Fluid Replacement: How Much Should I Drink?

Recently, a friend asked how much fluid should be replaced during endurance exercise.  It really depends on several factors which include air temperature, humidity, body weight, and acclimation to exercise in the heat.  Just a 2% drop in body weight, due to sweating and a lack of adequate fluids will cause a drop in performance.  Below is a chart that provides a good starting place based on air temperature and body weight.  

For instance, at 70 degrees (F) a 120 lb person would consume 4 ounces of fluid for every mile run and a 180 lb person would replace 5.9 ounces every mile.  The difference between the two runners is nearly two ounces.  Now, that seems like a small difference however, over the course of one hour that can add up to 16 ounces more for the heavier, 180 lb runner.  Calculating your fluid replacement needs before the workout based on air temperature and your body weight will make you feel and perform better.