Workshop Reference
Drill Bit Size Conversion(mm to inches & fractions)
Convert a metric drill diameter to decimal inches before comparing fractional, letter or numbered drill sizes. The nearest drill is a separate size choice, not an exact replacement.
Maintained by mmtoinches.im · Updated
NIST describes the standard inch as exactly equivalent to 25.4 mm
. NIST length definition. Method and limitations.
Looking for tap drill sizes? See Tap Drill Calculator & Chart.
Convert mm to decimal inches and nearest fractions with denominator control. Below are tap drill sizes, clearance/pilot recommendations, and letter/number series tables.
Conversion Formulas
Decimal inches: inches = mm ÷ 25.4
Nearest fraction: nearest = round(inches × denominator) ÷ denominator
Choose 1/16, 1/32, or 1/64 based on tolerance and availability.
Try the Fraction Converter
Open the mm to inches fraction converter to choose a denominator and see rounding error before drilling.
Drill Bit Size Chart
Switch denominators and view rounding error. Verify against your bit series (fraction/number/letter/metric).
| mm | Decimal inches | Nearest fraction (1/32) | Error (in) |
|---|---|---|---|
| 1 | 0.0394 | ≈ 1/32" | ±0.0081 |
| 1.5 | 0.0591 | ≈ 1/16" | ±0.0034 |
| 2 | 0.0787 | ≈ 3/32" | ±0.0150 |
| 2.5 | 0.0984 | ≈ 3/32" | ±0.0047 |
| 3 | 0.1181 | ≈ 1/8" | ±0.0069 |
| 3.5 | 0.1378 | ≈ 1/8" | ±0.0128 |
| 4 | 0.1575 | ≈ 5/32" | ±0.0012 |
| 4.5 | 0.1772 | ≈ 3/16" | ±0.0103 |
| 5 | 0.1969 | ≈ 3/16" | ±0.0094 |
| 5.5 | 0.2165 | ≈ 7/32" | ±0.0022 |
| 6 | 0.2362 | ≈ 1/4" | ±0.0138 |
| 6.5 | 0.2559 | ≈ 1/4" | ±0.0059 |
| 7 | 0.2756 | ≈ 9/32" | ±0.0057 |
| 7.5 | 0.2953 | ≈ 9/32" | ±0.0140 |
| 8 | 0.3150 | ≈ 5/16" | ±0.0025 |
| 9 | 0.3543 | ≈ 11/32" | ±0.0106 |
| 10 | 0.3937 | ≈ 13/32" | ±0.0125 |
| 11 | 0.4331 | ≈ 7/16" | ±0.0044 |
| 12 | 0.4724 | ≈ 15/32" | ±0.0037 |
Fractions are rounded by denominator. Verify against your stocked series (fraction/number/letter/metric).
Clearance and Pilot Hole Chart
Screw Clearance (Close/Free Fit)
| Screw | Close (in) | Close (mm) | Free (in) | Free (mm) |
|---|---|---|---|---|
| #4 | 0.1120" | 2.845 mm | 0.1270" | 3.226 mm |
| #6 | 0.1380" | 3.505 mm | 0.1530" | 3.886 mm |
| #8 | 0.1640" | 4.166 mm | 0.1790" | 4.547 mm |
| #10 | 0.1900" | 4.826 mm | 0.2050" | 5.207 mm |
| #12 | 0.2160" | 5.486 mm | 0.2310" | 5.867 mm |
| 1/4 | 0.2500" | 6.35 mm | 0.2650" | 6.731 mm |
| 5/16 | 0.3125" | 7.938 mm | 0.3275" | 8.319 mm |
| 3/8 | 0.3750" | 9.525 mm | 0.3900" | 9.906 mm |
General Guidance
- Close fit ≈ screw major diameter; free fit ≈ major + 0.010–0.020".
- Pilot holes: softwood ≈ 65–75% of major; hardwood ≈ 75–90%.
- Use brad-point or HSS wood bits for clean entry; clamp workpieces and use guides.
Wood Screw Pilot Holes (Softwood/Hardwood)
| Screw | Softwood (in) | Softwood (mm) | Hardwood (in) | Hardwood (mm) |
|---|---|---|---|---|
| #4 | 0.0784" | 1.991 mm | 0.0952" | 2.418 mm |
| #6 | 0.0966" | 2.454 mm | 0.1173" | 2.979 mm |
| #8 | 0.1148" | 2.916 mm | 0.1394" | 3.541 mm |
| #10 | 0.1330" | 3.378 mm | 0.1615" | 4.102 mm |
| #12 | 0.1512" | 3.84 mm | 0.1836" | 4.663 mm |
Values are approximate percentages of typical screw major diameters. Always test on scrap and adjust for your screws and materials.
Notes & Sources: Unified thread sizes per ASME B1.1; metric per ISO 261/965. Manufacturer charts may differ; verify and adjust for tooling and finish.
Letter Drill Bit Sizes(A–Z)
| Letter | Inches | mm |
|---|---|---|
| A | 0.234" | 5.944 mm |
| B | 0.238" | 6.045 mm |
| C | 0.242" | 6.147 mm |
| D | 0.246" | 6.248 mm |
| E | 0.250" | 6.35 mm |
| F | 0.257" | 6.528 mm |
| G | 0.261" | 6.629 mm |
| H | 0.266" | 6.756 mm |
| I | 0.272" | 6.909 mm |
| J | 0.277" | 7.036 mm |
| K | 0.281" | 7.137 mm |
| L | 0.290" | 7.366 mm |
| M | 0.295" | 7.493 mm |
| N | 0.302" | 7.671 mm |
| O | 0.316" | 8.026 mm |
| P | 0.323" | 8.204 mm |
| Q | 0.332" | 8.433 mm |
| R | 0.339" | 8.611 mm |
| S | 0.348" | 8.839 mm |
| T | 0.358" | 9.093 mm |
| U | 0.368" | 9.347 mm |
| V | 0.377" | 9.576 mm |
| W | 0.386" | 9.804 mm |
| X | 0.397" | 10.084 mm |
| Y | 0.404" | 10.262 mm |
| Z | 0.413" | 10.49 mm |
Number Drill Bit Sizes (#80–#1)
| Number | Inches | mm |
|---|---|---|
| #80 | 0.0135" | 0.343 mm |
| #79 | 0.0145" | 0.368 mm |
| #78 | 0.0160" | 0.406 mm |
| #77 | 0.0180" | 0.457 mm |
| #76 | 0.0200" | 0.508 mm |
| #75 | 0.0210" | 0.533 mm |
| #74 | 0.0225" | 0.571 mm |
| #73 | 0.0240" | 0.61 mm |
| #72 | 0.0250" | 0.635 mm |
| #71 | 0.0260" | 0.66 mm |
| #70 | 0.0280" | 0.711 mm |
| #69 | 0.0292" | 0.742 mm |
| #68 | 0.0310" | 0.787 mm |
| #67 | 0.0320" | 0.813 mm |
| #66 | 0.0330" | 0.838 mm |
| #65 | 0.0350" | 0.889 mm |
| #64 | 0.0360" | 0.914 mm |
| #63 | 0.0370" | 0.94 mm |
| #62 | 0.0380" | 0.965 mm |
| #61 | 0.0390" | 0.991 mm |
| #60 | 0.0400" | 1.016 mm |
| #59 | 0.0410" | 1.041 mm |
| #58 | 0.0420" | 1.067 mm |
| #57 | 0.0430" | 1.092 mm |
| #56 | 0.0465" | 1.181 mm |
| #55 | 0.0520" | 1.321 mm |
| #54 | 0.0550" | 1.397 mm |
| #53 | 0.0595" | 1.511 mm |
| #52 | 0.0635" | 1.613 mm |
| #51 | 0.0670" | 1.702 mm |
| #50 | 0.0700" | 1.778 mm |
| #49 | 0.0730" | 1.854 mm |
| #48 | 0.0760" | 1.93 mm |
| #47 | 0.0785" | 1.994 mm |
| #46 | 0.0810" | 2.057 mm |
| #45 | 0.0820" | 2.083 mm |
| #44 | 0.0860" | 2.184 mm |
| #43 | 0.0890" | 2.261 mm |
| #42 | 0.0935" | 2.375 mm |
| #41 | 0.0960" | 2.438 mm |
| #40 | 0.0980" | 2.489 mm |
| #39 | 0.0995" | 2.527 mm |
| #38 | 0.1015" | 2.578 mm |
| #37 | 0.1040" | 2.642 mm |
| #36 | 0.1065" | 2.705 mm |
| #35 | 0.1100" | 2.794 mm |
| #34 | 0.1110" | 2.819 mm |
| #33 | 0.1130" | 2.87 mm |
| #32 | 0.1160" | 2.946 mm |
| #31 | 0.1200" | 3.048 mm |
| #30 | 0.1285" | 3.264 mm |
| #29 | 0.1360" | 3.454 mm |
| #28 | 0.1400" | 3.556 mm |
| #27 | 0.1440" | 3.658 mm |
| #26 | 0.1470" | 3.734 mm |
| #25 | 0.1495" | 3.797 mm |
| #24 | 0.1520" | 3.861 mm |
| #23 | 0.1540" | 3.912 mm |
| #22 | 0.1570" | 3.988 mm |
| #21 | 0.1590" | 4.039 mm |
| #20 | 0.1610" | 4.089 mm |
| #19 | 0.1660" | 4.216 mm |
| #18 | 0.1695" | 4.305 mm |
| #17 | 0.1730" | 4.394 mm |
| #16 | 0.1770" | 4.496 mm |
| #15 | 0.1800" | 4.572 mm |
| #14 | 0.1820" | 4.623 mm |
| #13 | 0.1850" | 4.699 mm |
| #12 | 0.1890" | 4.801 mm |
| #11 | 0.1910" | 4.851 mm |
| #10 | 0.1935" | 4.915 mm |
| #9 | 0.1960" | 4.978 mm |
| #8 | 0.1990" | 5.055 mm |
| #7 | 0.2010" | 5.105 mm |
| #6 | 0.2040" | 5.182 mm |
| #5 | 0.2050" | 5.207 mm |
| #4 | 0.2090" | 5.309 mm |
| #3 | 0.2130" | 5.41 mm |
| #2 | 0.2210" | 5.613 mm |
| #1 | 0.2280" | 5.791 mm |
Data compiled from standard charts; for reference and verification, see manufacturer tables.
Materials and Drill Bit Types
Special Materials Tips
- Acrylic (PMMA): Use sharp brad-point or plastic bits; clamp firmly with backing. Step up gradually, peck to clear chips, keep heat low. Typical RPM: 800–2000 based on diameter; chamfer edges to reduce cracking.
- Hardwood (oak, maple): Prefer brad-point. Pilot ~85–95% of screw minor diameter; clearance ~102–108% of major. Reduce RPM for large diameters to avoid burning; support exit side to prevent tear-out.
- Stainless steel (304/316): Use cobalt or carbide split-point. Run on the low end of RPM windows (−20–40% vs mild steel), maintain steady feed, flood coolant if possible, and avoid dwelling to prevent work hardening.
- Aluminum: HSS or cobalt; 1.5–2× mild-steel RPM windows. Use lubricant to prevent chip welding; clear chips frequently.
RPM Windows by Diameter & Bit Material
| Diameter band | HSS (metal) | Cobalt (metal) | Carbide (metal) |
|---|---|---|---|
| ≤ 3 mm (≤ 1/8") | 500–900 RPM | 600–1000 RPM | 800–1400 RPM |
| 3–6 mm (1/8"–1/4") | 250–600 RPM | 300–700 RPM | 400–800 RPM |
| 6–10 mm (1/4"–3/8") | 150–400 RPM | 200–500 RPM | 250–600 RPM |
| 10–13 mm (3/8"–1/2") | 100–250 RPM | 150–300 RPM | 200–400 RPM |
| 13–20 mm (1/2"–3/4") | 60–180 RPM | 100–220 RPM | 150–300 RPM |
Windows are typical for mild steel. For aluminum, increase ≈1.5–2× with lubricant. For stainless, reduce ≈20–40%, use coolant, and steady feed. Wood generally runs 2–3× higher RPM; avoid burning and tear-out.
Cutting Speed & Feed Reference (SFM/IPR) + RPM Calculator
Recommended SFM by Material & Bit
| Material | HSS | Cobalt | Carbide |
|---|---|---|---|
| Mild steel | 60–100 | 80–140 | 200–350 |
| Stainless steel | 30–50 | 40–70 | 120–220 |
| Aluminum | 200–350 | 250–400 | 350–500 |
| Brass | 180–280 | 220–320 | 300–450 |
SFM values are typical reference ranges for drilling. Choose the low end for larger diameters and harder alloys; increase with carbide tooling and effective cooling.
IPR by Diameter Band (base, mild steel)
- ≤ 1/8": 0.0015–0.003 IPR
- 1/8"–1/4": 0.003–0.006 IPR
- 1/4"–3/8": 0.004–0.009 IPR
- 3/8"–1/2": 0.006–0.012 IPR
- 1/2"–3/4": 0.008–0.015 IPR
Adjust IPR for material/tooling: stainless −25% to −35%; aluminum +10% to +20%; carbide +10% to +15%.
RPM & Feed Calculator
Formula: RPM = (SFM × 12) ÷ (π × diameter). IPR = inches per revolution; IPM = RPM × IPR.
Plastics Drilling Strategies (Acrylic, Polycarbonate, PETG)
Acrylic (PMMA)
- Use plastic-specific or brad-point bits; avoid aggressive rake to reduce grabbing.
- Clamp with backing; leave protective film on; support exit side to prevent chipping.
- Step drill progressively; peck to clear chips; cool with air; avoid solvent heat-crack.
- Typical RPM windows: ≤1/8" 1500–2200; 1/8"–1/4" 900–1600; 1/4"–1/2" 450–900.
- Deburr and lightly chamfer holes to mitigate stress cracking.
Polycarbonate (PC)
- More ductile than acrylic; resists cracking but can melt. Use sharp split-point bits.
- Moderate RPM with firm feed; peck to prevent chip welding; cool with air.
- Typical RPM windows: ≤1/8" 1200–1800; 1/8"–1/4" 800–1400; 1/4"–1/2" 400–800.
- Use step drilling for large diameters; de-stress edges with a light chamfer.
PETG
- Easy to drill; lower melting point than PC. Keep sharp tooling; avoid dwelling.
- Run slightly lower RPM than acrylic to reduce heat; peck and clear chips.
- Typical RPM windows: ≤1/8" 1000–1600; 1/8"–1/4" 700–1200; 1/4"–1/2" 350–700.
- Back the work; final pass chamfer to minimize stress risers.
Note: Plastics vary by grade and thickness. Always test on scrap and tune RPM/feed to avoid heat and cracking.
Frequently Asked Questions
Larger denominators (e.g., 1/64) reduce rounding error, but availability matters. Woodworking commonly uses 1/16 or 1/32; precision jigs may prefer 1/64.
Bit series include intermediate diameters across metric/number/letter sizes and skip some fractions. Choose the closest stocked bit and verify with a gauge or calipers.
Clearance allows screws to pass freely; pilot guides screws or taps and is sized near material and thread needs. Hardwood pilots are typically closer to the screw’s major diameter.
Convert and drill confidently
Select mm to inches targets and denominator, then choose bits based on tap/clearance/pilot recommendations and material process. Document both units to avoid confusion.
Sources and calculation notes
Convert a metric drill diameter to decimal inches before comparing fractional, letter or numbered drill sizes. The nearest drill is a separate size choice, not an exact replacement.
Method: Divide millimeters by 25.4 to obtain inches. The factor is exact; displayed decimal values are rounded. Fractional inches are rounded to the selected denominator.
Scope: A unit conversion does not establish a manufacturing tolerance or guarantee that similarly sized parts fit. Keep the original measurement and its uncertainty.
Content maintained by mmtoinches.im. Updated and reference notes reviewed . How we calculate and present measurements. Sources support the stated definitions; examples and calculators are not certification of a product or compliance with a standard.