How to Convert Letter Drills to Fractionals

Letter drill sizes correspond to standard diameters for steel wire.

Evidence of the first metal twist drills dates from the Harappan civilization, an Asian dynasty that flourished around 2000 B.C. Around 4,000 years later, Stephen A. Morse patented an improved twist drill that produced accurately sized cylindrical holes in a range of materials. The Stubs Steel Wire Gauge, an early American standard for rod and wire, brought with it a corresponding selection of drill sizes, each identified by a number or a letter. Today, most designers specify dimensions in inches, and convert letter sizes accordingly.

Things You'll Need

• Drill size table
• Ruler

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Instructions

1. Note the letter designation of the drill size that you wish to convert. If you are unsure of the size, check the information etched onto the shank of the drill.

2. Scan the column headings on the drill size table and find the column titled “Drill Size Letter” or “Drill Letter.” Read down this column until you find the row containing the letter size of your drill. Place the ruler horizontally across the table directly under this row.

3. Scan the column headings again and find the column headed “Drill Size in Inches” or “Diameter in Inches.” Read down this column and note the value in the row directly above the ruler; this will usually be given in decimal inches. Write down this value, which is the diameter of the drill.

4. Multiply the diameter in decimal inches by 1,000 and note the result. This is the fractional drill size expressed in thousandths of an inch. For example, if the drill size is “A,” the diameter is 0.234 inches and the result is 234/1000 inch.

5. Check each step of your work. Record the result.

Tips & Warnings

• To multiply by 1,000 simply move the decimal point three places to the right.

• If you do not have a drill size table, find a copy online.

• Do not confuse the Stubs Steel Wire Gauge with the Stubs Iron Wire Gauge, also known as the English Standard Wire Gauge or Birmingham Gauge. Sizes from these two gauges are not the same.

References

• University of Hawai'i; Book Reviews; L.M. Young, et al.; 1978
• Freepatentsonline; United States Patent US0038119; Stephen A. Morse; April 1863
• “American Machinists’ Handbook”; Fred H. Colvin, et al; 1914

Resources

• U.S. Department of the Army; Fundamentals of Machine Tools -- Drilling Machines; October 1996
• CSGNetwork.com: Drill Size Conversion Table