Unified Drill Bit Size Conversion Chart

Main U.S. Drill Size Systems

Typical labeling systems include fraction of an inch, decimal inch, number, letter, and millimeters. A bit labeled “#7” is a number drill, not a 7 mm bit. Likewise, a “G” drill is a letter size, not a gauge number. Confusing these systems is one of the most common mistakes in bit selection.

These systems overlap in diameter but often do not have exact equivalents; a metric bit and a fractional bit of similar size may differ by a few thousandths of an inch.

Do not assume every manufacturer stocks every 0.1 mm increment; available sizes vary by brand and drill set.

Conversion: 1 in = 25.4 mm exactly.

Number drills range from #80 to #1. #80 is the smallest size, and #1 is the largest, filling gaps between common fractions. This system is particularly useful for tapping, precision drilling, machining, and clearance holes where a fractional size alone would leave too large a gap.

Letter drills run from A, the smallest, through Z, the largest. The current chart spans A at 0.234 in through Z at 0.413 in, picking up in diameter immediately above the largest number drill, #1 at 0.228 in.

ASME B94.11M-1993 remains in effect under stabilized maintenance and covers twist drill nomenclature, definitions, sizes, and tolerances for high-speed-steel drills, straight and taper shanks, inch and metric sizes, and combined drills and countersinks. The standard defines drill dimensions and tolerances; it does not tell you what hole size every screw, bolt, or tap requires, which is why this page links out to the specific-purpose hole charts throughout.

A nominal 1/4-in drill does not necessarily produce an exactly 0.2500-in finished hole in every condition. Actual hole size can be affected by drill runout, drill wear, machine or tool rigidity, workpiece material, point geometry, feed rate, and spindle or hand-drill alignment. Drill diameter and finished-hole tolerance are related but not identical concepts.

For close-tolerance work, drilling may be followed by reaming, boring, or honing to achieve final required dimensions. Do not assume a twist drill alone produces a precision-fit hole automatically. ASME B94.11M establishes drill dimensional tolerances, but achieving a tighter finished-hole tolerance typically requires an additional finishing operation.

This Drill Bit Size Chart answers “what diameter is this drill?” The Pilot Hole Size Chart answers “what hole should I drill before driving this screw?” These are related but distinct questions; pilot hole diameter depends on screw type, thread, and wood species, not simply the closest drill to the screw’s outside diameter.

Tap drill size depends on the thread’s major diameter, pitch or threads per inch, and desired thread engagement percentage. Common examples include #6-32 requiring a #35 drill, 1/4-20 requiring a #7 drill, and 3/8-24 requiring a Q drill. The complete size-by-thread reference belongs to the Tap Drill Chart; this page explains the conceptual relationship and links out rather than duplicating that dataset.

A 1/4-in bolt does not necessarily use a 1/4-in clearance drill. The appropriate drill depends on fastener size, whether a close, normal, or loose fit is required, and the applicable standard. See the Clearance Hole Size Chart for the complete fit-class reference.

Structural bolted connections use standardized hole categories, including standard, oversize, and slotted holes, governed by structural steel standards. The Bolt Hole Size Chart owns this structural-specific reference; this master drill chart simply explains that drilling-tool diameter and allowable structural hole category are separate questions requiring separate references.

A countersink uses a hole diameter, a conical recess diameter, and an included angle. A counterbore requires a base or clearance hole plus a larger cylindrical recess and a specified depth. See the Countersink Size Chart and Counterbore Size Chart for the complete angle and dimension tables rather than duplicating them here.

The correct drill depends on whether you need a pilot hole, a clearance hole, a tapped hole, or a countersink. Do not assume you should use a drill bit the same size as the screw; that is often wrong and can split wood or strip threads. Key inputs for wood screws include screw major diameter, root or minor diameter, hardwood versus softwood, threaded versus unthreaded shank, and required clearance. See the Wood Screw Size Chart and Pilot Hole Size Chart for the complete screw-specific reference.

Lag fasteners may need a lead or pilot hole for the threaded portion, a clearance hole through the attached member, and possibly a larger clearance hole under the unthreaded shank. Exact pilot sizes depend on material and fastener requirements; see the Lag Bolt Sizes Chart for lag-specific sizing.

Masonry and concrete anchors often require a hole diameter specified directly by the anchor manufacturer. Anchor categories include concrete screws, wedge anchors, sleeve anchors, and drop-in anchors, each with their own required drill diameter.

Consider the exact target hole diameter, whether a tap or clearance requirement applies, deburring needs, reaming allowance if precision matters, and whether the work is thin sheet or thick plate. This chart addresses diameter selection only; cutting speeds, feeds, and coolant practices are outside its scope.

Wood drilling can involve twist bits, brad-point bits, augers, spade bits, and Forstner bits. Their labeled diameter can be the same while geometry and application differ substantially; keep size selection separate from bit-type selection when planning a project.

The shank can differ from the nominal cutting diameter. Examples include reduced-shank bits, Silver and Deming-type larger bits, and hex-shank bits. Chuck capacity is determined by shank size, not necessarily by cutting diameter, so a drill’s cutting diameter can exceed what its shank designation might suggest at first glance.

Straight Shank vs Reduced Shank Drill Bits

A common user question is whether a 1/2-in chuck can drill a hole larger than 1/2 in. A reduced-shank bit can allow a larger cutting diameter while fitting a smaller chuck, subject to the specific tool and product rating.

This helps users reading drawings specifying decimals such as .125, .201, .257, or .332 identify the corresponding drill designation.

For a standard twist drill, measure carefully across the cutting diameter at the outer margins, not across the web or body of the bit. A caliper works well for general use, while a micrometer provides higher accuracy for precision applications.