilling
- Tapping and threading preparation
- Fastener installation
A conversion calculator is especially useful when working with equipment or plans that use a different measurement system than your available drill bits.
How to Use the Drill Bit Conversion Calculator
Using the calculator is straightforward.
Step 1: Enter the Drill Bit Size
If you are converting from inches, millimeters, or decimal inches, enter the numerical drill bit size in the input field.
For example:
- 0.25 inches
- 6.35 mm
- 0.125 inches
The calculator accepts decimal values for numerical inputs.
Step 2: Select the Starting Unit
Choose the measurement system that corresponds to your input.
The available options are:
- Inches
- Millimeters
- Fractional Inches
- Decimal Inches
- Wire Gauge
If you choose inches or decimal inches, enter the decimal diameter.
If you select millimeters, enter the diameter in millimeters.
If you choose fractional inches, you can select a listed fractional drill size.
If you select wire gauge, choose the appropriate gauge number from the available list.
Step 3: Click Calculate
Select the Calculate button. The calculator converts your selected measurement into decimal inches and uses that value to determine the other results.
Step 4: Review the Results
The results section displays:
- Decimal-inch diameter
- Millimeter diameter
- Nearest common fraction
- Nearest 1/64 fraction
- Nearest standard gauge
This gives you several ways to identify a comparable drill bit.
Drill Bit Conversion Formula
The fundamental conversion between inches and millimeters is:
1 inch = 25.4 millimeters
Therefore:
Inches to Millimeters
Millimeters = Inches × 25.4
For example, if a drill bit measures 0.25 inches:
0.25 × 25.4 = 6.35 mm
Therefore:
0.25 inch = 6.35 mm
Millimeters to Inches
To convert millimeters into inches:
Inches = Millimeters ÷ 25.4
For example, for a 10 mm drill bit:
10 ÷ 25.4 = 0.3937 inches
So a 10 mm drill bit has a decimal diameter of approximately 0.3937 inch.
Understanding Decimal Inches
Decimal inches represent a drill bit’s diameter as a decimal number rather than a fraction.
For example:
| Fractional Size | Decimal Inches |
|---|---|
| 1/16 | 0.0625 |
| 1/8 | 0.1250 |
| 3/16 | 0.1875 |
| 1/4 | 0.2500 |
| 5/16 | 0.3125 |
| 3/8 | 0.3750 |
| 1/2 | 0.5000 |
| 5/8 | 0.6250 |
| 3/4 | 0.7500 |
| 1 inch | 1.0000 |
Decimal measurements are useful when you need to compare sizes precisely or perform mathematical calculations.
For example, 5/16 inch is easier to use in a calculation after converting it to 0.3125 inch.
Understanding Fractional Drill Bit Sizes
Fractional drill sizes are expressed as fractions of an inch.
Common examples include:
- 1/16 inch
- 1/8 inch
- 3/16 inch
- 1/4 inch
- 5/16 inch
- 3/8 inch
- 7/16 inch
- 1/2 inch
- 5/8 inch
- 3/4 inch
- 7/8 inch
- 1 inch
The calculator includes a range of common fractional sizes, including increments such as 1/64, 1/32, and larger fractional sizes.
For instance:
3/8 = 0.375 inch
Converting the fraction to decimal makes it easier to compare with metric measurements.
The calculator also determines a nearest common fraction for decimal measurements.
What Is the Nearest 1/64 Fraction?
Many imperial drill bit measurements can be expressed in increments of 1/64 inch.
The calculator determines the nearest 1/64-inch value by rounding the decimal-inch measurement to the closest sixty-fourth.
The basic calculation is:
Nearest 1/64 = Round(Decimal Inches × 64) ÷ 64
Suppose the drill diameter is 0.250 inch.
First:
0.250 × 64 = 16
Then:
16 ÷ 64 = 1/4
So the nearest 1/64 fraction is:
1/4 inch
For a measurement that does not correspond exactly to a standard fraction, the calculator rounds it to the closest 1/64-inch increment.
This can be useful when trying to identify an imperial drill bit that is closest to a metric measurement.
Understanding Wire Gauge Drill Sizes
Wire gauge drill sizes are commonly identified by numbers such as #1 through #80. Unlike ordinary fractional sizes, the gauge number does not directly state the diameter.
A smaller gauge number generally corresponds to a larger diameter, while a larger gauge number corresponds to a smaller diameter.
For example, the calculator’s gauge list includes values such as:
| Gauge | Diameter in Inches |
|---|---|
| #1 | 0.2280 |
| #10 | 0.1935 |
| #20 | 0.1610 |
| #30 | 0.1285 |
| #40 | 0.0980 |
| #50 | 0.0700 |
| #60 | 0.0400 |
| #70 | 0.0280 |
| #80 | 0.0135 |
The calculator compares the decimal-inch diameter with the listed gauge sizes and identifies the closest available gauge.
This is particularly useful when working with numbered drill index sets.
Worked Example: Convert 1/4 Inch to Millimeters
Suppose you have a 1/4-inch drill bit and want to know its metric equivalent.
First convert the fraction to decimal:
1/4 = 0.25 inch
Then convert inches to millimeters:
0.25 × 25.4 = 6.35 mm
The result is:
- Decimal Inches: 0.2500 in
- Millimeters: 6.35 mm
- Nearest Fraction: 1/4 in
- Nearest 1/64 Fraction: 1/4 in
- Nearest Standard Gauge: determined by the closest gauge in the calculator’s gauge table
This example demonstrates why decimal inches are useful as the common intermediate measurement.
Worked Example: Convert 10 mm to Inches
Suppose a project specifies a 10 mm drill bit, but you want to compare it with imperial drill sizes.
Use:
Inches = 10 ÷ 25.4
Therefore:
10 ÷ 25.4 ≈ 0.3937 inch
The calculator reports approximately:
0.3937 in
The nearest 1/64-inch size is approximately 25/64 inch, which is 0.390625 inch.
This shows that a 10 mm drill bit does not correspond exactly to a common 1/64-inch size.
When precision is important, it is generally better to use the specified drill diameter rather than automatically substituting the nearest imperial size.
Drill Bit Conversion Reference Table
The following table provides useful approximate conversions for several familiar sizes.
| Fraction | Decimal Inches | Millimeters |
|---|---|---|
| 1/16 | 0.0625 | 1.59 mm |
| 3/32 | 0.0938 | 2.38 mm |
| 1/8 | 0.1250 | 3.18 mm |
| 5/32 | 0.1563 | 3.97 mm |
| 3/16 | 0.1875 | 4.76 mm |
| 7/32 | 0.2188 | 5.56 mm |
| 1/4 | 0.2500 | 6.35 mm |
| 9/32 | 0.2813 | 7.14 mm |
| 5/16 | 0.3125 | 7.94 mm |
| 11/32 | 0.3438 | 8.73 mm |
| 3/8 | 0.3750 | 9.53 mm |
| 13/32 | 0.4063 | 10.32 mm |
| 7/16 | 0.4375 | 11.11 mm |
| 15/32 | 0.4688 | 11.91 mm |
| 1/2 | 0.5000 | 12.70 mm |
| 9/16 | 0.5625 | 14.29 mm |
| 5/8 | 0.6250 | 15.88 mm |
| 3/4 | 0.7500 | 19.05 mm |
| 7/8 | 0.8750 | 22.23 mm |
| 1 | 1.0000 | 25.40 mm |
These values are useful for quick reference, while the calculator can provide more precise conversions for decimal inputs.
Fractional vs. Decimal vs. Metric Drill Sizes
Each measurement system has advantages.
Fractional Inches
Fractional sizes are common in imperial drill sets and are easy to recognize when working with traditional fasteners and hardware.
For example:
1/4 inch, 5/16 inch, and 3/8 inch
are familiar fractional sizes.
Decimal Inches
Decimal inches are convenient for precise calculations and comparisons.
A fractional measurement can be converted into decimal form:
3/8 = 0.375
Decimal values are also useful when comparing imperial and metric measurements.
Millimeters
Metric drill bits are identified by their diameter in millimeters.
Examples include:
- 3 mm
- 4 mm
- 5 mm
- 6 mm
- 8 mm
- 10 mm
- 12 mm
Metric sizes are widely used in engineering, manufacturing, automotive applications, and international specifications.
Wire Gauge
Gauge numbers are especially common for small drill sizes. They provide a compact identification system, but the relationship between gauge number and diameter is not immediately obvious.
Choosing the Correct Drill Bit Size
Conversion is only one part of selecting a drill bit. The appropriate diameter depends on the job.
For a simple clearance hole, the drill diameter needs to accommodate the fastener without unnecessarily enlarging the hole.
For a pilot hole, the appropriate diameter depends on factors such as:
- Fastener diameter
- Thread dimensions
- Material
- Thread type
- Desired thread engagement
- Manufacturer specifications
- Hole depth
- Application requirements
For tapping operations, the correct drill size should generally be selected from the tap manufacturer’s recommended drill chart rather than relying solely on a generic conversion.
A converted measurement tells you how sizes compare; it does not automatically determine the correct hole size for every application.
Why the Nearest Size Is Not Always the Correct Size
One important feature of a drill bit conversion calculator is identifying the closest size. However, nearest does not necessarily mean interchangeable.
For example, a metric drill diameter may fall between two fractional sizes. Selecting whichever is numerically closest may not be appropriate for a precision hole.
The required tolerance can be extremely important in:
- Machining
- Bearing installations
- Reaming
- Tapping
- Dowel holes
- Precision assemblies
- CNC work
If a technical drawing specifies an exact diameter or tolerance, follow that specification rather than simply selecting the closest converted drill size.
Practical Applications of Drill Bit Conversion
A drill bit conversion tool can be useful in many situations.
Woodworking
Woodworkers may encounter plans that specify imperial drill sizes while their drill index uses metric measurements. Conversion makes it easier to compare available bits.
Metalworking
Metalworkers frequently need accurate hole diameters. Converting between metric and imperial measurements can help when working with imported equipment or mixed fastener systems.
Automotive Repairs
Automotive components can involve a combination of metric and imperial specifications, particularly when working on older equipment or parts from different manufacturers.
DIY Projects
Home improvement instructions may use a measurement system different from the tools you have available. A quick conversion can help you identify a comparable drill size.
Manufacturing
Manufacturing environments often require consistent interpretation of engineering specifications. Decimal-inch and metric measurements may both appear in documentation.
Tips for Better Drill Size Conversions
1. Check the Original Specification
Always start with the exact measurement provided by the manufacturer, drawing, fastener specification, or project instructions.
2. Understand the Difference Between Exact and Approximate
A conversion between inches and millimeters can be mathematically exact when using the defined relationship of 25.4 mm per inch. However, converting to a common fractional or gauge size may produce an approximation.
3. Use Decimal Inches for Comparisons
Decimal inches provide a common numerical format that makes it easier to compare different sizes.
4. Be Careful With Small Drill Bits
Small diameter differences become proportionally significant in precision work. A seemingly minor difference may affect hole fit or thread preparation.
5. Verify Critical Applications
For engineering, machining, tapping, or safety-critical work, verify the required diameter against the appropriate technical specification.
6. Consider Tool Availability
If you need to substitute one drill size for another, consider whether the application allows a slightly larger or smaller hole before making the substitution.
Advantages of Using an Online Drill Bit Conversion Calculator
An online calculator offers several practical benefits.
Fast conversion: You do not need to manually perform inch-to-millimeter calculations.
Multiple formats: You can view decimal, metric, fractional, and gauge information together.
Reduced calculation errors: Automatic calculations eliminate many common arithmetic mistakes.
Easy comparison: The results make it easier to compare a known measurement with common drill sizes.
Convenient for mixed-unit projects: It is particularly useful when project specifications and available drill bits use different measurement systems.
Useful reference: The calculator can serve as a quick digital alternative to carrying a printed conversion chart.
Important Notes About Drill Bit Measurements
Drill bit dimensions can vary slightly because of manufacturing tolerances, standards, and intended applications. The nominal diameter listed for a drill bit should therefore be distinguished from the actual measured diameter.
Additionally, different drill-bit systems are designed for different purposes. Fractional, letter, number, and metric drill sets are not simply interchangeable naming systems in every situation.
The calculator’s nearest standard gauge result is based on the gauge values included in the tool. Likewise, its nearest fractional result is based on the common fractional sizes included in its conversion list.
For highly precise work, use an appropriate measuring instrument such as a micrometer or caliper to verify the actual diameter of a drill bit.
Drill Bit Conversion Calculator: Quick Summary
The Drill Bit Conversion Calculator simplifies the process of comparing common drill bit sizing systems.
It can convert:
- Inches to millimeters
- Millimeters to inches
- Fractional inches to decimal inches
- Decimal inches to fractional sizes
- Decimal inches to the nearest 1/64 fraction
- Decimal inches to the nearest standard wire gauge
The primary mathematical relationship is:
1 inch = 25.4 mm
For inches-to-millimeters conversion:
mm = inches × 25.4
For millimeters-to-inches conversion:
inches = mm ÷ 25.4
For 1/64-inch approximation:
Nearest fraction = Round(decimal inches × 64) ÷ 64
These calculations provide a convenient way to compare drill bit diameters across different measurement systems.
Frequently Asked Questions
1. What is a Drill Bit Conversion Calculator?
A Drill Bit Conversion Calculator converts drill bit diameters between inches, millimeters, fractional inches, decimal inches, and wire gauge. It also identifies nearby common fractional and gauge sizes.
2. How do I convert inches to millimeters?
Multiply the inch measurement by 25.4. For example, 1/4 inch equals 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 ÷ 25.4 is approximately 0.3937 inch.
4. What is 1/4 inch in millimeters?
A 1/4-inch drill bit is 0.2500 inch, which equals 6.35 mm.
5. What is 1/8 inch in millimeters?
An 1/8-inch drill bit is 0.125 inch, which equals approximately 3.18 mm.
6. What is 3/8 inch in millimeters?
A 3/8-inch drill bit is 0.375 inch. Multiplying by 25.4 gives approximately 9.53 mm.
7. What does a drill bit gauge number mean?
A drill gauge number identifies a particular drill diameter. In the numbered drill system, smaller gauge numbers generally correspond to larger drill diameters, while higher numbers correspond to smaller diameters.
8. What is the difference between fractional and decimal drill sizes?
Fractional drill sizes use fractions such as 1/4 or 3/8 inch, while decimal drill sizes express the same diameter numerically, such as 0.250 or 0.375 inch.
9. What is the nearest 1/64 drill size?
The nearest 1/64 drill size is the fractional inch measurement obtained by rounding a decimal diameter to the closest increment of 1/64 inch.
10. Can I use the nearest fractional size instead of the exact metric size?
Not always. The appropriate substitution depends on the application’s required tolerance. For precision drilling, machining, tapping, or fitting, use the specified diameter whenever possible.
11. What is 5/16 inch as a decimal?
5/16 inch equals 0.3125 inch.
12. What is 1/2 inch in millimeters?
1/2 inch equals 0.5 inch. Multiplying by 25.4 gives 12.70 mm.
13. Why are drill bits available in different sizing systems?
Different industries and regions use different measurement standards. Fractional, metric, decimal, and gauge systems allow drill manufacturers and users to identify diameters according to different conventions.
14. Is a converted drill size always exact?
An inch-to-millimeter conversion is exact based on the defined relationship of 25.4 mm per inch. However, converting that value to the nearest common fraction or wire gauge is an approximation unless the sizes match exactly.
15. Should I verify the drill size before precision work?
Yes. For precision applications, verify the required hole diameter, tolerance, material requirements, and manufacturer specifications. A conversion calculator is a useful reference, but the application’s technical requirements should determine the final drill size.