What are Number and Letter drill sizes?

They are inch-based gauge systems. Number (wire) gauge runs from #1 (0.228 in) down to #80 (0.0135 in); larger numbers mean smaller diameters. Letter gauge runs from A (0.234 in) up to Z (0.413 in), filling the range just above #1.

How do I convert a drill size to millimeters?

Multiply the diameter in inches by 25.4. For example, a 1/4 inch drill is 0.25 x 25.4 = 6.35 mm, and a #7 drill (0.201 in) is about 5.11 mm. This chart lists the mm equivalent for every size.

Is this the same as a tap drill chart?

No. This is a general drill diameter conversion chart between Number, Letter, fraction, and mm. A tap drill chart instead tells you which drill to use before cutting a specific thread. See the Tap Drill Size reference for that.

Why does the chart show the closest size and not an exact match?

Number, Letter, fractional, and metric series rarely share identical diameters, so an exact equivalent usually does not exist. The lookup returns the nearest standard size in each system and the difference, so you can pick the closest available drill.

Are these nominal or measured diameters?

These are the nominal (standard) diameters defined for each series, shown in inches with the mm equivalent. Actual cut holes vary with drill condition, runout, and material, so always test on scrap for critical fits.

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Reference

Drill Size Chart

Number (1–80), Letter (A–Z), fractional inch, and metric mm drill diameters in one chart. Enter a diameter to find the closest standard drill in each system.

Closest size lookup

Enter a target diameter — the closest standard drill in each system is shown below.