Drill Bit Size Chart — Decimal Equivalents

Drill Bit Size Chart

Complete printable drill bit size chart with decimal inch and millimeter equivalents for all four standard series: number (#97–#1), letter (A–Z), fractional (1/64"–2"), and metric (1.0–50.0 mm). All values verified against Machinery’s Handbook, 29th Edition.

A drill bit size chart is a reference table listing every standard twist drill diameter with its decimal inch and millimeter equivalents. Drill bits are organized into four series defined by ANSI/ASME B94.11M-1993: number drills (#97–#1), letter drills (A–Z), fractional drills (1/64" increments), and metric drills (millimeter sizes per ISO 235). This chart includes all four series sorted by diameter, with search, filtering, and the nearest fractional equivalent for every size.

Drill bit sizes: number, letter, fractional, and metric drills with decimal equivalents per ANSI/ASME B94.11M-1993

Source: Machinery’s Handbook 29th Ed. — Drill sizes: pp. 868–890, Table 1 (ANSI/ASME B94.11M-1993). Metric drill sizes per ISO 235.

How to Use This Drill Bit Size Chart

Use the Series dropdown to filter by number, letter, fractional, or metric drills, or leave it on “All” to see every standard drill size sorted by diameter. Type a drill designation or decimal value into the Search field to find a specific size instantly. Click any table value to copy it to your clipboard. Click a row to highlight it for reference.

If you need to find the correct drill for a tapped hole, see our tap drill size chart, which maps thread sizes to drill diameters at adjustable engagement percentages.

For pipe threads, see our NPT pipe thread size chart.

Example: Finding the Right Drill for a 0.201" Hole

You need to drill a 0.201" hole in a steel part. Type “0.201” in the search field — the chart shows a #7 drill at 0.2010". If you only have fractional drills, the nearest fraction column shows 13/64" (0.2031"), which is 0.002" oversize. For this application, the #7 number drill is the better match.

For a metric example: you need a 6.5 mm hole. Search “6.5” to find the 6.5 mm metric drill (0.2559"). The nearest fractional drill is 1/4" (0.2500"), which is 0.15 mm undersize — the metric drill is the correct choice when precision matters.

Drill Size Terminology

Number Drills (#97–#1)
Wire-gauge-based sizes defined by ANSI/ASME B94.11M. #97 is the smallest (0.0059") and #1 is the largest (0.2280"). Number drills provide the finest increments in the small-diameter range, making them essential for tap drilling, precision holes, and miniature work. The numbering runs opposite to diameter: a higher number means a smaller drill.
Letter Drills (A–Z)
Twenty-six sizes from A (0.2340") to Z (0.4130") that bridge the gap between the largest number drill (#1 at 0.2280") and fractional drills. Letter drills are commonly used as tap drills for thread sizes in the 1/4"–7/16" range.
Fractional Drills
Drills sized in 1/64" increments, ranging from 1/64" (0.0156") upward. These are the most widely stocked drill sizes and the default choice for general-purpose work. Above 1", only selected fractional sizes are standard.
Metric Drills
Drills sized in millimeters per ISO 235. Standard metric drills are available in 0.05 mm increments from 1.0–2.5 mm, 0.1 mm increments from 2.5–10.0 mm, and 0.5–1.0 mm increments above 10 mm. Metric drills are essential when working with ISO-standard drawings or tapping metric threads.

Frequently Asked Questions

What are number, letter, and fractional drill bit series?

Drill bits are organized into four standard series. Number drills (#97 through #1) are wire-gauge-based sizes that provide the finest diameter increments, especially in the small-diameter range below 0.228". Letter drills (A through Z) cover diameters from 0.234" to 0.413", bridging the gap between number and fractional drills. Fractional drills are sized in 1/64" increments (e.g., 1/16", 3/32", 1/8") and are the most commonly stocked in shops. Metric drills are sized in millimeters per ISO 235. All four series are defined in ANSI/ASME B94.11M-1993.

How do I convert drill bit sizes to decimal inches?

Number and letter drill sizes do not follow a simple mathematical formula — each size has a specific decimal equivalent defined by ANSI/ASME B94.11M-1993 (see the chart above). For fractional drills, divide the fraction: a 7/32" drill is 7 ÷ 32 = 0.2188". For metric drills, divide the millimeter size by 25.4: a 5.5 mm drill is 5.5 ÷ 25.4 = 0.2165". This chart shows the decimal equivalent for every standard drill size.

What size drill bit do I need for a pilot hole?

Pilot hole size depends on the application. For wood screws, a pilot hole is typically 50–70% of the screw’s root diameter. For self-tapping screws in sheet metal, match the pilot hole to the screw’s root diameter. For machine screws with tapped threads, use a tap drill chart to find the correct drill for your thread size and engagement percentage — see our tap drill size chart for the complete lookup table. The drill bit size chart above helps you identify the nearest standard drill once you know the target diameter.

What is the difference between a drill size chart and a tap drill chart?

A drill bit size chart is a reference of all standard drill diameters organized by series (number, letter, fractional, metric) with their decimal and millimeter equivalents. A tap drill chart tells you which specific drill to use before tapping a particular thread size — it maps thread specifications (like 1/4-20 UNC) to the correct drill diameter for a target thread engagement percentage. This page is a drill size chart; for tap-specific lookups, see our tap drill size chart.

What are the standard metric drill bit sizes?

Standard metric drill sizes are defined by ISO 235. Common sizes range from 1.0 mm to 50.0 mm. From 1.0 to 2.5 mm, drills are available in 0.05 mm increments. From 2.5 to 10.0 mm, increments widen to 0.1 mm. Above 10 mm, increments are 0.5 mm up to 32 mm, then 1.0 mm up to 50 mm. Use the Series filter above set to “Metric” to view the complete list with inch equivalents.

Related Calculators

References

  • Oberg, E. et al. Machinery’s Handbook, 29th Edition, Industrial Press, 2012, pp. 868–890. ANSI Straight Shank Twist Drill sizes (number, letter, fractional, and metric drill diameters per ANSI/ASME B94.11M-1993).
  • ANSI/ASME B94.11M-1993 — Twist Drills (Inch and Metric). Defines standard drill diameters for number, letter, fractional, and metric series.
  • ISO 235 — Twist drills with parallel shanks; metric sizes. Defines the standard metric drill size series.

Data last verified: March 2026

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