Women's Running posted an easy way to calculate how much water you need to be drinking after a run in order to stay properly hydrated.
Take the test to find out if you are hydrating correctly:
Step 1: Immediately before you go for a run, weigh yourself (naked) on an accurate scale.
Step 2: Go for a 30- minute run. Don’t drink any water during the run.
Step 3: When you return home, weigh yourself again (yes, naked).
Step 4: Subtract your pre-run weight from your post-run weight. Multiply the number of pounds lost by two. That’s the number of pints of water you need to drink per hour (during your run or immediately after) in order to stay properly hydrated.