Knurling Calculator
This tool can be used to calculate an appropriate diameter for knurling, assuming you know the circular pitch of your knurl and the diameter of your stock. If your knurl doesn't have the pitch written on the side (or if you want more confidence in the accuracy), coat the knurl with some stamping ink or machinists blue and run it along a piece of paper for several turns. Count the number of gaps between lines and divide by the distance between them and you'll have the circular pitch, which is used in the form below. There's a nice picture describing this process (along with a description of the required calculations) at this link.
All the fields that expect a number (diameter, pitch etc), will accept fractions (e.g. 1 1/4) and numbers followed by an explicit unit (" or mm), so if you chose to do so you can specify the diameter in metric and the knurl pitch in imperial or viceversa.
Units:  
Original Diameter (mm) : 
mm


Knurl Pitch  toothtotooth (mm) : 
mm


Knurling Diameter (mm) : 
mm

??? mm

Background
This calculator uses the following equations  let me know if you think they're wrong! Regardless of the selected unit, all entered data is converted into millimetres when calculating the results; if you've selected imperial units, the result is converted back into imperial units for display (although both inches and millimetres are displayed in the right hand column regardless of the selected unit).
Key
\(\qquad P\) is the knurl pitch
\(\qquad D_O\) is the stock original diameter
\(\qquad D_K\) is the knurling diameter
\(\qquad k_1\) is a constant (equal to the inverse of the diametrical pitch)
\(\qquad k_2\) is another constant, defined below.
\(\qquad floor\) is a function that rounds a value (e.g. 1.54) down to the nearest integer (e.g. 1.0)
Calculations
$$ k_1 = \frac{P}{\pi} $$ $$ k_2 = floor \left( \frac{D_O}{k_1} \right) $$ $$ D_K = k_2 \cdot k_1 $$