Drill Bit Sizes Explained: Fractional, Metric, and Number Systems

Drill bits come in four sizing systems: fractional inch, metric, wire gauge (number), and letter. Most homeowners only ever use fractional inch bits — sizes like 1/8″, 1/4″, 3/8″ — but knowing how the other systems work helps you read charts, match pilot holes to fasteners, and buy the right bit the first time. This guide decodes all four systems and shows you exactly which size to reach for in common situations.

What You’ll Need

• A set of drill bits (or a chart/reference when shopping)
• A drill bit gauge or calipers (optional but helpful for identifying unmarked bits)
• Knowledge of your fastener size (screw diameter, bolt size) for pilot hole selection
• Material you’re drilling into — pilot hole sizes differ for wood, metal, and masonry

Safety Precautions

• Always use the correct bit type for the material. A wood bit in metal will dull instantly and can break. A masonry bit in wood tears rather than cuts. Match the bit type before worrying about the size.
• Secure thin workpieces when using large bits. Large bits (1/2″ and above) generate significant torque. Always clamp thin metal, wood panels, or sheet material before drilling.
• Wear safety glasses. Bit breakage is most likely when the wrong size or wrong type is used at excessive speed.
• Drill at appropriate speeds. Larger bits need slower speeds to avoid overheating. Small bits in metal need adequate speed or they’ll rub rather than cut. Our guide on drill overheating prevention covers speed selection.

The Four Drill Bit Sizing Systems

1. Fractional Inch (Most Common in the US)

This is the system you see on standard twist bit sets sold at hardware stores. Sizes are expressed as fractions of an inch — 1/16″, 1/8″, 3/16″, 1/4″, 5/16″, 3/8″, 7/16″, 1/2″, and so on. Most consumer sets go from 1/16″ to 1/2″ in 1/64″ increments.

The important thing to understand is that the denominator increases by powers of 2, so the bits get progressively larger in smaller steps as you go up in the sequence. A 1/4″ bit (0.250″) is exactly twice the diameter of a 1/8″ bit (0.125″).

When to use this system: General woodworking, drywall anchors, standard home projects. If someone tells you to “drill a 1/4-inch hole,” they’re using this system.

2. Metric (Common in Imported Tools and Automotive)

Metric bits are sized in millimeters: 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 4mm, 5mm, 6mm, 8mm, 10mm, 12mm, and so on. They’re common in electronics, automotive work, and any project with metric fasteners.

Conversion tip: 1 inch = 25.4mm. So a 6mm bit ≈ 15/64″ (close to 1/4″), and a 3mm bit ≈ 7/64″. When working with metric fasteners from European or Asian manufacturers, metric bits ensure a precise fit that fractional approximations miss.

For more on comparing these two systems in detail, see our guide on fractional vs metric drill bits.

3. Wire Gauge Number System (Numbered Bits, #1–#80)

This system is used primarily in metalworking, machining, and precision work. Bit sizes run from #1 (largest at 0.2280″) to #80 (smallest at 0.0135″). Unlike what you might expect, the numbers get larger as the bits get smaller.

The most common reason a homeowner encounters number bits: drilling pilot holes for small machine screws. A #6 machine screw, for example, typically needs a #36 drill bit (0.1065″) for a clearance hole in metal, and a smaller number for a tapped/pilot hole.

4. Letter System (A–Z)

Letter drill bits fill the gaps between number bits and fractional bits. They run from A (0.2340″) to Z (0.4130″). Like the number system, they’re primarily used in metalworking and machining for precise clearance and tap drill holes.

A homeowner’s most likely encounter with letter bits: matching to fastener specifications in technical manuals or when the exact hole size falls between two fractional sizes.

Practical Reference: Common Sizes You’ll Actually Use

How to Read a Drill Bit Set

Most consumer drill bit sets are labeled on the shank — the cylindrical end you insert into the chuck. The size is stamped into the metal, usually readable with good lighting or a magnifying glass. On cheap bits, the marking wears off quickly.

If your bit is unlabeled, use a drill bit gauge — a small metal plate with holes of known sizes. Insert the bit into progressively smaller holes until it fits snugly. This is the most reliable method for identifying mystery bits from old mixed sets.

Digital calipers can also measure bit diameter directly at the flutes (the cutting edge), though the shank and flute diameter sometimes differ by 0.001–0.003″ on lower-quality bits.

For long-term organization, our article on how to store drill bits properly covers storage systems that keep your bits labeled and accessible.

Drill Bit Types and What Size Applies to Each

The sizing systems above apply to standard twist bits, but different bit types use their own size conventions:

• Spade bits / paddle bits: Sized by the hole diameter they produce — 3/4″, 1″, 1-1/2″, up to 1-1/2″. Common for wood boring.
• Forstner bits: Also sized by hole diameter — 1/4″ to 4″ common range. Produces flat-bottomed holes. See our comparison of spade bit vs Forstner bit.
• Masonry bits: Sized like fractional twist bits but designed for concrete, brick, and block. Use the same size as your anchor requires.
• Hole saws: Sized by the hole diameter — 2-1/8″ for door locksets, 3/4″ for pipe pass-throughs, etc.
• Countersink bits: Sized by the pilot drill diameter inside — e.g., a #8 countersink fits #8 screws.

Pro Tips and Common Mistakes

• Don’t guess on pilot hole size — look it up. Using a bit that’s too large removes the material the screw threads grip. Too small and you’ll split hardwood or snap a small screw. Use a pilot hole chart for each fastener diameter.
• Match bit coatings to material. Black oxide bits are fine for wood. Cobalt or carbide-tipped bits are needed for hardened steel. Titanium coatings improve tool life in general use but aren’t necessary for occasional work.
• Bit wobble means the wrong size chuck or a worn bit. If your bit wobbles in the chuck, either the shank is worn (replace the bit) or you need a different chuck size. See our article on drill bit wobble causes and fix.
• The “sharp” sound vs “scraping” sound test. A properly sized, sharp bit in wood or metal makes a clean cutting sound. A dull or oversized bit scrapes and generates heat. Stop and reassess.
• Step drill bits skip the size confusion entirely for sheet metal. One step bit covers multiple sizes in one tool — great for electrical work and thin panels. See our step drill bit guide for details.

Frequently Asked Questions

What size drill bit for a 1/4-inch screw?

For a 1/4-20 bolt clearance hole, use a 17/64″ bit (0.266″). For a pilot hole in wood for a 1/4″ lag screw, use a 3/16″ or 7/32″ bit depending on wood hardness. For a 1/4″ concrete anchor, use a 1/4″ masonry bit.

What’s the most common drill bit size for home use?

1/8″ and 3/16″ bits get the most use in typical home projects — pilot holes for screws, hanging hardware, and drywall anchors. A basic 13-piece or 21-piece fractional set from 1/16″ to 1/2″ covers 95% of residential tasks.

How do I know what drill bit size to use for a wall anchor?

Wall anchor packages specify the required drill bit size on the packaging — always check. Light-duty plastic anchors typically need 3/16″ to 1/4″ holes. Toggle bolts and heavy-duty anchors may need 1/2″ or larger. Never drill larger than specified.

Are metric and inch drill bits interchangeable?

Close in size but not identical. A 6mm metric bit (0.2362″) is very close to a 15/64″ fractional bit (0.2344″), but not exactly the same. For most woodworking, the difference is negligible. For precision metalwork, tap drilling, or exact clearance holes, use the correct system for your fastener.

What drill bit do I need for concrete or masonry?

You need a masonry bit — identifiable by its wide, arrow-shaped carbide tip. The bit size should match your anchor exactly. Use a hammer drill for concrete over 1/2″ depth. A regular drill will work for shallow holes in soft brick or mortar with a masonry bit, but hammer mode significantly reduces effort and bit wear.

Conclusion

The fractional inch system covers most home projects. The metric system matters when your fasteners are metric. Number and letter sizes appear mostly in metalworking and precision applications. When in doubt, use a pilot hole chart for your specific fastener and material combination — it saves split wood, stripped screws, and wasted time.

Continue building your skills with these related guides:

• Fractional vs Metric Drill Bits
• Spade Bit vs Forstner Bit: When to Use Each
• How to Store Drill Bits Properly
• Drill Bit Wobble: Causes and Fixes
• How to Remove a Stuck Drill Bit