Drill Bit Size Chart

Fractional Drill Bit Sizes in Decimal Inches and Millimeters

Standard fractional drill bits advance in sixteenth-inch increments across basic sets and thirty-second or sixty-fourth increments in complete shop indexes. Most common drill bit sets sold in North America use fractional inches, ranging from 1/16 inch up to 1/2 inch or 1 inch.

Per the National Institute of Standards and Technology (NIST), the inch has been defined as exactly 25.4 millimeters since July 1, 1959, under Federal Register Notice 59-5442. Every millimeter value in this table derives directly from that exact 25.4 ratio. For a broader mathematical breakdown of fractional measurements, refer to our fraction to decimal chart.

Number and Letter Drill Bit Sizes

Number and letter gauge drill bits fill the intermediate diameter gaps between standard fractional inch sizes for precise machining and thread tapping. In this system, numbers run backwards from #80 at 0.0135 inches up to #1 at 0.228 inches. Letter sizes then take over, running alphabetically from A at 0.234 inches up to Z at 0.413 inches.

Machinists use number and letter sizes because fractional steps leave gaps that are too wide when cutting threads. For instance, a 1/4-20 machine screw thread requires a hole diameter close to 0.201 inches. A 3/16 inch bit (0.1875 inches) is too tight, while a 7/32 inch bit (0.2188 inches) leaves the hole too loose for strong threads. A #7 drill bit sits at 0.201 inches, providing the necessary clearance.

The table below highlights commonly referenced gauge sizes used for drilling tap holes in general metalworking and home repairs. Millimeter values are converted using the NIST 25.4 standard.

Converting Between Fraction, Number, Letter, and Metric Sizes

You can convert any fractional or gauge drill bit to millimeters by multiplying its decimal-inch value by exactly 25.4. Metric drill bits do not use fractions or letter codes. They are manufactured and labeled directly in millimeters, typically stepping up by 0.1 millimeter increments in small sizes and by half-millimeter or whole-millimeter increments above 6 millimeters.

To find the standard inch equivalent of a metric drill bit, divide the millimeter size by 25.4. For example, a 6 mm bit divided by 25.4 equals 0.2362 inches, which falls between a 15/64 inch bit (0.2344 inches) and a letter B bit (0.2380 inches). When working on household projects, a difference of a few thousandths of an inch rarely matters, but precision hole sizing in metal demands exact cross-referencing.

If you are measuring an unmarked bit from an old toolbox, measure the shank diameter with a caliper or check your layout against our guide on how to read a tape measure.

Drill Bit Sizes for Wood Screw Pilot Holes

A wood screw pilot hole should match the root diameter of the screw core in softwood and slightly exceed it in hardwood to prevent splitting. The root diameter, also called the minor diameter, is the width of the solid central shank beneath the threads. If your pilot hole matches the outer thread diameter, the threads will have no wood fiber to bite into. If the hole is too small, driving the screw creates excessive friction that can snap the shank or split the board.

Softwoods like pine and cedar compress easily, allowing the screw threads to push wood fibers aside into a tighter pilot hole. Hardwoods such as oak and maple have dense wood fibers that resist compression, which requires a pilot bit roughly 1/64 inch larger than the softwood size. Tapered wood screws work best with specialized tapered pilot bits, but standard straight twist drill bits work reliably for everyday woodworking.

The table below lists common pilot hole sizes for standard wood screws. Because screw geometry differs between brands and thread styles, treat these numbers as practical starting points and drill a test hole in scrap wood first.

Drill Bit Sizes for Tapping Machine Screw Threads

A tap drill cuts a hole that leaves approximately 75 percent thread engagement when a tap cuts internal threads into metal or plastic. Thread engagement refers to the percentage of thread depth that contacts the matching machine screw. Aiming for 100 percent thread engagement creates excessive cutting torque that frequently snaps brittle taps inside the workpiece without adding much holding power.

Machine screws are identified by their nominal diameter and thread pitch under Unified National Coarse (UNC) or Unified National Fine (UNF) thread series standards. For example, a 1/4-20 UNC screw has a 1/4 inch nominal diameter and 20 threads per inch. A 10-32 UNF screw has a number 10 diameter with 32 fine threads per inch.

Because standard fractional bits rarely match the precise diameter needed for 75 percent thread engagement, tap drill sets rely heavily on number and letter drill bits. Always check the tap packaging or the tool manufacturer's data sheet to confirm the recommended drill size before cutting metal.

Drill Bit Sizes for Drywall and Masonry Anchors

Wall anchors require the exact drill bit diameter specified on the anchor packaging to hold securely without spinning or pulling free. Unlike wood screws, expansion anchors rely on friction against the walls of the drilled hole to resist pullout loads. Anchor designs vary widely across plastic sleeves, self-drilling anchors, toggle bolts, and concrete sleeve anchors.

Drilling an oversized hole causes the anchor sleeve to spin in place when you attempt to drive the screw, destroying drywall gypsum or concrete grip. Drilling an undersized hole prevents the anchor from sliding into the wall flush, and forcing it can crush plastic collars or crack brittle concrete and brick. Masonry anchors require carbide-tipped masonry bits and a hammer drill to cut accurate holes without blowing out the back of the brick or block.

Because there is no universal chart linking anchor screws to hole sizes across different brands, always follow the drill diameter molded into the plastic body or printed on the original retail packaging. Keep the drill bit perpendicular to the wall face throughout drilling to keep the entrance hole round and straight.

How to Choose the Right Drill Bit Size

Choosing the correct drill bit size starts with identifying whether the hole must clear a shank, create threads, or support an expansion sleeve. For a clearance hole where a bolt passes completely through material without threading, choose a bit 1/64 to 1/32 inch larger than the bolt shank diameter. For a pilot hole in wood, select a bit that covers the central root diameter while exposing the threaded ridges when held up to a light source.

At DIY Repair Guide, we compiled these reference tables for standard workshop woodworking, maintenance, and basic machine screw fastening. These tables are not intended for production CNC machining or structural engineering connections that mandate specific class-of-fit tolerances or formal testing certifications. In those manufacturing environments, engineers calculate specific percentage limits for tap clearance based on alloy tensile strengths.

Our recommendations assume standard standard-pitch fasteners and typical DIY materials. If you are installing specialty fasteners like self-tapping structural lag screws or ceramic tile anchors, follow the fastener manufacturer's published engineering guide. To verify your setup, drill a test hole in a scrap cutoff of the same material before committing to your finished piece.

• Clearance holes: Pick a drill bit 1/64 inch to 1/32 inch larger than the fastener diameter so the screw shank slides through freely.
• Wood pilot holes: Select a bit matching the central core diameter so threads bite into solid wood without splitting the board.
• Machine screw taps: Use the specific number, letter, or fractional drill bit specified on your tap packaging to maintain 75 percent thread engagement.
• Wall anchors: Inspect the anchor sleeve or retail packaging and use the exact fractional or metric bit diameter stamped by the manufacturer.

Common questions

How do I know what size drill bit to use?

Match the bit diameter to your specific task. For a wood screw pilot hole, hold the bit behind the screw and ensure the bit hides the central core while leaving the threads exposed. For taps and wall anchors, use the exact size stamped on the tap shank or anchor packaging.

What size drill bit for a 1/4-20 tap?

A standard 1/4-20 UNC tap requires a #7 drill bit, which has a decimal diameter of 0.201 inches (5.11 mm). If a #7 bit is unavailable, a 13/64 inch bit (0.2031 inches) serves as a close fractional alternative for general repairs in softer materials.

What drill bit size is closest to 3mm?

A 1/8 inch drill bit is the closest standard fractional size to 3 mm. At 0.125 inches (3.175 mm), a 1/8 inch bit is just 0.175 mm larger than a 3 mm bit.

Do I need a different drill bit for metal versus wood?

Yes, metal drilling requires high-speed steel (HSS), cobalt, or carbide bits with a flatter 135-degree split point to penetrate hard materials without wandering. Wood drilling works well with standard 118-degree twist bits, brad point bits with sharp centering spurs, or spade bits for large holes.

What size pilot hole do I drill for a deck screw?

Most standard deck screws are #8 or #10 gauge. Use a 1/8 inch drill bit for #8 deck screws and a 9/64 or 5/32 inch bit for #10 deck screws when working in dense framing lumber or composite decking.

Can I substitute a standard fractional bit for a tap drill size?

You can substitute a close fractional bit in softer materials like plastic or aluminum, but hard metals require exact number or letter sizes. An undersized fractional bit can snap your tap, while an oversized bit strips out under load.

Related

• Fraction to Decimal Chart
• How to Read a Tape Measure