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Smart Drill Size Finder — Fractional, Number, Letter & Metric Cross-Reference

Enter Any Drill Size Format and Get Equivalents in Every Standard System

Free universal drill size finder with over 570 standard drill sizes across all four systems. Enter a drill size in any format — fractional, number (#29), letter (F), or metric (5.0mm) — and instantly get decimal equivalents plus the nearest match in every other system. Smart parser auto-detects input format.

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Includes a neighbors table (3 above, 3 below), tap drill cross-reference for 27 common threads, and a full filterable drill chart.

Pro Tip: Keep a copy of the number and letter drill sizes taped inside your toolbox lid. Number drills (#1-#60) and letter drills (A-Z) fill the gaps between fractional sizes — there is a standard drill within 0.002 inches of nearly any diameter you need.

Calculate tap drill sizes for any engagement percentage

Tap Drill Calculator →

Get counterbore and clearance hole dimensions

Counterbore & Clearance Calculator →

Calculate speeds and feeds for drilling operations

Speeds & Feeds Calculator →

Look up drill and tap combinations

Drill & Tap Calculator →
Start Using This Tool See How It Works

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Smart Drill Size Finder
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How It Works

  1. Enter Any Drill Size

    Type a drill size in any format: "3/16" for fractional, "#29" for number, "F" for letter, "5.0mm" for metric, or "0.1875" for decimal inches. Auto-detection handles the rest.

  2. View Decimal Equivalents

    See the drill diameter in decimal inches and millimeters with nearest standard drill and deviation.

  3. Check All Four Systems

    Result panel shows nearest match in fractional, number, letter, and metric systems with deviations.

  4. Review Neighbors Table

    See the 3 drills smaller and 3 drills larger regardless of system, sorted by decimal diameter.

  5. Cross-Reference Tap Drills

    Tap drill reference shows which drills serve as tap drills for 27 common thread sizes.

Built For

  • Machinists converting between number, letter, fractional, and metric drill sizes
  • CNC programmers looking up decimal equivalents for drill call-outs on older drawings
  • Tool crib operators identifying unlabeled drills by measured diameter
  • Maintenance mechanics finding substitute drill sizes from available inventory
  • Engineering students learning the four drill size systems
  • Purchasing agents cross-referencing drill sizes across suppliers

Features & Capabilities

Smart Input Parser

Auto-detects fractional, number, letter, metric, and decimal formats. Just type and it figures out the system.

570+ Drill Database

28 fractional, 60 number, 26 letter, and metric drills from 0.5mm to 25mm. Complete standard coverage.

Neighbors Table

3 drills smaller and 3 larger sorted by decimal diameter across all systems.

Tap Drill Cross-Reference

27 common thread sizes linked to standard tap drills with automatic flagging.

Full Filterable Chart

Browse complete database sorted by diameter with columns for all four naming systems.

PDF Export

Export results as a branded PDF for toolbox reference or job documentation.

Frequently Asked Questions

Historical reasons. Fractional covers common sizes in 1/64" increments. Number drills (#1-#60) and letter drills (A-Z) fill the gaps. Metric uses millimeter diameters. Together they provide a standard drill within about 0.002" of any diameter.
A #7 drill is 0.2010 inches (5.105mm), commonly used as the tap drill for 1/4-20 UNC at approximately 75% engagement.
Measure the diameter with a micrometer to the nearest 0.001", then enter that decimal value. The calculator finds the nearest standard size and shows deviation.
If diameters match within tolerance, yes. A 6.0mm (0.2362") is very close to 15/64" (0.2344") — difference is only 0.0018". For general drilling this is interchangeable; for precision work, use the exact specified size.
Disclaimer: Drill size data based on published ANSI/ASME standards. Actual diameters may vary by manufacturer and condition. Always verify critical hole sizes with calibrated measurement tools.

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