Drill Bit Sizes: Charts, Charts, and Quick Picks

A 1/4-inch bit cuts a 1/4-inch hole, and that nominal cutting diameter,not the drill body or motor,sets the size. A 1/4-inch bit aims for a 1/4-inch cut, though wear, feed, and material can nudge the result. That small gap affects screw fit, taps, and clearance.

Anyone drilling wood, metal, plastic, or sheet goods needs the right size system for the job. The sections below cover chart reading, size order, and practical picks without the usual guesswork.

The Four Drill Size Systems in Everyday Use

Four naming systems can sit in the same shop drawer, and each one still points to diameter. Fractional inch sizes cover common U.S. work, metric drills serve drawings in millimeters, and number and letter gauges fill the gaps between them.

Engineering ToolBox and similar shop charts group these systems because no single set lands every hole size exactly. That is why the chart matters more than the label on the shank.

Fractional Inch Sizes Lead Most U.S. Work

1/8, 3/16, 1/4, and 3/8 show up in wood shops, job sites, and general metal work. The spacing is easy to read, but it leaves gaps, so a bit like 7/32 can matter more than expected.

Fractional drill bit sizes are handy for anchors, lag screws, and ordinary clearance holes. The label tells you the cutting diameter, while flute length and shank style tell you something else entirely.

Metric Millimeters Follow the Drawing

Metric drill bit sizes are printed in millimeters, so a 5.0 mm bit is a 5.0 mm diameter tool. ISO-based fastener and tap specs often start in millimeters, which keeps the shop note and the drill chart in the same unit.

The inch conversion is close, not exact. A 6.0 mm bit lands near 15/64 inch, but that doesn’t make the two twins.

Number and Letter Gauges Fill the Small-Gap Zone

Number and letter drill sizes matter once fractions get too coarse. Number drills run through tiny diameters for electronics, tapping, and fine pilot holes, while letter drills fill the middle band between number and fractional series.

Machinists use them because the steps are tighter. A #7, a letter F, and a 1/4 bit sit in different spots on the same size line, even though the labels look unrelated.

Nominal Diameter Is the Real Meaning of Size

Drill bit size means the cutting diameter at the margins of the bit. It doesn’t describe power, flute length, or shank length, and a reduced-shank bit can still cut a smaller hole than its body suggests.

Milwaukee Tool, DEWALT, and other tool makers sort bits by diameter for that reason. Once the label is tied to the cut size, the chart starts making sense, and the next step is reading it without guessing.

That connection makes the numbers practical, and the chart becomes easier to read at a glance.

Reading a Drill Size Chart Without Guesswork

A good chart starts with the hole you need, not the bit already in hand. Once the target diameter is known, fractional, metric, number, or letter listings can be matched by decimal equivalent.

NIST defines the inch-to-millimeter relationship used in these conversions, and that fixed unit relationship keeps the math honest. The chart does the translation, while the job is to pick the size that fits the function.

Start With the Hole Diameter You Need

Fasteners, taps, and drawings all ask for a final hole size. A clearance hole for a bolt, a pilot hole for a screw, and a tap drill each call for a different diameter even when the hardware label looks similar.

That distinction matters because the drill chart is a tool, not a guess. The required hole diameter comes first, and the printed bit size comes after.

Use Decimal Equivalents To Compare Systems

Decimal equivalents put 1/4, 6.0 mm, #7, and letter F into one line of numbers. That makes cross-system comparison easier than staring at three different naming schemes.

McMaster-Carr, Kennametal, and Engineering ToolBox all present size tables this way because decimals reveal the actual cutting diameter. Once the decimal is visible, the chart stops feeling like a code sheet.

Treat Cross-System Matches As Close Fits

Metric and imperial drill sizes are approximate equivalents, not exact twins. A 1/4 inch bit is 0.250 inch, while 6 mm is about 0.236 inch, and that difference can matter in close work.

That is why a print, a tap chart, or a clearance chart beats eyeballing the nearest label. To stay out of fit trouble, match the spec, not the nearest-looking number.

Label	Decimal Diameter	Rough Match
1/4 inch	0.250 inch	6.35 mm
7/32 inch	0.219 inch	5.56 mm
3/8 inch	0.375 inch	9.53 mm
1/2 inch	0.500 inch	12.70 mm

That table is enough for a bench check, but it doesn’t replace a full drill size chart. The next step is sorting the labels from small to large, because that’s where mistakes start in a hurry.

Once the labels are clear, a simple sequence reveals which size comes first and which comes last.

Ordering Sizes From Smallest to Largest at a Glance

Larger labels usually mean larger diameters, but the numbering system can hide that fact. A 1/8 bit is smaller than a 3/16 bit, and a #40 is smaller still, even though the symbols feel unrelated.

Britannica notes that drill bits are made in standard sizes, and the size order follows nominal diameter, not label style. Once that rule clicks, quick comparisons become easy.

Quick Comparisons Resolve the Common Mix-Ups

• ▸5/16 beats 1/4: 5/16 inch is larger because 0.3125 inch exceeds 0.250 inch.
• ▸7/32 loses to 1/4: 7/32 inch is smaller because 0.21875 inch sits below 0.250 inch.
• ▸1/2 beats 3/8: 1/2 inch is larger because 0.500 inch exceeds 0.375 inch by a wide margin.
• ▸Metric needs decimals: 8 mm is larger than 5/16 inch, but 6 mm is smaller than 1/4 inch.
• ▸Number drills run small: a #60 bit is far smaller than common fractional sizes used for hardware.

That list clears up the common shop-floor comparisons fast. The trick is to move from the label to the decimal, then back to the application, which is where the real choice happens.

Spot the Bigger Bit Across Different Systems

A side-by-side chart makes the answer visible in seconds. Number sizes shrink as the number rises, letter sizes grow as the letters move forward, and fractional sizes grow as the fraction rises.

That pattern is the same in most drill size charts, including shop charts from McMaster-Carr and IRWIN. Once the direction of each series is known, the biggest bit is obvious even before conversion.

With the order set, the remaining choice is matching the bit to the task before cutting begins.

Picking the Right Bit for the Job

Hole function matters more than the printed size. A screw pilot, a through-hole for a bolt, and a tap drill all ask for different diameters, even when the hardware looks close in hand.

MIT machine-shop notes and U.S. Department of Energy machining resources both stress that point. The drill size follows the job, and the chart only translates the job into a diameter.

Pilot Holes Guide Screws and Reduce Splitting

A pilot hole gives the screw a path and cuts down on splitting in wood or brittle stock. In hardwood, the pilot often needs to stay smaller than the screw root, not the outer thread width.

That choice lowers driving torque and keeps the screw from wandering. A split-point bit can help the bit stay centered, but the size still has to suit the screw.

Clearance Holes Let Fasteners Pass Cleanly

A clearance hole is sized so the bolt or screw shank passes through without binding. Close, normal, and loose clearance all use different diameters, and the hardware label alone doesn’t settle it.

For a 3/8-inch bolt, the right hole may be larger than 3/8 inch, depending on fit and alignment needs. That is why a clearance hole chart belongs near the drill press.

Tap Drills Follow Thread Pitch, Not Just Diameter

A tap drill leaves material for internal threads to form. The right size depends on the thread pitch and the thread engagement wanted, so 1/4-20 and 1/4-28 don’t share the same tap drill.

That pitch-sensitive rule is the part many charts hide in plain sight. For example, 1/4-20 UNC in steel often uses 13/64 inch, while M6 x 1.0 in aluminum often uses 5.0 mm. The nominal thread size is only part of the decision.

“Tap drills are a compromise. A smaller hole gives more thread engagement, but it also raises tap torque and breakage risk in harder stock.

That compromise is a materials problem as much as a geometry problem. Steel, aluminum, brass, and plastic all respond differently, so the same nominal thread can need a different working diameter in the shop.

Even a correct diameter can behave differently once the material and tool shape start influencing the cut.

Material, Wear, and Geometry Change the Final Hole

The label on the box is only the starting point. Real holes can land slightly larger or smaller because drill wear, feed rate, runout, and point angle all change how the cut behaves.

Sandvik Coromant and Penn State machining materials both point to that same mechanism. The bit diameter stays nominal, but the hole finished may not.

Wear Changes the Cutting Edge

As the lips and margins dull, the tool rubs more than it cuts. That can enlarge the hole, smear softer stock, or leave a rough edge that no chart can predict.

Soft aluminum and gummy plastics can drift larger faster than harder steels because the worn edge starts pushing material aside. Tool wear is a size problem before it is a finish problem.

Feed Rate and Drill Angle Shift the Result

Fast feed can push the bit off line, while slow feed with poor chip clearing can rub and heat the cut. Both affect final size, straightness, and surface quality.

Point angle matters too. A 118-degree point and a 135-degree split point don’t cut the same way, even though the nominal diameter stays unchanged; the geometry changes centering and thrust, not the listed size.

Runout and Deflection Add Small Errors

Runout means the bit does not spin on a perfect centerline. Even a small wobble can make a hole oversize, especially with long drills or thin stock.

That is why stub length drills feel steadier than long series drills. The diameter label stays the same, but rigidity changes how faithfully the cut follows the label.

For tight tolerance work, a drilled hole is often only a rough step. Reaming comes after drilling when the final diameter needs a cleaner finish and a tighter size band.

That extra finishing step points to a quick reference that keeps common sizes and checks close at hand.

A Compact Reference for Shop and DIY Work

Metric makes sense when the drawing, tap, or fastener already speaks millimeters. Imperial is easier when the hardware bin, tape measure, and wrenches live in inch fractions.

NYU Tandon and MIT shop resources both point to the same shop rule: use the system that matches the job, then convert only when the spec forces a crossover.

The Most Useful Decimal Habits

• ▸Know 1/4: 1/4 inch equals 0.250 inch, a fast anchor for quick checks.
• ▸Know 1/2: 1/2 inch equals 0.500 inch, which helps size larger clearances.
• ▸Know 1 mm: 1 mm is 0.03937 inch, the base for metric conversion.
• ▸Know 6 mm: 6 mm equals 0.236 inch, close enough to help with rough selection.
• ▸Know 13/64: 13/64 inch equals 0.203125 inch, a common tap-drill reference point.

Those numbers cover a lot of hand work without opening a calculator. A drill size conversion chart still does the final check, but these anchor values cut the time in half.

A Bench-Friendly Way To Choose Fast

Start with the hole type, then read the chart for the target diameter, then pick the closest bit in the same system. That order works for pilot holes, clearance holes, and tap drills.

For mixed hardware bins, mark the decimal equivalent on the case or drawer. That one habit keeps metric drill bit sizes and fractional inches from turning into a guessing game.

Drill bit sizes stay simple once diameter becomes the only thing that matters. Match the hole function, read the decimal, and check the chart before pulling the trigger on the drill.

FAQ