Workshop Reference
Drill Bit Size Conversion(mm to inches & fractions)
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
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.