Drill Bit Sizes: How to Choose the Right Size

Drill bit sizes can feel simple until a drawing calls for a #7, a tap chart names an F drill, a packaged part lists 6.8 mm and the index on the bench is arranged in sixty-fourths. Those markings are not competing opinions about the same tool. They are different size systems, each used where it gives a shop, a drawing or a manufacturing process the right level of control.

Quick answer: select the hole requirement first, then match the bit's actual diameter, material and geometry to the work. A bit is not chosen by its label alone, and a nearby size is not automatically an acceptable substitute.

A reliable selection begins with one question: what does the finished hole need to do? A clearance hole for a bolt, a pilot hole for a screw, a tap drill, a hole for a rivet or a close-tolerance fit all start from different requirements. Once that requirement is clear, the size label becomes useful rather than confusing. This guide explains the systems, the checks that matter and the details worth sending when a shop needs the right cutting tool.

What a drill bit size actually describes

The size stamped on a standard twist drill describes its nominal outside diameter. It does not promise that every hole will measure exactly that size after drilling. Material, tool condition, spindle runout, rigidity, feed, coolant, point geometry and workholding can all influence the result. A drill can also wander if it starts on a rounded or uneven surface. That is why a drawing that calls for a precise finished hole may specify drilling followed by reaming, boring or another finishing process.

For ordinary maintenance work, the nominal diameter is still the right place to begin. Read the specification on the hardware, anchor, tap or component. Measure an existing hole only when it is known to be the intended size and not a worn or altered field repair. If the job has a manufacturer instruction, drawing or tap chart, use that source before selecting a tool from visual comparison.

The OSHA drill press training guidance also reinforces the practical side of drilling: secure the work, make adjustments with the machine stopped and keep hands clear of the cutting area. Selection and safe setup belong together. A correctly sized bit cannot compensate for a workpiece that is not properly held.

The four common drill size systems

Most industrial drill indexes contain one or more of four systems. Fractional and metric sizes state a dimension directly. Number and letter sizes are standardized series that fill many of the useful gaps between common fractional sizes. The label on the bit, the drawing and the application all need to agree on the same actual diameter.

System	How it is marked	Useful range or example	What to remember
Fractional inch	1/16, 1/4, 3/8	1/64 in to 1 in in common jobber ranges	Fractions increase as the diameter increases.
Number	#80 through #1	#80 is 0.0135 in; #1 is 0.228 in	The numbers run backward: a lower number is a larger drill.
Letter	A through Z	A is 0.234 in; Z is 0.413 in	Letters bridge much of the range above #1 and below common larger fractions.
Metric	3 mm, 6.8 mm, 10 mm	Stated directly in millimetres	Convert before substituting for an inch-based requirement.

A number drill is not a wire gauge measurement even though it may be called a wire-gauge drill. It is a drill-size series. The important practical point is its direction: #80 is tiny, and the drill gets larger as the number moves down toward #1. Letter drills then continue through the intermediate diameters from A to Z. The Slugger drill size chart provides decimal-inch and millimetre references for the fractional, number, letter and metric systems, which is useful when a shop has to translate a callout rather than guess at the nearest fraction.

Assortment of twist drill bits in a range of sizes — Read the size marking and compare the actual diameter, rather than treating all nearby drills as interchangeable.

Start with the finished-hole requirement

There is no universal best drill size for a fastener. A bolt can need a clearance hole sized for the required fit. A thread needs a tap drill selected from the thread form and pitch. A self-tapping screw may need a pilot hole that depends on the screw maker's instruction and the material. A rivet, dowel, bushing or hole saw needs its own matching range. The part, not the available index, determines the starting diameter.

For a clearance hole, identify the bolt diameter, the grade of clearance required by the assembly and whether the hole will be finished after drilling. A close assembly may use a different hole than a general-purpose field installation. For a threaded hole, use the tap maker's chart for that exact thread, including metric pitch or inch threads per inch. Do not select from a generic fraction simply because it looks close to the tap diameter. The drill must leave the material needed for the thread form.

For a replacement or repair, take the specification from the hardware package, equipment manual or existing drawing when possible. If none is available, document the mating part, its measured diameter, material thickness and the job purpose before ordering a special size. A clear photograph of the marking on the old tool or a measurement with an appropriate gauge can prevent a close-but-wrong replacement.

Use decimal equivalents before making a substitution

Fractional and metric labels may sit very near one another without being identical. A 1/4 in bit is 6.35 mm. A 6 mm bit is smaller, while a 6.5 mm bit is larger. Whether either can stand in depends on the allowed tolerance, not on whether it looks nearly the same in an index. That difference matters little for a rough opening through thin sheet, but it can matter a great deal for an anchor, tap drill, press fit or component with a specified hole size.

The same principle applies to number and letter drills. A #7 drill is 0.201 in, for example, while a nearby 13/64 in drill is 0.2031 in. That small change is not automatically harmless in a tapping operation. A decimal chart gives the actual comparison, but the relevant tap chart or product instruction decides whether the alternate size is acceptable.

When converting, record both systems on the work order or bin label: for example, “6.8 mm, 0.2677 in.” That habit makes it easier for a buyer, machinist or maintenance technician to verify the request against the available stock. It also separates an intentional alternate from a mistaken conversion.

Match the drill style to the material and hole

Diameter is only one part of drill selection. A general-purpose high-speed steel twist drill may be a practical choice for many routine materials, while cobalt, coating, point geometry, flute length, shank style or specialty construction can matter on tougher metals, thin sheet, deep holes or restricted setups. The right size in the wrong style can still run hot, dull early or leave an unsuitable result.

For common shop drilling in steel, aluminum and many other materials, a sharp twist drill with a stable setup is often the starting point. Tougher alloys, stainless steel and work-hardening materials need more attention to the tool recommendation, cutting speed, feed, coolant and consistent pressure. Letting a tool rub and generate heat can damage the cutting edge and work-harden some materials, making the next pass harder rather than easier.

Hole depth matters too. A jobber drill is a versatile general-purpose format, but deep-hole work may call for a different flute length or drilling sequence. A reduced-shank drill can provide a larger cutting diameter in a smaller-capacity chuck. A step drill is useful for selected thin-material holes, but it does not replace a twist drill, hole saw or reamer when the application needs their specific geometry or finish.

Bi-metal hole saw for making larger diameter openings — Larger openings may call for a hole saw or another dedicated cutter, not an oversized twist drill.

Point condition and setup affect the hole

A sharp, correctly selected drill still needs a stable setup. Clamp the workpiece so it cannot spin, shift or lift with the bit. Mark the location accurately and use a center punch, spotting operation or suitable pilot method where the material and tolerance call for it. Starting a drill on a rounded surface or an unrestrained thin piece makes it harder to hold location and can create a safety risk.

Keep the tool aligned with the intended hole. Excessive side pressure, a loose chuck, a bent drill or a worn spindle can increase runout and leave an oversized or rough hole. Use a feed that lets the cutting edges cut rather than skate. When chips pack in the flutes, withdraw as appropriate to clear them and use cutting fluid or coolant where the material and process require it. Do not treat heat discoloration, squealing or rapidly changing chips as normal signs of progress. They are reasons to stop and check the setup.

For a hole with a controlled fit, drilling is often the material-removal step rather than the final sizing step. Leave the correct amount of stock and use the specified reamer, boring method or finishing tool. That approach is more dependable than repeatedly forcing a twist drill to meet a precision requirement it was not selected to hold.

Three common jobs, three different selection paths

Through-hole for a bolt: Start with the bolt size and the assembly's required clearance. Confirm material thickness, whether the bolt must align closely and whether there is a washer, slot or finishing operation. Choose the bit by the documented hole diameter, then use a tool style suitable for the material.

Hole for a tap: Start with the exact tap callout, including diameter and pitch or threads per inch. Use the tap manufacturer's drill recommendation, not a guess based on the outside diameter of the fastener. Verify the actual decimal equivalent if the chart uses a number or letter drill. Use the correct tapping procedure and do not force a tap into a hole that feels wrong.

Large opening for cable, conduit or access: Start with the needed finished opening and the material. A hole saw, step drill, chassis punch or other dedicated cutter may be better than enlarging a small hole with a standard twist drill. Plan for edge protection when wire or cable passes through metal. The site's cable-management options include grommets and protective hardware that help protect a completed route.

Build a clearer cutting-tool request

A vague request for “a drill bit” forces the buyer to guess at the size system, material and intended result. A useful request states the actual size, size system, material, hole purpose, tool style when known and quantity. Add the machine or chuck limitation when it affects the shank choice. For example: “6.8 mm drill for an M8 x 1.25 tap in steel,” or “1/2 in reduced-shank drill for a 3/8 in chuck.”

Electro-Fast carries cutting-tool options including Norseman jobber, cobalt, specialty and reduced-shank drills, along with step drills, taps, reamers, dies, extractors and Morse cutting tools. The Drill Bits & Cutting Tools list is a practical place to browse the available family, while Parts Search helps when you already know a part number. For a job with an unusual callout, include the drawing note or tool marking in a quote request so the match can be checked before the work starts.

Packaged reciprocating saw blades for material cutting — Match the cutting tool to the work. Drills, hole saws and reciprocating blades each solve different cutting jobs.

Read the marking on the shank, not the color

Finish color can indicate a coating or treatment, but it is not a dependable way to identify either the drill diameter or its intended material. Different manufacturers use different finishes, and a tool that has seen service may be discoloured by heat, coolant or storage. The shank marking, package label and product information are the useful sources for size, construction and part identification.

When a marking is faint, clean the shank carefully and use good light before trying to measure it. A drill gauge can help identify a loose fractional drill, while calipers or a micrometer can establish the outside diameter where the tool is in sound condition. Measure the cylindrical body, not the cutting lips, and understand that a worn, damaged or modified tool may no longer represent its original nominal size. For a critical replacement, the original part number is better evidence than an approximate measurement.

Keep drill indexes organized by their actual system. Mixing fractional, number, letter and metric drills in one unlabeled drawer turns a simple stock check into a conversion problem. A readable index label makes it easier to return a tool to the correct location and spot gaps before a job starts. For commonly repeated work, write the required drill callout beside the tap, fixture or work instruction rather than depending on memory.

Choose the process for the material thickness

Material thickness changes the drilling plan. Thin sheet can grab as a twist drill breaks through, distort around the hole or leave a burr that affects the next assembly step. Backing material, a suitable step drill, controlled feed and deburring may help produce a cleaner result, depending on the material and application. A large hole in thin sheet often benefits from a cutter designed for that job rather than a sequence of increasingly larger twist drills.

Thicker material creates more chip volume and more heat. Confirm that the flute length can reach the required depth while the machine still supports the drill correctly. Use a stable setup and clear chips as needed rather than packing them into the flutes. If the hole is deep, positioned near an edge or drilled through an irregular surface, slow down the planning process. The setup may need a different tool length, a pilot operation, a fixture or a staged drilling sequence.

Material condition matters as much as material name. Rust, scale, coatings, layered assemblies, hardened spots and prior welds can change how a bit enters and cuts. Where a fastener must pass through more than one layer, confirm that the intended hole location and diameter work for the complete stack, not only the exposed outer piece. If a hole is part of a repair on equipment, inspect the opposite side for lines, harnesses, hoses and components before drilling.

Inspect and replace drills before they spoil the job

A dull or chipped drill can create more work than it saves. Warning signs include excessive pressure, blueing from heat, squealing, long wrapped chips, a rough edge, wandering at the start or a hole that repeatedly measures oversize. Stop when the tool is no longer cutting cleanly. Pushing harder can damage the workpiece, work-harden the material or create an unsafe condition.

Inspect the cutting lips and point for chipping, uneven wear or a damaged web. Check the shank for scoring that could affect chuck grip. A drill that has been sharpened may be useful when it has been ground correctly for the application, but an uneven grind can cut an oversized hole or pull to one side. For critical work, replace or professionally recondition a questionable tool instead of making it carry a tolerance it cannot reliably hold.

Store drills dry, clean and separated enough that their cutting edges do not knock together. Return them to their marked locations after use, remove chips before closing an index and keep broken tools out of the active set. Those small habits protect the size identification and make a complete tool kit available when a repair is on the clock.

When to stop and verify the specification

Pause before drilling when the hole affects a pressure boundary, a structural member, a guarded machine, an electrical enclosure, a fuel or hydraulic path, or a component with a manufacturer service procedure. The required location, hole size, edge distance and finishing method may be controlled by the equipment design. An ordinary drill chart cannot replace that requirement.

The same pause is worthwhile when a part marking, drawing callout or fastener thread does not agree with the tool available. Record the exact information, compare the decimal dimensions and get the correct tool or confirmation before cutting. A few minutes spent verifying a drill size is usually less costly than repairing an enlarged, misplaced or incorrectly threaded hole.

That verification is especially valuable on repeat jobs. Once a size and process are confirmed, keep the supporting chart, tool number and application note with the work instruction. The next technician can repeat the successful process without rebuilding the decision from a mixed drill index or an incomplete memory of the earlier repair. A documented process also makes stock replenishment more accurate, because the buyer can request the same drill series and size instead of a merely similar alternative. It also gives the team a repeatable reference when the original installer is unavailable, helps reduce downtime, and supports consistent workmanship across future repairs.

Drill-bit selection checklist

Define the hole: identify whether it is clearance, pilot, tap, rivet, access or a controlled-fit hole.
Confirm the actual diameter: translate number, letter, fractional or metric markings through a decimal chart when needed.
Check the material: select the drill construction and geometry for the workpiece, not only the diameter.
Check the machine and setup: verify chuck capacity, shank style, workholding, alignment and the required drilling method.
Plan the finished edge: account for deburring, reaming, grommets, corrosion protection or the next operation before drilling.

That short process prevents the most common mistakes: choosing a near size without checking the decimal difference, using a clearance-hole assumption for a tap drill, selecting a tool without considering the material, or treating a final precision hole as an ordinary drilling operation. It also makes the parts request clear enough for the right tool to reach the bench the first time.

Frequently asked questions

How do I know what drill bit size to use?

Start with the required hole diameter or the fastener, anchor, tap or component specification. Then confirm the material, hole type and tolerance. The bit label tells you the tool diameter, but the correct choice comes from the job requirement, not from the closest-looking bit in an index.

Are number and letter drill bits smaller or larger as the label increases?

Number drills get smaller as the number rises, so a #80 is very small and a #1 is much larger. Letter drills run upward from A through Z. Neither system is a simple fraction, which is why a decimal-equivalent chart is useful when the drawing or tap chart calls out a number or letter size.

Can I substitute a metric drill for a fractional drill?

Only when the actual decimal difference is acceptable for the job. A nearby metric size can be useful for a clearance hole or rough work, but it may be unsuitable for a tap drill, dowel, bearing fit or other controlled hole. Compare the stated diameter before substituting.

Why does a drilled hole sometimes measure larger than the bit?

Runout, bit wear, drill condition, workholding, feed, material and the drill point can all affect the finished hole. Where the size matters, use the appropriate drilling process and leave material for a reamer or other finishing operation when the specification calls for it.