Complete Guide to Drill Bit Sizing

Understanding the Three Sizing Systems

Drill bits in the United States use three distinct sizing systems that can initially seem bewildering but serve specific purposes. Fractional sizing (1/16", 1/8", 1/4", etc.) represents the most familiar system, using common fractions measured in sixtyforths of an inch. This system works well for general purpose drilling and aligns with common fastener sizes and woodworking dimensions.

Number gauge sizing runs from #80 (the smallest at 0.0135 inches) to #1 (0.2280 inches), with smaller numbers indicating larger diameters. This seemingly backwards system originated in wire gauge standards where higher numbers meant thinner wire after more drawing operations. Number drills fill the gaps between fractional sizes and are essential for precision work, tapping operations, and machine shop applications where exact hole sizes matter.

Letter sizing uses A through Z, with A being smallest (0.2340 inches) and Z largest (0.4130 inches). Letter drills bridge the gap between large number drills and small fractional sizes, primarily used in machine shops for tapping, reaming, and precision work where exact hole dimensions are critical. The three systems overlap considerably—for instance, a #7 drill (0.2010 inches) is nearly identical to a 13/64" drill (0.2031 inches)—but each fills specific niches in professional and hobbyist drilling applications.

Why Multiple Sizing Systems Exist

The existence of three parallel sizing systems reflects the evolution of manufacturing and metalworking practices over centuries. Fractional sizing dominated early American manufacturing, aligning with carpentry and general construction where fractions were standard. As precision metalworking and machine shops developed, the need for more granular size increments became apparent—jumping from 1/8" (0.125") to 9/64" (0.1406") left a gap of 0.0156 inches, significant in precision work.

Number and letter drills fill these gaps, providing size increments as small as 0.001 inches in some ranges. This precision enables exact tap drill sizing—drilling holes to precise dimensions before cutting threads with taps. A 1/4"-20 bolt requires a tap drill of exactly 0.2010 inches (#7 drill) for optimal thread engagement. No fractional drill provides this exact size; the nearest fractional equivalents (13/64" at 0.2031" or 3/16" at 0.1875") are either too large or too small for proper threading.

Modern manufacturing has standardized on metric sizing internationally, but American industry retains these imperial systems due to enormous installed infrastructure, existing tooling, and standardized fasteners all based on these measurements. Understanding all three systems remains essential for anyone working with American-made equipment, fasteners, or following American technical specifications.

Fractional Drill Bits: The General Purpose Standard

Fractional drill bits represent the most commonly available and widely used sizing system for general drilling applications. Hardware stores and home centers stock fractional sets almost exclusively, as they serve the needs of construction, woodworking, and general home maintenance adequately. The progression by 1/64" increments from 1/16" through 1/2" (and larger) provides sufficient granularity for most non-precision work.

Common fractional sizes appear repeatedly across various applications. The 1/8" bit drills pilot holes for small screws, starter holes for larger bits, and general purpose small diameter holes. A 1/4" bit is ubiquitous for wall anchors, many types of hardware, and medium-sized fasteners. The 3/8" size handles larger anchors, electrical work, and plumbing applications. These workhorse sizes see daily use in construction and home improvement.

Fractional drill sets typically come in two formats: sets jumping by 1/16" increments (1/16", 1/8", 3/16", 1/4", etc.)—about 9 bits from 1/16" to 1/2"—or more complete sets with 1/64" increments throughout the range. The 1/16" increment sets suffice for rough work but leave frustrating gaps for precision tasks. Complete fractional sets with all 1/64" sizes (about 29 bits from 1/16" to 1/2") provide much greater versatility and eliminate the frequent problem of not having quite the right size.

Number and Letter Drills: Precision Sizing

Number gauge drill bits excel in machine shop work, metalworking, and applications requiring precision hole sizing. The complete number drill set (#80 through #1) contains 60 different sizes spanning 0.0135" to 0.2280"—granularity impossible with fractional sizing. This precision enables exact tap drill selection, precision fitting of pins and shafts, and hole sizing for close-tolerance assemblies.

Common number drills serve specific standard purposes. A #43 drill (0.0890") is the standard tap drill for #4-40 machine screws, the ubiquitous size in electronics and computer hardware. Number #36 (0.1065") taps #6-32 threads common in electrical boxes and fixtures. Number #7 (0.2010") is the precise tap drill for 1/4"-20 threads, one of the most common bolt sizes. These specific sizes make number drills essential for anyone cutting threads or working with machine screws.

Letter drills continue where number drills end, covering 0.2340" (A) through 0.4130" (Z). This range overlaps with fractional sizes from roughly 1/4" to 7/16" but provides much finer increments. Letter drills are particularly important for tap drills in larger thread sizes and for precision reaming operations where hole size must be controlled within thousandths of an inch. Industrial machine shops maintain complete letter drill sets; home workshops can usually get by with a smaller selection of the most commonly needed letters.

Metric Drill Bits: International Standard

Metric drill bits use straightforward millimeter sizing, proceeding in logical increments: 1mm, 1.5mm, 2mm, 2.5mm, 3mm, and so forth. This simplicity and the mathematical relationship to metric fasteners make metric drills the global standard outside North America. Even in the U.S., imported equipment, furniture, and electronics increasingly use metric fasteners, making metric drill bits increasingly necessary in American workshops.

Metric and imperial sizes don't align perfectly, though some approximate equivalents exist. A 3mm drill (0.1181") is very close to 1/8" (0.1250"), close enough for many purposes. A 5mm drill (0.1969") approximates 3/16" (0.1875") within reasonable tolerances for non-precision work. However, for tapping metric threads or precision fitting of metric fasteners, you need actual metric drills—approximations with imperial sizes will produce incorrect fits.

Metric tap drills follow systematic rules based on thread pitch. An M6 x 1.0 bolt (6mm diameter with 1.0mm thread pitch) requires a 5.0mm tap drill. An M8 x 1.25 bolt needs a 6.8mm tap drill. These specific sizes aren't available in fractional or number/letter systems, making metric drills essential when working with metric threads. Complete metric drill sets typically include sizes from 1mm through 13mm in 0.5mm or 1mm increments, with expanded sets offering 0.1mm increments for precision work.

Specialized Drill Bit Types and Sizes

Beyond standard twist drills, specialized bit types serve specific materials and applications, each with their own size ranges. Brad point bits feature a sharp center point and cutting spurs that prevent wandering in wood, providing cleaner, more accurate holes than standard twist drills. Available primarily in fractional sizes from 1/8" to 1/2", brad point bits are essential for fine woodworking, cabinetry, and any application requiring precisely located, clean holes in wood.

Spade bits (also called paddle bits) cut large diameter holes in wood quickly and cheaply. Ranging from 1/4" to 1-1/2" or larger, spade bits handle rough work like drilling for electrical cables, plumbing, and lumber framing. They cut aggressively but leave rougher holes than Forstner bits, making them unsuitable for visible work but perfect for utility drilling where appearance doesn't matter.

Forstner bits produce flat-bottomed holes with clean edges, ideal for hinge mortises, dowel joints, and anywhere you need a precise, clean hole in wood. Available in fractional sizes from 1/4" through 2-1/4" (and larger specialty sizes), Forstners are more expensive than spade bits but produce vastly superior results in visible work. The flat bottom allows overlapping holes and drilling to precise depths, impossible with twist or spade bits.

Step drill bits feature multiple diameter steps on a single conical bit, allowing one bit to drill multiple hole sizes. Sized by their step diameters (typically ranging from 1/8" through 1-3/8" on a single bit), step drills excel at sheet metal work, electrical boxes, and enlarging existing holes. Their self-centering design and deburring action produce clean holes in thin materials more easily than standard twist drills.

Tap Drill Selection and Threading

Tapping threads requires drilling holes to very specific dimensions—too small and the tap breaks; too large and threads won't engage properly. Tap drill charts specify the exact drill size for each thread size and pitch. For a 1/4"-20 bolt (1/4" diameter with 20 threads per inch), the tap drill is #7 (0.2010"). This size leaves approximately 75% of the thread height remaining after tapping, providing optimal strength and ease of tapping.

The formula for tap drill sizing in imperial threads is: Tap Drill Diameter = Major Diameter - (1.0825 / Threads Per Inch). For 1/4"-20: 0.250" - (1.0825 / 20) = 0.250" - 0.054" = 0.196". The nearest standard drill is #7 at 0.2010", slightly larger than the calculation for easier tapping with adequate thread engagement. Metric tap drills follow similar principles but use the thread pitch (distance between threads) instead of threads per inch.

Professional machinists maintain complete number and letter drill sets specifically for tap drill sizes, as these provide the precision needed. DIYers can get by with purchasing individual tap drill sizes as needed—if you're tapping 1/4"-20 threads, buy a #7 drill specifically for that purpose. Over time, you'll accumulate the most common tap drills: #43 for #4-40, #36 for #6-32, #29 for #8-32, #25 for #10-24, and #7 for 1/4"-20, covering the majority of common thread sizes.

Building Your Drill Bit Collection

Rather than buying massive combination sets with bits you'll rarely use, build your collection strategically. Start with a fractional set spanning 1/16" through 1/2" in 1/64" increments—this forms your general purpose foundation handling most woodworking, construction, and home maintenance tasks. Quality matters significantly; cheap bits dull quickly and drill inaccurately. Invest in good high-speed steel (HSS) or cobalt bits from reputable brands like DeWalt, Bosch, Milwaukee, or Irwin.

Add specialized bits as specific needs arise. If you work with wood regularly, acquire a brad point set. For large holes in wood, buy spade bits in the sizes you need—a 3/4" spade bit costs $3-5 individually rather than buying a complete set with sizes you won't use. Add masonry bits if you drill concrete or brick. Step drills serve anyone doing electrical work or metalworking.

For precision work, metalworking, or frequent tapping, invest in number and letter drill indexes. These sets come in protective cases with each size labeled, making it easy to find and return bits. A complete #1 through #60 number drill set costs $30-60 depending on quality, while letter drill sets run similar prices. These aren't necessary for casual DIY work but become essential if you regularly tap threads or perform machine work.

Metric drill sets are increasingly important even for American workshops. Buy a set from 1mm through 13mm if you work on imported equipment, furniture, or electronics. The complete set occupies little space and costs $20-40 for decent quality. Having metric drills on hand prevents frustration when faced with metric fasteners that don't quite match any imperial size.

Practical Sizing Tips and Substitutions

When you don't have the exact size bit needed, understanding equivalents and acceptable substitutions helps complete work without unnecessary tool purchases. Drill bit size conversion charts showing decimal equivalents across all systems are invaluable—you'll frequently discover that a number or letter drill you own is effectively identical to a fractional size you need. A #7 drill (0.2010") substitutes perfectly for 13/64" (0.2031") in non-precision applications; the 0.002" difference is negligible.

For clearance holes (holes for bolts or screws to pass through without threading), you can err slightly larger without problems. A 1/4" bolt requires a clearance hole of about 17/64" (0.2656") for easy fit, but using a 1/4" drill (0.250") or even 9/32" (0.2813") works fine depending on how loose a fit you want. Tap drills, however, must be exact—substituting nearby sizes produces poor threads or broken taps.

Pilot holes for wood screws allow more flexibility. Charts specify ideal pilot hole sizes based on screw gauge and wood hardness, but being slightly larger or smaller usually works. In softwoods, you can often omit pilot holes entirely for larger screws far from board edges. In hardwoods, drill pilot holes religiously to prevent splitting, but exact size matters less than simply having a hole to guide the screw and reduce wood splitting pressure.

Maintaining Your Drill Bits

Proper care extends drill bit life dramatically and maintains drilling accuracy. Store bits in protective cases or indexes rather than loose in toolboxes where they bang against each other and dull cutting edges. Many bits come in protective plastic cases; keep them there. Dedicated drill indexes (wooden blocks with holes, metal cases, or plastic organizers) protect bits and keep sizes organized and easily identifiable.

Sharp bits drill easily with light pressure; dull bits require excessive force, generate heat, and produce poor holes. Recognize when bits need sharpening: difficulty starting holes, excessive pressure required, burning smell, discolored metal from heat, or rough hole surfaces all indicate dullness. Small bits (#60 through 1/8") are economical to replace rather than sharpen. Larger bits (1/4" and up) justify sharpening, either with a drill bit sharpener tool or by a professional sharpening service.

Use appropriate speeds for different materials and bit sizes. Small bits in metal require high speeds (1500-3000 RPM) while large bits need slower speeds (350-600 RPM) to prevent overheating. Wood tolerates higher speeds than metal. Excessive speed overheats and ruins bits quickly; too slow causes grabbing and potential breakage. Cutting fluid or lubricant when drilling metal significantly extends bit life and improves hole quality.

Making Your Selection

Assess your typical drilling tasks honestly before buying drill bits. Home maintenance and general construction needs are met with a quality fractional set in 1/64" increments plus a few spade bits for large wood holes and masonry bits for concrete. This basic collection handles hanging pictures, assembling furniture, basic carpentry, and home repairs for under $50.

Woodworkers benefit from adding brad point bits for clean, accurate holes and Forstner bits for hinge mortises and joinery work. Metalworkers and mechanics need number drills for precision work and tap drills, plus cobalt or titanium-coated bits for drilling harder metals. Electricians find step drills invaluable for panel boxes and conduit work. Match your collection to your actual work rather than buying comprehensive sets you'll never fully utilize.

Quality deserves emphasis. Cheap bits from hardware store bargain bins dull after a few holes and drill inaccurately. Professional grade high-speed steel (HSS) bits from reputable brands cost 2-3 times more but last 10 times longer and perform far better. For frequently used sizes and demanding applications, cobalt steel bits (M35 or M42 grade) cost more but stay sharp much longer in hard materials like stainless steel. Buy quality in sizes you'll use often; cheap bits are false economy that cost more through replacement and poor results than investing correctly initially.