Turks head is a knot. Other sites can tell you how to tie it, this one just tells you about how much cord to start with, and if your planned knot can be tied. Other sites can do that too. Ashley has a chart, but the basic rule is leads and bights may have no common factor. To tie, I print out a "crossing chart"(page down on T.J. Bartruffs site for example), cut it to the right width for the number of bights, tape it into a tube, stuff it full of rags, and start pinning the cord down. There is another turks head calculator on the web, but I couldn't figure it out, so I wrote my own.
Cord estimate is theoretical and approximate, according to my theories, which may be wrong. Use at your own risk. This is an estimate only. Don't hold me responsible if you come up short.
To find the cord diameter without calipers or micrometers, wrap a piece of white thread around it several times, mark across the thread with a black marker, measure between marks with a ruler, and divide by pi.
John Chambers.