Drill Bit Size Chart for Aircraft Mechanics – Complete Guide

If you've ever stood at a workbench trying to figure out which drill to grab for a rivet hole or a bolt clearance, you know how important it is to have a reliable drill bit size chart nearby. For aircraft mechanics, choosing the wrong drill size isn't just inconvenient — it can mean an oversized hole, a scrapped part, or worse, a write-up that grounds an aircraft.

This guide covers everything an AMT needs to know about drill bit sizing — number drills, letter drills, fraction drills, and how to read a decimal equivalent chart. We've also linked our free searchable drill chart so you can look up any size instantly on your phone in the hangar.

The Three Types of Drill Bits Used in Aviation

Aircraft mechanics work with three numbering systems for drill bits. Understanding all three is essential because maintenance manuals, SRMs, and shop drawings can reference any of them.

1. Number Drills (#80 through #1)

Number drills are the smallest category, ranging from #80 at 0.0135 inches up to #1 at 0.228 inches. The system is counterintuitive at first — a higher number means a smaller drill. Once it clicks, it becomes second nature.

The most commonly used number drills in airframe sheet metal work are:

• #30 (0.1285") — the standard drill for a 1/8" (AN470AD4) rivet hole
• #40 (0.098") — the standard drill for a 3/32" (AN470AD3) rivet hole
• #21 (0.159") — used for 5/32" rivets
• #11 (0.191") — used for 3/16" rivets
• #52 (0.0635") — common for safety wire hole drilling in hardware

2. Letter Drills (A through Z)

Letter drills pick up where number drills leave off, starting at A (0.234") and going up to Z (0.413"). They're used for larger bolt clearance holes and some structural applications.

A few letter drills that come up frequently:

• E (0.250") — exactly 1/4 inch, same as a 1/4" fraction drill
• F (0.257") — clearance hole for AN4 (1/4-28) bolts
• W (0.386") — clearance hole for AN6 (3/8-24) bolts
• U (0.368") — same diameter as 3/8", used for AN7 bolt applications

3. Fraction Drills (1/64" and up)

Fraction drills are specified in fractions of an inch — typically in 64ths. They're used when a precise fractional diameter is required, such as for countersink pilots, bushing installations, or structural repairs specified in the SRM.

The most important fraction to remember: 1 inch = 25.4 mm exactly. Everything else follows from there.

How to Read a Drill Bit Size Chart

A standard drill bit size chart has four columns: the drill size (number, letter, or fraction), the fraction equivalent if applicable, the decimal equivalent in inches, and the metric equivalent in millimeters.

The decimal equivalent column is the most useful because it lets you compare any drill size directly, regardless of which system it comes from. When your AMM says "drill to 0.201 inches," you look down the decimal column and find #7. When it says "drill for a #4 rivet," you know that means a 1/8" rivet which requires a #30 drill.

Choosing the Right Drill for Rivet Holes

Per AC 43.13-1B, rivet holes should be drilled to a diameter that gives 0.002" to 0.004" clearance over the rivet shank. This ensures the rivet can be inserted easily but still fills the hole properly when driven.

• 3/32" rivet (AN-3) → drill with #40 (0.098")
• 1/8" rivet (AN-4) → drill with #30 (0.1285")
• 5/32" rivet (AN-5) → drill with #21 (0.159")
• 3/16" rivet (AN-6) → drill with #11 (0.191")
• 1/4" rivet (AN-8) → drill with F (0.257")

Never upsize a rivet hole without SRM authorization. If a hole is overdrilled, the approved repair is to use the next larger rivet size — but only if edge distance and pitch requirements are still met.

Drilling Tips for Aircraft Sheet Metal

• Use a center punch first — even a light dimple prevents the drill from walking on aluminum
• Use cutting fluid or Boelube on aluminum to extend drill life and get cleaner holes
• Drill at the correct speed — aluminum likes high RPM, steel needs slower speeds
• Deburr every hole — sharp edges in aluminum cause fatigue cracks over time
• Check drill sharpness — a dull drill generates heat and can work-harden the material
• Use a drill stop when drilling through multiple layers to control depth
• Never use a drill bit that shows wear or chipping on safety-critical structure

Quick Reference: Most Common Aviation Drill Sizes

Use Our Free Drill Chart Tool

Instead of carrying a paper chart, bookmark our searchable drill bit size chart on your phone. You can search by number, decimal, fraction, or metric size — and it works without an internet connection once the page is loaded.

We also have a full rivet size reference chart and AN hardware reference if you need bolt clearance sizes or rivet-to-drill size pairings.