A drill bit size chart lets you look up the exact diameter of any standard drill bit in decimal inches and metric millimeters. Four systems are used in practice: fractional inch (1/64\" to 1\"+), wire gauge numbers (#80 to #1), letters (A to Z), and metric (mm). This reference combines all four, sorted by size, so you can find the right bit regardless of which system your hardware uses.
Find Closest Drill Bit
Enter a target diameter — decimal inches (e.g. 0.375) or mm (e.g. 9.5mm) — to find the nearest standard drill bits.
Tap Drill Cross-Reference
Common thread sizes with the correct tap drill bit for 75% thread engagement.
| Thread Size | TPI / Pitch | Tap Drill | Drill Decimal (in) | Drill (mm) |
|---|
All Drill Bit Sizes
| Designation | Decimal (in) | Metric (mm) |
|---|
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How to Use the Drill Bit Size Chart
This free drill bit size chart covers all four standard systems — fractional, number, letter, and metric — sorted by diameter so you can compare across systems at a glance. Whether you're drilling for a tap thread, fitting a specific bolt, or simply matching an existing hole, this tool helps you find the right bit quickly.
Step 1: Search or Filter by Type
Use the search box to type any size format: a fraction like 3/8, a metric size like 9.5mm, a number like #7 or just 7, or a letter like F. The table filters in real time. Use the filter tabs to narrow the list to only fractional, number, letter, or metric bits.
Step 2: Sort by Any Column
Click any column header to sort the table. By default, all bits are sorted smallest to largest by decimal inch size. Click "Decimal (in)" or "Metric (mm)" headers to sort by diameter. Click the header again to reverse the sort. This makes it easy to find the largest bit in a number series or compare overlapping ranges between systems.
Step 3: Find the Closest Bit to Any Target Size
If you know the exact diameter you need — say, 0.386" for a specific component — enter it in the Find Closest Bit tool. It works in both decimal inches and millimeters (add "mm" suffix). The tool returns the five nearest standard drill bits across all systems, with the offset so you can see which is oversized vs. undersized.
Understanding the Four Drill Bit Systems
Fractional inch bits are the most common in US woodworking, construction, and general use. They're sized in fractions of an inch from 1/64" up to 1" and beyond. Wire gauge number bits (#80 to #1) are primarily used in metalworking and precision machining — #80 is the smallest at 0.0135" and #1 is 0.228". Letter bits (A through Z) bridge the gap between number bits and fractional bits, from 0.234" to 0.413". Metric bits are the global standard, measured in millimeters, and are essential when working with metric fasteners and hardware.
Using the Tap Drill Cross-Reference
When threading a hole, you first drill with a tap drill — a bit slightly smaller than the thread's outer diameter. The tap drill size ensures about 75% thread engagement, which gives sufficient strength without overworking the tap. For example, to tap a 1/4-20 UNC thread, you need a #7 drill bit (0.201"). To tap an M6×1.0 metric thread, you need a 5mm drill. Use the tap drill table in this chart for the most common thread sizes in both inch (UNC/UNF) and metric (M) standards.
Drill Bit Size Chart Tips
When buying drill bits, the fractional and metric systems have the widest availability. Number and letter bits are typically sold as indexed sets in machinist supply stores. For the closest match to a target size, always prefer a slightly undersized bit — you can always enlarge a hole, but you can't shrink it. For metric hardware in the US, metric bits are inexpensive and widely available at hardware stores. Keep a complete indexed set of number bits (#1–#60) for precision work, and supplement with letter bits for larger clearance holes.
Frequently Asked Questions
Is this drill bit size chart free to use?
Yes, completely free with no account required. All data is built into the page and works offline — nothing is sent to any server.
What is the difference between number, letter, and fractional drill bits?
Number drill bits (#80 to #1) are the smallest sizes, ranging from 0.0135" to 0.228". Letter drill bits (A to Z) cover 0.234" to 0.413". Fractional bits use inch fractions (1/64" to 1"+). Metric bits are measured in millimeters. All four systems overlap in size ranges and are used in different trades.
What drill bit size do I need for a tap?
The tap drill size is the bit you use to drill a hole before threading it. For example, a 1/4-20 tap requires a #7 drill bit (0.201"), and a 10-32 tap requires a #21 drill bit (0.159"). Use the Tap Drill Cross-Reference section of this chart to find the correct drill for common thread sizes.
How do I find the closest drill bit to a decimal size?
Use the 'Find Closest Bit' tool on this page. Enter any decimal inch value (e.g., 0.375 for 3/8") and it will instantly show the nearest standard drill bits across all four systems — fractional, number, letter, and metric.
What is the most common drill bit system used in the US?
Fractional inch drill bits are the most common in general US woodworking and construction. Number and letter bits are more common in machining and metalworking. Metric bits are standard in most of the world and increasingly common in the US for metric fastener work.
What size drill bit is equivalent to 3/8 inch?
3/8" equals exactly 0.3750 decimal inches, or 9.525 mm. The closest metric size is 9.5mm (0.3740") or you could use 9.6mm if available. In letter drill bits, the closest is letter U at 0.368" or W at 0.386".
Are drill bit sizes the same worldwide?
The metric system (mm) is used worldwide. Fractional inch, number, and letter drill systems are primarily American. When working with metric hardware outside the US, use the metric mm column. The decimal inch equivalents provided in this chart let you compare any system at a glance.