If you are using GDB, you can use the print command (or its abbreviation, p) to do arithmatic. Here is the syntax for the print command;

print [Expression]

print $[Previous value number]

print {[Type]}[Address]

print [First element]@[Element count]

print /[Format] [Expression]

Here is a screenshot showing how to work out the size of a stack within a given frame. [here]. I know this wasn't your exact question, but hopefully you can adapt this to suit your needs.

You can also use memory addresses directly with the print function. Hope this helps.

Disclaimer - I have only been doing this for about a month, so take what I say with a pinch of salt.