The old Hebrew and Greek languages did not have separate symbols for denoting numbers, but used the letters of the alphabets themselves. It's really very simple: the first letter denotes 1, the second denotes 2, the third 3 and so on for the first ten letters.
Letter Value 1st 1 2nd 2 3rd 3 4th 4 5th 5 6th 6 7th 7 8th 8 9th 9 10th 10
It's really very simple. Then it gets harder but only slightly. The values of the eleventh through nineteenth letters increase as multiples of ten. That is:
Letter Value 11th 20 12th 30 13th 40 14th 50 15th 60 16th 70 17th 80 18th 90 19th 100
In a fashion similar to that after reaching 10, previously, after 100, the values then increase by 100.
Letter Value 20th 200 21st 300 22nd 400
and so on.
As the old Hebrew only has 22 letters, the single highest factor is 400. The Greek had 28 letters making the single highest factor 800.
It is the position of the letter in the alphabet the determines its theomatic value, rather than the letter itself. So while the ordering of the Greek alphabet is not disputed, I contend the true order of the old 'paleo'-Hebrew alphabet is different that that prescribed today. Click here for a comparison chart.

For example, the ordering above (if you are familiar with the symbols) dictates a different thematic value for "aleim" (Elohim - using pointings) than the modern hebrew alphabet.
|
|
Ordering |
Ordering |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
So the thematic value using the modern ordering is 86, but using (what I insist is the correct) paleo ordering is 343. Other words, like "ab," which comprises letters which share the same position, have, of course, the same thematic value (3, in this example).
HIB computes thematic values using either (or both) of these orderings.