Drill Bit Sizes Chart - Complete Reference Guide for 2026
Here's something peculiar: A #7 drill bit is bigger than a 1/4" bit. A 6mm bit doesn't match any standard fractional size. And that F-sized bit in your index? It's exactly 0.257 inches - a dimension that makes sense to absolutely nobody until you understand the system behind it.
The drill bit sizing chaos isn't random. It's actually four different measurement systems crashing into each other - fractional inches from British tradition, metric from the ISO standard, number gauges from American wire manufacturing, and letter sizes from aviation specs. Whether you're using an impact driver or drill, understanding these systems matters. Every hardware store rack displays this collision of standards, sorted smallest to largest, creating that familiar confusion when you're standing there with a 10mm bolt wondering whether you need a 3/8" bit or a 25/64" bit for the job.
The numbers tell an interesting story about global manufacturing. American factories still default to fractional inches. European and Asian manufacturers run metric. Aviation and precision machining use number and letter gauges. Amazon listings show drill indexes with all four systems because nobody wants to stock four different sets. The result: that 115-piece drill bit set where half the bits are duplicates with different labels.
Understanding the Four Drill Bit Sizing Systems
Walk into any hardware store and you'll encounter drill bits labeled in fractions, decimals, numbers, and letters. This isn't marketing confusion - it's the result of different industries developing their own standards over the past century.
The fractional inch system dominates American construction and woodworking. These bits run from 1/64" up to 1" or larger, always in fractional increments. The jumps between sizes follow a pattern - typically doubling the denominator to add sizes between existing ones. That's why you see 1/4", then 17/64", then 9/32" - they're filling gaps in 64ths of an inch.
Metric sizing follows millimeter increments, typically from 1mm to 13mm for standard sets, extending to 25mm or larger for specialty work. The metric system's decimal nature makes the math simpler - a 6.5mm bit is exactly halfway between 6mm and 7mm. No fraction conversion needed.
Number gauge drill bits run from #80 (0.0135") to #1 (0.2280"), with the numbers working backwards - smaller numbers mean bigger bits. This system emerged from wire manufacturing, where the same backwards numbering still applies. These sizes fill specific gaps between fractional sizes, particularly in the smaller ranges where precision matters.
Letter gauge bits run from A (0.234") to Z (0.413"), giving 26 additional sizes between common fractional measurements. Originally developed for aircraft manufacturing, these sizes appear in tap and die sets because they match specific thread specifications that don't align with standard fractions.
Complete Drill Bit Size Chart - Fractional, Number, Letter, and Metric
The measurement overlap creates interesting patterns. A 1/4" bit measures 6.35mm - close enough to 6mm that some manufacturers label them interchangeably. But that 0.35mm difference matters when precision counts. Machine shops keep conversion charts posted on walls because guessing costs money when tolerances are tight.
Smallest to Largest - Combined Systems
| Size | Decimal Inches | Millimeters | System Type |
|---|---|---|---|
| #80 | 0.0135" | 0.343mm | Number |
| #79 | 0.0145" | 0.368mm | Number |
| 1/64" | 0.0156" | 0.397mm | Fractional |
| #78 | 0.0160" | 0.406mm | Number |
| #77 | 0.0180" | 0.457mm | Number |
| #76 | 0.0200" | 0.508mm | Number |
| #75 | 0.0210" | 0.533mm | Number |
| #74 | 0.0225" | 0.572mm | Number |
| #73 | 0.0240" | 0.610mm | Number |
| #72 | 0.0250" | 0.635mm | Number |
| #71 | 0.0260" | 0.660mm | Number |
| #70 | 0.0280" | 0.711mm | Number |
| #69 | 0.0292" | 0.742mm | Number |
| #68 | 0.0310" | 0.787mm | Number |
| 1/32" | 0.0313" | 0.794mm | Fractional |
| #67 | 0.0320" | 0.813mm | Number |
| #66 | 0.0330" | 0.838mm | Number |
| #65 | 0.0350" | 0.889mm | Number |
| #64 | 0.0360" | 0.914mm | Number |
| #63 | 0.0370" | 0.940mm | Number |
| #62 | 0.0380" | 0.965mm | Number |
| #61 | 0.0390" | 0.991mm | Number |
| #60 | 0.0400" | 1.016mm | Number |
| #59 | 0.0410" | 1.041mm | Number |
| #58 | 0.0420" | 1.067mm | Number |
| #57 | 0.0430" | 1.092mm | Number |
| #56 | 0.0465" | 1.181mm | Number |
| 3/64" | 0.0469" | 1.191mm | Fractional |
| #55 | 0.0520" | 1.321mm | Number |
| #54 | 0.0550" | 1.397mm | Number |
| #53 | 0.0595" | 1.511mm | Number |
| 1/16" | 0.0625" | 1.588mm | Fractional |
| #52 | 0.0635" | 1.613mm | Number |
| #51 | 0.0670" | 1.702mm | Number |
| #50 | 0.0700" | 1.778mm | Number |
| #49 | 0.0730" | 1.854mm | Number |
| #48 | 0.0760" | 1.930mm | Number |
| 5/64" | 0.0781" | 1.984mm | Fractional |
| #47 | 0.0785" | 1.994mm | Number |
| #46 | 0.0810" | 2.057mm | Number |
| #45 | 0.0820" | 2.083mm | Number |
| #44 | 0.0860" | 2.184mm | Number |
| #43 | 0.0890" | 2.261mm | Number |
| #42 | 0.0935" | 2.375mm | Number |
| 3/32" | 0.0938" | 2.381mm | Fractional |
| #41 | 0.0960" | 2.438mm | Number |
| #40 | 0.0980" | 2.489mm | Number |
| #39 | 0.0995" | 2.527mm | Number |
| #38 | 0.1015" | 2.578mm | Number |
| #37 | 0.1040" | 2.642mm | Number |
| #36 | 0.1065" | 2.705mm | Number |
| 7/64" | 0.1094" | 2.778mm | Fractional |
| #35 | 0.1100" | 2.794mm | Number |
| #34 | 0.1110" | 2.819mm | Number |
| #33 | 0.1130" | 2.870mm | Number |
| #32 | 0.1160" | 2.946mm | Number |
| #31 | 0.1200" | 3.048mm | Number |
| 1/8" | 0.1250" | 3.175mm | Fractional |
| #30 | 0.1285" | 3.264mm | Number |
| #29 | 0.1360" | 3.454mm | Number |
| #28 | 0.1405" | 3.569mm | Number |
| 9/64" | 0.1406" | 3.572mm | Fractional |
| #27 | 0.1440" | 3.658mm | Number |
| #26 | 0.1470" | 3.734mm | Number |
| #25 | 0.1495" | 3.797mm | Number |
| #24 | 0.1520" | 3.861mm | Number |
| #23 | 0.1540" | 3.912mm | Number |
| 5/32" | 0.1563" | 3.969mm | Fractional |
| #22 | 0.1570" | 3.988mm | Number |
| #21 | 0.1590" | 4.039mm | Number |
| #20 | 0.1610" | 4.089mm | Number |
| #19 | 0.1660" | 4.216mm | Number |
| #18 | 0.1695" | 4.305mm | Number |
| 11/64" | 0.1719" | 4.366mm | Fractional |
| #17 | 0.1730" | 4.394mm | Number |
| #16 | 0.1770" | 4.496mm | Number |
| #15 | 0.1800" | 4.572mm | Number |
| #14 | 0.1820" | 4.623mm | Number |
| #13 | 0.1850" | 4.699mm | Number |
| 3/16" | 0.1875" | 4.763mm | Fractional |
| #12 | 0.1890" | 4.801mm | Number |
| #11 | 0.1910" | 4.851mm | Number |
| #10 | 0.1935" | 4.915mm | Number |
| #9 | 0.1960" | 4.978mm | Number |
| #8 | 0.1990" | 5.055mm | Number |
| #7 | 0.2010" | 5.105mm | Number |
| 13/64" | 0.2031" | 5.159mm | Fractional |
| #6 | 0.2040" | 5.182mm | Number |
| #5 | 0.2055" | 5.220mm | Number |
| #4 | 0.2090" | 5.309mm | Number |
| #3 | 0.2130" | 5.410mm | Number |
| 7/32" | 0.2188" | 5.556mm | Fractional |
| #2 | 0.2210" | 5.613mm | Number |
| #1 | 0.2280" | 5.791mm | Number |
| A | 0.2340" | 5.944mm | Letter |
| 15/64" | 0.2344" | 5.953mm | Fractional |
| B | 0.2380" | 6.045mm | Letter |
| C | 0.2420" | 6.147mm | Letter |
| D | 0.2460" | 6.248mm | Letter |
| 1/4" | 0.2500" | 6.350mm | Fractional |
| E | 0.2500" | 6.350mm | Letter |
| F | 0.2570" | 6.528mm | Letter |
| G | 0.2610" | 6.629mm | Letter |
| 17/64" | 0.2656" | 6.747mm | Fractional |
| H | 0.2660" | 6.756mm | Letter |
| I | 0.2720" | 6.909mm | Letter |
| J | 0.2770" | 7.036mm | Letter |
| K | 0.2810" | 7.137mm | Letter |
| 9/32" | 0.2813" | 7.144mm | Fractional |
| L | 0.2900" | 7.366mm | Letter |
| M | 0.2950" | 7.493mm | Letter |
| 19/64" | 0.2969" | 7.541mm | Fractional |
| N | 0.3020" | 7.671mm | Letter |
| 5/16" | 0.3125" | 7.938mm | Fractional |
| O | 0.3160" | 8.026mm | Letter |
| P | 0.3230" | 8.204mm | Letter |
| 21/64" | 0.3281" | 8.334mm | Fractional |
| Q | 0.3320" | 8.433mm | Letter |
| R | 0.3390" | 8.611mm | Letter |
| 11/32" | 0.3438" | 8.731mm | Fractional |
| S | 0.3480" | 8.839mm | Letter |
| T | 0.3580" | 9.093mm | Letter |
| 23/64" | 0.3594" | 9.128mm | Fractional |
| U | 0.3680" | 9.347mm | Letter |
| 3/8" | 0.3750" | 9.525mm | Fractional |
| V | 0.3770" | 9.576mm | Letter |
| W | 0.3860" | 9.804mm | Letter |
| 25/64" | 0.3906" | 9.922mm | Fractional |
| X | 0.3970" | 10.084mm | Letter |
| Y | 0.4040" | 10.262mm | Letter |
| 13/32" | 0.4063" | 10.319mm | Fractional |
| Z | 0.4130" | 10.490mm | Letter |
| 27/64" | 0.4219" | 10.716mm | Fractional |
| 7/16" | 0.4375" | 11.113mm | Fractional |
| 29/64" | 0.4531" | 11.509mm | Fractional |
| 15/32" | 0.4688" | 11.906mm | Fractional |
| 31/64" | 0.4844" | 12.303mm | Fractional |
| 1/2" | 0.5000" | 12.700mm | Fractional |
| 33/64" | 0.5156" | 13.097mm | Fractional |
| 17/32" | 0.5313" | 13.494mm | Fractional |
| 35/64" | 0.5469" | 13.891mm | Fractional |
| 9/16" | 0.5625" | 14.288mm | Fractional |
| 37/64" | 0.5781" | 14.684mm | Fractional |
| 19/32" | 0.5938" | 15.081mm | Fractional |
| 39/64" | 0.6094" | 15.478mm | Fractional |
| 5/8" | 0.6250" | 15.875mm | Fractional |
| 41/64" | 0.6406" | 16.272mm | Fractional |
| 21/32" | 0.6563" | 16.669mm | Fractional |
| 43/64" | 0.6719" | 17.066mm | Fractional |
| 11/16" | 0.6875" | 17.463mm | Fractional |
| 45/64" | 0.7031" | 17.859mm | Fractional |
| 23/32" | 0.7188" | 18.256mm | Fractional |
| 47/64" | 0.7344" | 18.653mm | Fractional |
| 3/4" | 0.7500" | 19.050mm | Fractional |
| 49/64" | 0.7656" | 19.447mm | Fractional |
| 25/32" | 0.7813" | 19.844mm | Fractional |
| 51/64" | 0.7969" | 20.241mm | Fractional |
| 13/16" | 0.8125" | 20.638mm | Fractional |
| 53/64" | 0.8281" | 21.034mm | Fractional |
| 27/32" | 0.8438" | 21.431mm | Fractional |
| 55/64" | 0.8594" | 21.828mm | Fractional |
| 7/8" | 0.8750" | 22.225mm | Fractional |
| 57/64" | 0.8906" | 22.622mm | Fractional |
| 29/32" | 0.9063" | 23.019mm | Fractional |
| 59/64" | 0.9219" | 23.416mm | Fractional |
| 15/16" | 0.9375" | 23.813mm | Fractional |
| 61/64" | 0.9531" | 24.209mm | Fractional |
| 31/32" | 0.9688" | 24.606mm | Fractional |
| 63/64" | 0.9844" | 25.003mm | Fractional |
| 1" | 1.0000" | 25.400mm | Fractional |
Common Metric Sizes and Their Fractional Equivalents
The metric-to-fractional conversion creates persistent confusion in hardware stores. A 10mm bolt seems like it should use a 10mm drill bit for the shaft clearance. But 10mm converts to 0.3937 inches - larger than 3/8" (0.375") but smaller than 13/32" (0.4063"). Machine shops keep digital calipers handy because eyeballing the difference between 10mm and 3/8" leads to stuck bolts or sloppy fits.
| Metric Size | Decimal Inches | Closest Fractional | Difference |
|---|---|---|---|
| 1mm | 0.0394" | 3/64" (0.0469") | +0.0075" |
| 1.5mm | 0.0591" | 1/16" (0.0625") | +0.0034" |
| 2mm | 0.0787" | 5/64" (0.0781") | -0.0006" |
| 2.5mm | 0.0984" | 3/32" (0.0938") | -0.0046" |
| 3mm | 0.1181" | 1/8" (0.1250") | +0.0069" |
| 3.5mm | 0.1378" | 9/64" (0.1406") | +0.0028" |
| 4mm | 0.1575" | 5/32" (0.1563") | -0.0012" |
| 4.5mm | 0.1772" | 11/64" (0.1719") | -0.0053" |
| 5mm | 0.1969" | 13/64" (0.2031") | +0.0062" |
| 5.5mm | 0.2165" | 7/32" (0.2188") | +0.0023" |
| 6mm | 0.2362" | 15/64" (0.2344") | -0.0018" |
| 6.5mm | 0.2559" | 17/64" (0.2656") | +0.0097" |
| 7mm | 0.2756" | 9/32" (0.2813") | +0.0057" |
| 7.5mm | 0.2953" | 19/64" (0.2969") | +0.0016" |
| 8mm | 0.3150" | 5/16" (0.3125") | -0.0025" |
| 8.5mm | 0.3346" | 21/64" (0.3281") | -0.0065" |
| 9mm | 0.3543" | 23/64" (0.3594") | +0.0051" |
| 9.5mm | 0.3740" | 3/8" (0.3750") | +0.0010" |
| 10mm | 0.3937" | 25/64" (0.3906") | -0.0031" |
| 11mm | 0.4331" | 7/16" (0.4375") | +0.0044" |
| 12mm | 0.4724" | 15/32" (0.4688") | -0.0036" |
| 13mm | 0.5118" | 33/64" (0.5156") | +0.0038" |
| 14mm | 0.5512" | 35/64" (0.5469") | -0.0043" |
| 15mm | 0.5906" | 19/32" (0.5938") | +0.0032" |
| 16mm | 0.6299" | 5/8" (0.6250") | -0.0049" |
| 17mm | 0.6693" | 43/64" (0.6719") | +0.0026" |
| 18mm | 0.7087" | 45/64" (0.7031") | -0.0056" |
| 19mm | 0.7480" | 47/64" (0.7344") | -0.0136" |
| 20mm | 0.7874" | 25/32" (0.7813") | -0.0061" |
| 21mm | 0.8268" | 53/64" (0.8281") | +0.0013" |
| 22mm | 0.8661" | 55/64" (0.8594") | -0.0067" |
| 23mm | 0.9055" | 29/32" (0.9063") | +0.0008" |
| 24mm | 0.9449" | 15/16" (0.9375") | -0.0074" |
| 25mm | 0.9843" | 63/64" (0.9844") | +0.0001" |
Tap Drill Sizes - The Critical Measurements
Tap drill sizing represents where precision actually matters. Too small and you'll snap the tap. Too large and the threads won't hold. The 75% thread engagement rule drives most of these calculations - you want the drill to remove enough material for clean threading while leaving enough base material for strength.
| Thread Size | Tap Drill Size | Decimal Equivalent | Thread Pitch |
|---|---|---|---|
| #0-80 UNF | 3/64" | 0.0469" | 80 TPI |
| #1-64 UNC | #53 | 0.0595" | 64 TPI |
| #1-72 UNF | #53 | 0.0595" | 72 TPI |
| #2-56 UNC | #50 | 0.0700" | 56 TPI |
| #2-64 UNF | #50 | 0.0700" | 64 TPI |
| #3-48 UNC | #47 | 0.0785" | 48 TPI |
| #3-56 UNF | #45 | 0.0820" | 56 TPI |
| #4-40 UNC | #43 | 0.0890" | 40 TPI |
| #4-48 UNF | #42 | 0.0935" | 48 TPI |
| #5-40 UNC | #38 | 0.1015" | 40 TPI |
| #5-44 UNF | #37 | 0.1040" | 44 TPI |
| #6-32 UNC | #36 | 0.1065" | 32 TPI |
| #6-40 UNF | #33 | 0.1130" | 40 TPI |
| #8-32 UNC | #29 | 0.1360" | 32 TPI |
| #8-36 UNF | #29 | 0.1360" | 36 TPI |
| #10-24 UNC | #25 | 0.1495" | 24 TPI |
| #10-32 UNF | #21 | 0.1590" | 32 TPI |
| #12-24 UNC | #16 | 0.1770" | 24 TPI |
| #12-28 UNF | #14 | 0.1820" | 28 TPI |
| 1/4"-20 UNC | #7 | 0.2010" | 20 TPI |
| 1/4"-28 UNF | #3 | 0.2130" | 28 TPI |
| 5/16"-18 UNC | F | 0.2570" | 18 TPI |
| 5/16"-24 UNF | I | 0.2720" | 24 TPI |
| 3/8"-16 UNC | 5/16" | 0.3125" | 16 TPI |
| 3/8"-24 UNF | Q | 0.3320" | 24 TPI |
| 7/16"-14 UNC | U | 0.3680" | 14 TPI |
| 7/16"-20 UNF | 25/64" | 0.3906" | 20 TPI |
| 1/2"-13 UNC | 27/64" | 0.4219" | 13 TPI |
| 1/2"-20 UNF | 29/64" | 0.4531" | 20 TPI |
| 9/16"-12 UNC | 31/64" | 0.4844" | 12 TPI |
| 9/16"-18 UNF | 33/64" | 0.5156" | 18 TPI |
| 5/8"-11 UNC | 17/32" | 0.5313" | 11 TPI |
| 5/8"-18 UNF | 37/64" | 0.5781" | 18 TPI |
| 3/4"-10 UNC | 21/32" | 0.6563" | 10 TPI |
| 3/4"-16 UNF | 11/16" | 0.6875" | 16 TPI |
| 7/8"-9 UNC | 49/64" | 0.7656" | 9 TPI |
| 7/8"-14 UNF | 13/16" | 0.8125" | 14 TPI |
| 1"-8 UNC | 7/8" | 0.8750" | 8 TPI |
| 1"-14 UNF | 15/16" | 0.9375" | 14 TPI |
Pilot Holes for Wood Screws - Material Matters
Wood screw pilot holes follow a different logic than machine screws. The pilot hole needs to clear the shank while letting threads bite into the wood. Hardwoods and softwoods require different approaches - oak might need a pilot hole 75% of the screw diameter, while pine works with 50% or less. The wood's moisture content changes these numbers too. Kiln-dried lumber splits easier than green wood, requiring larger pilot holes.
| Screw Size | Hardwood Pilot | Softwood Pilot | Shank Clearance |
|---|---|---|---|
| #2 (0.086") | 5/64" | 1/16" | 3/32" |
| #3 (0.099") | 1/16" | 1/16" | 7/64" |
| #4 (0.112") | 1/16" | 1/16" | 7/64" |
| #5 (0.125") | 5/64" | 1/16" | 1/8" |
| #6 (0.138") | 3/32" | 5/64" | 9/64" |
| #7 (0.151") | 3/32" | 5/64" | 5/32" |
| #8 (0.164") | 7/64" | 3/32" | 11/64" |
| #9 (0.177") | 1/8" | 3/32" | 3/16" |
| #10 (0.190") | 1/8" | 7/64" | 3/16" |
| #12 (0.216") | 9/64" | 7/64" | 7/32" |
| #14 (0.242") | 5/32" | 1/8" | 1/4" |
| #16 (0.268") | 3/16" | 9/64" | 17/64" |
| #18 (0.294") | 13/64" | 5/32" | 19/64" |
| #20 (0.320") | 15/64" | 3/16" | 21/64" |
Wire Gauge Drill Sizes - Electrical Reality
Electrical work brings its own drill bit requirements. Wire gauge sizes determine conduit holes, and the National Electrical Code specifies minimum clearances. A 12 AWG Romex cable measures about 0.3" in diameter with sheathing, but electricians drill 3/4" holes for single runs. The extra space prevents insulation damage during pulling and allows for thermal expansion.
| Wire Gauge | Wire Diameter | Typical Hole Size | NEC Minimum |
|---|---|---|---|
| 18 AWG | 0.040" | 1/4" | 3/16" |
| 16 AWG | 0.051" | 1/4" | 3/16" |
| 14 AWG | 0.064" | 5/16" | 1/4" |
| 12 AWG | 0.081" | 3/8" | 5/16" |
| 10 AWG | 0.102" | 7/16" | 3/8" |
| 8 AWG | 0.128" | 1/2" | 7/16" |
| 6 AWG | 0.162" | 5/8" | 1/2" |
| 4 AWG | 0.204" | 3/4" | 5/8" |
| 2 AWG | 0.258" | 7/8" | 3/4" |
| 1 AWG | 0.289" | 1" | 7/8" |
| 1/0 AWG | 0.325" | 1-1/8" | 1" |
| 2/0 AWG | 0.365" | 1-1/4" | 1-1/8" |
| 3/0 AWG | 0.410" | 1-3/8" | 1-1/4" |
| 4/0 AWG | 0.460" | 1-1/2" | 1-3/8" |
Masonry and Concrete Bit Sizes - Carbide Territory
Masonry bits operate in a different universe from standard drill bits. The carbide tips measure slightly larger than the shaft to clear concrete dust. A 3/8" masonry bit might have a tip measuring 0.400" - that extra width keeps the shaft from binding. Hammer drill settings add another variable. The percussive action requires different speeds than rotation-only drilling.
Standard concrete anchors specify exact bit sizes. If you're drilling into concrete walls, you'll want to check out the best drill for concrete walls before starting. A 3/8" wedge anchor needs a 3/8" hole, but a 3/8" sleeve anchor wants a 1/2" hole. The difference comes from expansion mechanisms - wedge anchors expand against the hole sides, while sleeve anchors compress and bulge.
| Anchor Size | Bit Size Required | Hole Depth | Common Application |
|---|---|---|---|
| 1/4" Drop-in | 3/8" | 1-1/8" | Light electrical |
| 3/8" Drop-in | 1/2" | 1-5/8" | Conduit straps |
| 1/2" Drop-in | 5/8" | 2" | Heavy electrical |
| 1/4" Wedge | 1/4" | 2-1/4" | Light fixtures |
| 3/8" Wedge | 3/8" | 3-1/4" | Equipment mounting |
| 1/2" Wedge | 1/2" | 4-1/2" | Structural connections |
| 5/8" Wedge | 5/8" | 5-1/2" | Heavy equipment |
| 3/4" Wedge | 3/4" | 6-1/4" | Structural steel |
| 1/4" Sleeve | 1/4" | 1-3/4" | Hollow block |
| 3/8" Sleeve | 1/2" | 2-1/2" | Brick facing |
| 1/2" Sleeve | 5/8" | 3" | Block walls |
| #6 Tapcon | 5/32" | 1-1/4" | Light duty |
| #8 Tapcon | 3/16" | 1-1/2" | General purpose |
| 1/4" Tapcon | 3/16" | 1-3/4" | Medium duty |
| 3/8" Tapcon | 5/16" | 2-1/4" | Heavy duty |
Sheet Metal Drilling - Speed and Heat Management
Sheet metal thickness drives drill bit selection differently than solid materials. A #30 drill (0.1285") works for #8 sheet metal screws in 20-gauge steel. But 16-gauge steel needs a #29 (0.1360") for the same screw. The difference: thread engagement in thinner material requires tighter tolerances.
Step bits dominate sheet metal work for good reason. They create gradually larger holes without the grabbing that twist drills produce. A typical step bit might progress from 1/8" to 1/2" in 1/32" increments. Each step acts as a depth stop, preventing punch-through damage.
| Metal Gauge | Thickness | #6 Screw | #8 Screw | #10 Screw | #12 Screw |
|---|---|---|---|---|---|
| 26 Gauge | 0.018" | #33 | #30 | #26 | #20 |
| 24 Gauge | 0.024" | #33 | #30 | #26 | #20 |
| 22 Gauge | 0.030" | #32 | #29 | #25 | #19 |
| 20 Gauge | 0.036" | #32 | #29 | #25 | #18 |
| 18 Gauge | 0.048" | #31 | #28 | #24 | #17 |
| 16 Gauge | 0.060" | #30 | #27 | #22 | #16 |
| 14 Gauge | 0.075" | #29 | #26 | #20 | #13 |
| 12 Gauge | 0.105" | #27 | #23 | #17 | #9 |
| 11 Gauge | 0.120" | #26 | #21 | #15 | #7 |
| 10 Gauge | 0.135" | #25 | #19 | #11 | #4 |
Reading Drill Index Labels - Decoding the Numbers
Stand in front of a drill index at any hardware store and you'll see the labeling chaos firsthand. That Irwin 29-piece set shows fractions on top, decimals below, and millimeters on a separate row. The DeWalt 21-piece kit uses color coding - black for fractional, gold for the pilot point sizes that don't match standard fractions.
Import sets add another layer of confusion. Chinese manufacturers often label bits with approximate sizes. That "6.5mm" bit might actually measure 6.35mm (1/4") because they're using fractional tooling with metric labels. Japanese sets typically maintain tighter tolerances - when Makita labels something 6.5mm, it measures 6.5mm.
The index case order follows decimal size, not the labeling system. That's why #1 (0.2280") sits between 7/32" (0.2188") and 15/64" (0.2344"). Letter drills fill similar gaps - F drill (0.2570") bridges the space between 1/4" (0.2500") and 17/64" (0.2656").
Professional indexes include decimal equivalents for everything. Machine shops won't use a bit labeled only in fractions because precision work requires knowing exact measurements. A thousandth of an inch matters when cutting threads or fitting bearings.
Digital Calipers vs Charts - Modern Measurement
Digital calipers changed how shops handle drill bit sizing. Instead of squinting at worn markings on a bit shank, you measure the actual cutting diameter. This reveals wear patterns - a heavily used 1/4" bit might measure 0.245" after months of drilling steel. That five thousandths of wear creates sloppy holes.
But charts remain relevant. Calipers tell you what size a bit is. Charts tell you what size you need. The two-column tap drill chart stays taped to drill press stations because nobody memorizes that 1/4"-20 threads need a #7 drill. Even with digital readouts, the relationships between thread pitch and hole size require reference materials.
Temperature affects measurements too. A precision ground drill bit expands about 0.0001" per inch of length for every 100°F temperature rise. Machine shops doing precision work maintain temperature-controlled tool storage. That 0.5000" reamer at 70°F measures 0.5002" at 120°F - enough difference to matter in bearing fits.
The Reality of Drill Bit Standards
The sizing standards tell an industrial history story. Fractional inches trace to British Commonwealth manufacturing. Metric emerged from the 1875 Treaty of the Meter. Number and letter gauges solved specific American manufacturing problems in the early 1900s. We're stuck with all four because retooling entire industries costs more than printing conversion charts.
Modern manufacturing adds new confusion. CNC machines work in decimal inches or millimeters, but operators still order fractional drill bits. A programmer enters 0.2500 for a hole size. The setup sheet calls for a 1/4" drill. The tool crib might stock 6.35mm bits labeled as 1/4". Everyone pretends these are identical, and mostly they are - until tolerance stack-up causes problems.
The aerospace industry maintains the tightest standards. They specify drill bits by decimal size to ten-thousandths. A hole for an AN3 bolt requires 0.1890" to 0.1910" - exactly #11 drill territory. No equivalents accepted. No metric substitutions. The specifications exist because someone, somewhere, learned the hard way that 0.002" matters at 30,000 feet.
Hardware store reality differs from precision manufacturing. That bargain 115-piece drill set includes metric and fractional sizes so close together you need calipers to tell them apart. 15/64" and 6mm? They're 0.0018" different. For hanging pictures, that's the same hole. For fitting a precision shaft, it's the difference between success and scrap metal.
The best drill indexes acknowledge this reality. They group similar sizes together, show decimal equivalents for everything, and include a conversion chart inside the lid. Because standing at a drill press trying to remember whether 3/8" is bigger than 10mm wastes time, and time is what drill bits are supposed to save.