Transform values of hash functions like MD5 into numbers I have read about the hash function MD5. I found an online calculator which counts the value of MD5 for different data. But the MD5 function returns as result strings like 735b90b4568125ed6c3f678819b6e058, so I can't apply any numerical operation to this value, e.g. mod.

How can I transform values like these into numbers?
The output 735b90b4568125ed6c3f678819b6e058 is just a hexadecimal representation of a 128-bit (in binary) number, which corresponds directly to an integer between $0$ and $2^{128}-1$.  Therefore, you can think of MD5 as returning a numeric value.  You should be able to make the conversion from hexadecimal  string to integer trivially using any programming language (if not, Stack Overflow is the place to go to).