Four tables that get used at the bench and at the desk: inches to millimetres, fractions to decimals, drill sizes, and tap drills for unified threads. Every figure is either exact arithmetic or taken from a published standard table, and the arithmetic is checked by a machine before a page is published.
Work out which quantity you already know and which one you need. If the drawing gives a fraction and the micrometer reads a decimal, use the fraction to decimal table. If a supplier quotes a millimetre size and the stock is imperial, use the inch to millimetre table and read across to the nearest sixty-fourth. If the job is a tapped hole, start with the thread designation, find the tap drill, then look that drill up in the size table to get the millimetre value.
Read the columns in the order they are printed. The fraction column is always the reduced fraction, so 2/64 appears as 1/32; that is the form used on drawings and the form a rule is marked in. The decimal column carries six places for the sixty-fourths because 1/64 does not terminate, and rounding it to four places would shift the millimetre value in the third decimal.
The inch is defined as exactly 25.4 millimetres, so every conversion on these pages is exact multiplication or division rather than an approximation. Drill sizes and thread data are not derived from a formula: they are fixed standard series, so they are read from a published table and cross-checked against a second independent source before they are used. Where the two sources disagree by more than half a thousandth of an inch, the figure is rejected rather than averaged.
Number drills run the opposite way to intuition: #1 is the largest at 0.2280 inch and #80 is the smallest at 0.0135 inch, so a higher number is a smaller hole. Letter drills fill the gaps between the number and fractional series and are not interchangeable with either. Tap drills depend on the thread series as well as the nominal size, so a coarse and a fine thread of the same diameter take different drills. And a fractional inch size and its decimal equivalent are the same size, not two sizes that happen to be close.
No estimated values and no interpolated rows. Where a size depends on a standard we have not verified, it is left out instead of being guessed. That is why the list is short and why some familiar charts have no counterpart here yet. A wrong number in a reference table is worse than a missing row, because the person reading it has no way to tell that it is wrong.