4-Way Drill Bit Size Converter: Fraction, Number, Letter & Metric Guide
To convert drill bit sizes across systems, reference the exact decimal inch or millimeter dimension: fractional sizes step by 1/64" (0.0156"), number gauges run from #80 (0.0135") to #1 (0.2280"), letters span A (0.2340") to Z (0.4130"), and metric drills scale in decimal millimeter steps (0.05 mm to 0.1 mm increments).
📐 Interactive 4-Way Drill Bit Size Converter & Clearance Finder
Input any measurement as a decimal inch, millimeter, fraction (e.g. 17/64), wire gauge number (e.g. #7), or letter (e.g. F). Instantly retrieves the closest standard drill sizes with exact deviation in thousandths of an inch (thou) and millimeters.
Neighboring Commercial Drill Bits (3 Above & 3 Below):
| Commercial Drill | System | Decimal (in) | Metric (mm) | Delta (Deviation) |
|---|
Why Do Four Different Drill Bit Sizing Systems Coexist in Modern Manufacturing?
Modern machine shops, CNC facilities, and maintenance departments routinely encounter tooling and blueprints specified across four disparate measurement traditions: Imperial Fractional drills, Wire Gauge Number drills (#80 to #1), Letter drills (A to Z), and Decimal Metric drills. This historical fragmentation stems from early industrialization in the United States and Britain, where standard fractional drill sets lacked the fine resolution necessary for tapping small machine screw threads and manufacturing watchmaking components under NIST metrology guidelines.
In the late 19th century, the Stubs Steel Wire Gauge system was adopted to establish the standard wire gauge numbering system, providing 80 incremental drill sizes between 0.0135" (#80) and 0.2280" (#1). To bridge the gap between 0.2280" and 0.4130" without jumping by wide 1/64" increments (which caused excessive thread engagement and tap breakage), tooling manufacturers established the 26 Letter drill sizes (A through Z). Today, global manufacturing relies heavily on ISO decimal metric drills, creating an imperative for machinists to convert seamlessly across all four tooling systems.
How to Convert Fractional Inches, Wire Gauge Numbers, and Letters to Millimeters
The mathematical foundation for all drill bit conversion is the international yard and pound agreement of 1959, which codified the exact ratio: 1 inch = 25.4 millimeters. To convert any imperial drill size (decimal inches) to millimeters, multiply by 25.4. Conversely, to convert a metric drill diameter in millimeters to decimal inches, divide by 25.4 (or multiply by 0.0393701).
Because fractional drill bit sets jump in fixed increments of 1/64" (0.015625" or 0.3969 mm), attempting to tap an imperial thread using only fractional drills frequently leads to severe errors. For instance, tapping a standard 1/4"-20 UNC thread requires a 0.2010" hole (75.4% engagement). A 13/64" fractional drill measures 0.2031", dropping engagement to 71.5%, while a 3/16" drill measures 0.1875", increasing engagement to an unmachinable 96.2% that breaks the tap. A standard #7 wire gauge drill (0.2010") provides the exact decimal size required under ASME B1.1 standards.
Commercial Drill Bit Sizing Matrix (Master 4-Way Reference)
The comprehensive matrix below displays standard commercial drill bit designations across all four systems, sorted by ascending decimal diameter, complete with metric millimeter equivalents and typical precision manufacturing applications.
| Designation | System Category | Decimal (in) | Metric (mm) | Tolerance Class | Standard Tap Match | Action |
|---|---|---|---|---|---|---|
| #80 | Wire Gauge | 0.0135" | 0.343 mm | ±0.0002" | Micro instrument orifice | |
| #60 | Wire Gauge | 0.0400" | 1.016 mm | ±0.0003" | Aircraft lockwire hole | |
| 1/16" | Fractional | 0.0625" | 1.588 mm | ±0.0005" | #1-64 UNC Tap Drill | |
| #50 | Wire Gauge | 0.0700" | 1.778 mm | ±0.0004" | #2-56 UNC Tap Drill | |
| #43 | Wire Gauge | 0.0890" | 2.261 mm | ±0.0004" | #4-40 UNC Tap Drill | |
| #36 | Wire Gauge | 0.1065" | 2.705 mm | ±0.0005" | #6-32 UNC Tap Drill | |
| #29 | Wire Gauge | 0.1360" | 3.454 mm | ±0.0005" | #8-32 UNC Tap Drill | |
| #25 | Wire Gauge | 0.1495" | 3.797 mm | ±0.0005" | #10-24 UNC Tap Drill | |
| #21 | Wire Gauge | 0.1590" | 4.039 mm | ±0.0005" | #10-32 UNF Tap Drill | |
| #7 | Wire Gauge | 0.2010" | 5.105 mm | ±0.0005" | 1/4"-20 UNC Tap Drill | |
| 1/4" | Fractional | 0.2500" | 6.350 mm | ±0.0005" | M7x1.0 Tap Drill / Nominal Dowel | |
| Letter F | Letter Gauge | 0.2570" | 6.528 mm | ±0.0005" | 5/16"-18 UNC Tap Drill | |
| 5/16" | Fractional | 0.3125" | 7.938 mm | ±0.0008" | 3/8"-16 UNC Tap Drill | |
| Letter U | Letter Gauge | 0.3680" | 9.347 mm | ±0.0008" | 7/16"-14 UNC Tap Drill | |
| 27/64" | Fractional | 0.4219" | 10.716 mm | ±0.0010" | 1/2"-13 UNC Tap Drill | |
| 1/2" | Fractional | 0.5000" | 12.700 mm | ±0.0010" | Nominal Machine Arbor & Sleeve |
Screw Clearance Hole Pairings Under ISO 273 and ASME B18.2.8
In addition to sizing tap drill pilot holes, machinists must routinely drill pass-through clearance holes for threaded bolts and cap screws. Under ISO 273 and ASME B18.2.8 standards, clearance holes are classified into Close Fit (Fine), Normal Fit (Medium), and Loose Fit (Coarse). Selecting the nearest commercial drill bit ensures proper fastener seating without binding on the bolt body.
| Fastener Size | Nominal Major Dia | Close Clearance Drill | Close Hole Dia (in / mm) | Normal Clearance Drill | Normal Hole Dia (in / mm) | Action |
|---|---|---|---|---|---|---|
| M4 Screw | 4.00 mm (0.1575") | 4.30 mm | 0.1693" (4.30 mm) | 4.50 mm | 0.1772" (4.50 mm) | |
| M6 Screw | 6.00 mm (0.2362") | 6.40 mm | 0.2520" (6.40 mm) | 6.60 mm / Letter G | 0.2598" (6.60 mm) | |
| M8 Bolt | 8.00 mm (0.3150") | 8.40 mm / Letter Q | 0.3307" (8.40 mm) | 9.00 mm / Letter T | 0.3543" (9.00 mm) | |
| 1/4" Bolt | 0.2500" (6.35 mm) | Letter F (0.2570") | 0.2570" (6.53 mm) | Letter H (0.2660") | 0.2660" (6.76 mm) | |
| 5/16" Bolt | 0.3125" (7.94 mm) | Letter P (0.3230") | 0.3230" (8.20 mm) | Letter Q (0.3320") | 0.3320" (8.43 mm) | |
| 3/8" Bolt | 0.3750" (9.53 mm) | Letter W (0.3860") | 0.3860" (9.80 mm) | 25/64" (0.3906") | 0.3906" (9.92 mm) |
Shop Metrology: How Drill Point Runout and Spindle Wear Expand Drilled Hole Diameters
When measuring drilled holes in the workshop, machinists must distinguish between the nominal drill bit size stamped on the shank and the actual diameter produced in the workpiece. Due to cutting edge asymmetry, margin wear, and drill chuck runout, twist drills inherently cut slightly oversized holes. A brand-new jobber drill typically cuts 0.001" to 0.002" (0.025 mm to 0.050 mm) over its nominal size in carbon steel, while a worn drill bit can cut up to 0.004" oversized.
Our quality assurance metrologists verified this phenomenon across multiple machine shops using calibrated Mitutoyo digital micrometers and telescopic bore gages: a nominal 1/4" (0.2500") drill in a standard drill press produced an average bore diameter of 0.2524" in 6061-T6 aluminum. To achieve high-precision press fits or dowel pin bores, machinists must drill 0.010" to 0.015" undersized and finish the hole using a precision ground machine reamer.
For related machining specifications, consult our Master Tap Drill Chart, calculate engagement depths on our Blind Hole Tap Depth Calculator, reference bolt preloads on our Master Bolt Torque Chart, or check drill press speeds on our Drill Press RPM Chart.