Drill Bit Size Chart

The size of a drill bit is primarily measured by the diameter at the cutting tip, which determines the size of the drilled hole. 

In the U.S., we use the imperial measurement system where drill bit sizes are commonly specified in fractional inches. However, drill bits are actually sized using four different systems: fractional, number, letter, and metric.

If you’ve ever wondered why a #30 bit is smaller than a #10, or why there is both a 5/16″ and a letter “P” bit that are almost the exact same size, read on… 

The following chart is a master list of drill bit sizes in both the imperial (inches) and metric (mm) systems. It is sorted from smallest to largest by decimal inches and shows the corresponding metric diameter of each size. The last column in the table shows the closest alternative size when an exact match is not available. You can click the buttons at the top to filter the chart and show only imperial, fractional, or metric sizes.

📌Note: Don’t confuse “drill size” with “shank size.”
The size in the chart is the hole diameter the bit cuts and not the diameter of the shank that fits into your drill chuck.

Drill Bit Sizes Explained

While the metric system serves as the standard for drill bit sizes globally, in North America, you will commonly see four different systems. These include legacy inch-based systems (fractional, letter, and number gauges) as well as the metric system.

1. Fractional sizes (inch)

Fractional drill bit sizes are specifies in fractions of an inch, such as 1/4″, 5/16″, etc. These are the most familiar sizes for general construction, carpentry, woodworking, DIY, plumbing and home maintenance work.

Fractional drill bits are labeled in fractions of an inch, typically in 1/64″ increments, from 1/64″ up to 1″ and beyond.

Examples: 1/16″, 1/8″, 5/32″, 5/16″, 3/8″, 1/2″, etc.

💡How to view them in the chart: Click on the Fractional Only button above the chart to see the list of fractional sizes.

2. Number sizes (#1–#80, plus #97 for micro sizes)

Number sizes are based on the old wire gauge system and are still used as a standard for small bits. They are commonly available from #80 (0.0135″) to #1 (0.228″), although sizes as small as #97 (0.0059″) are listed in the ASME B94.11M Twist Drills standard.

• The #1 drill bit measures 0.2280 inches (5.791 mm) in diameter.
• The #80 drill bit measures 0.0135 inches (0.343 mm) in diameter.

These sizes are commonly used for precision drilling, and tap-drill applications.

3. Letter sizes (A–Z)

Letters from A through Z are also used for sizing bits, with A being the smallest and Z the largest. They fill many of the size gaps between fractional and number drills.

• Drill Bit Size A is 0.234″ (5.95mm)
• Drill Bit Size Z is 0.413″ (10.49mm)

Letter bits are mostly used in aerospace engineering, and precision metalworking. If you need a size that’s “between 1/4″ and 17/64″,” there’s likely a letter bit (F & G) that fits your need.

4. Metric Drill Bits

Metric drill bits are sized directly in millimeters, such as 2 mm, 6.8 mm, etc. The metric system is the global standard, used everywhere outside of the United States (and to a limited extent in Canada due to cross-border manufacturing).

Standard metric drill bits are typically available in 0.1 mm increments from 1 mm up to 10 mm, shifting to 0.5 mm increments from 10 mm up to 25 mm and larger.

Common Examples:

• Fine range (0.1 mm steps): 1 mm, 1.1 mm, 1.2 mm, 1.3 mm,… 13 mm
• Larger range (0.5 mm steps): 10.0 mm, 10.5 mm, 11 mm,… 25 mm

📌Note: You can find individual non-standard sizes such as 10.2 mm which is the tap drill size for M12 screw hole.

If you’re working on imported equipment or assemblies with metric fasteners, metric bits are essential.

Using Alternative Size

Closest Alternative column in the chart shows the nearest drill size from a different sizing system. This is useful when the exact size is not available.

For example, when you need the nearest metric equivalent to a fractional, number, or letter drill. Keep in mind that a close alternative is not always interchangeable for precision work, especially for tapping or reaming.

Because the sizing systems overlap, several drill bit sizes are very close to one another. The chart above lets you compare their actual diameters and find the nearest alternative when the exact size is not available.

While the four sizing systems overlap, in most cases they don’t match the dimensions exactly. This often trips people. Here a few examples:

Nominal size	Actual inch	Actual mm	Closest other size
1/4″	0.2500″	6.35 mm	6.4 mm (metric)
6 mm	0.2362″	6.00 mm	“A” bit (0.234″)
5/16″	0.3125″	7.94 mm	8 mm (metric, 0.315″)
8 mm	0.3150″	8.00 mm	5/16″ (0.3125″)
#30	0.1285″	3.26 mm	3.3 mm (metric)

Inch to mm Conversion

Conversion Formula

1 inch = 25.4mm (millimeters)

To convert any inch size to mm, multiply by 25.4. For millimeters to inches, divide by 25.4.
For example: 3/8″ = 0.375 × 25.4 = 9.53 mm. 10 mm = 10 ÷ 25.4 = 0.394″.