Drill Bit Size Chart: Inch, Metric, Number & Letter Sizes
Use this drill bit size chart to compare standard drill sizes and find the decimal-inch and millimeter diameter of fractional, number, letter and metric drill bits.
| Drill Size | Standard | Decimal Inches | Millimeters |
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How to Use the Drill Bit Size Chart
This drill size chart lists standard drill bit sizes in order from the smallest diameter to the largest. Use the search field to find a specific size, decimal diameter or measurement. The filters allow you to display fractional, number, letter or metric drill sizes separately.
Each row provides the nominal drill size, its measurement system, its decimal-inch diameter and its equivalent diameter in millimeters. This makes the chart useful as both a drill bit size table and a drill size conversion chart.
When selecting a drill, use the actual decimal diameter to compare sizes from different systems. Two drill bits with different nominal designations may have very similar diameters, but they are not necessarily interchangeable for every machining operation.
Standard Drill Bit Sizes and Measurement Systems
Standard drill bit sizes used in machining and metalworking are commonly identified as fractional inch, number, letter or metric sizes. These systems overlap in diameter, which is why a combined drill diameter chart is useful when comparing available tools.
Fractional drill bits are designated by inch fractions. Number drills use sizes from #80 through #1, with smaller numbers representing larger diameters. Letter drills run from A through Z and increase in diameter as the letters progress. Metric drill bits are identified directly by their diameter in millimeters.
Fractional Drill Bit Sizes in Inches
Fractional drill bit sizes are expressed as fractions of an inch, such as 1/16, 1/8, 1/4 and 1/2 inch. They are widely used for general drilling, fabrication, construction, maintenance and machining in the United States.
A fractional drill chart makes it easier to compare drill bit fractions by converting each fraction to a decimal-inch measurement. For example, a 1/4-inch drill has a decimal diameter of 0.2500 inch and a metric equivalent of 6.35 mm.
Fractional sizes are commonly supplied in increments of 1/64 inch. Some specialized drill indexes may also contain sizes in smaller increments, depending on the type of tooling and its intended application.
Number Drill Sizes
Number drill sizes—also called numbered drill bit sizesor wire gauge sizes—range from #80 through #1. A #80 drill is the smallest size in the standard number series, while a #1 drill is the largest. This means number drill diameters increase as the designation number decreases.
Number drills are frequently used when a fractional size does not provide the required hole diameter. They are common in machine shops, toolrooms, screw-machine work, tapping applications and other operations requiring closely spaced drill sizes.
Use the number drill size chart above to compare each numbered drill with nearby fractional, letter and metric sizes. Always compare the decimal drill diameter when selecting the closest alternative.
Letter Drill Sizes
Letter drill sizes range from A through Z. Unlike number drills, letter drill bits increase in diameter as the letters advance. An A drill has a diameter of 0.2340 inch, while a Z drill has a diameter of 0.4130 inch.
Letter-size drill bits fill many of the diameter gaps between common fractional sizes. They are often used for machining, tapping and applications in which a standard fractional drill does not produce the desired hole size.
The letter drill size chart can also be searched by letter or decimal diameter, making it easier to locate sizes such as an F drill bit and compare them with adjacent drill sizes.
Metric Drill Bit Sizes in Millimeters
Metric drill bit sizes are designated by their nominal diameter in millimeters. Common metric drill sizes may advance in increments of 0.1 mm, although other increments are available for precision and specialized applications.
The chart lists standard drill sizes in millimeters alongside their decimal-inch equivalents. This is useful when a drawing specifies a metric hole but the available drill index contains inch, number or letter drills.
To compare a metric drill with an inch drill, use the decimal columns rather than relying only on the nominal size. The closest available size should still be evaluated against the required hole tolerance and the intended operation.
Converting Drill Bit Sizes Between Inches and Millimeters
To convert a drill diameter from inches to millimeters, multiply the inch diameter by 25.4. To convert a drill diameter from millimeters to inches, divide the millimeter diameter by 25.4.
For example, a 1/4-inch drill converts to 6.35 mm:
0.2500 inch × 25.4 = 6.35 mm
A 6 mm drill converts to approximately 0.2362 inch:
6 mm ÷ 25.4 = 0.2362 inch
The interactive drill bit size conversion chart performs these comparisons for you by displaying both measurements for every listed size.
Choosing the Correct Drill Bit Diameter
The correct drill diameter depends on the hole specification and the operation performed after drilling. A clearance hole, tapped hole, reamed hole and press-fit hole may each require a different starting drill size even when they relate to the same finished component.
Drilled holes may also finish slightly larger than the stated drill diameter because of drill runout, tool wear, machine condition, workpiece material and cutting conditions. When hole size and finish are critical, drilling may be followed by boring or reaming.
For threaded holes, use a dedicated tap drill chart or tap drill calculator rather than choosing a drill solely from this general drill bit chart. The proper tap drill depends on the thread diameter, pitch and desired thread engagement.
What Sizes Are Included in a Drill Index?
The contents of a drill index depend on its measurement system. A fractional drill index commonly contains sizes from 1/16 through 1/2 inch in 1/64-inch increments. A number drill index may contain all 80 sizes from #80 through #1, while a letter drill index contains 26 sizes from A through Z.
Combination drill indexes may contain fractional, number and letter sizes in one case. Metric drill indexes are generally organized by millimeter diameter and may use 0.1 mm, 0.5 mm or other increments.
Use the drill index chart above to identify a missing drill, compare neighboring diameters or determine which measurement system contains the closest standard size.
Important Drill Size Selection Note
This drill bit size guide is intended as a general machining and measurement reference. Verify the required drill diameter against the part drawing, thread specification, hole tolerance, tool manufacturer’s recommendations and established shop practices before drilling.
