I came across this article on the web which seems quite useful in determining a person's sweat rate:
So tomorrow I am going to try this out by running at roughly my race pace over multi-terrain for one hour. The weather is set to be chilly over the weekend so that will obviously affect the results but it is a good cool weather guideline. I will repeat the test when/if we get some warmer weather!
Step 1: Weigh yourself before you walk, bike, jog, run, hike, Rollerblade,
play basketball, soccer or work in the garden. For example, let's say you weigh
154 pounds before you start exercising.
Step 2: Exercise for one hour in the weather conditions and at the same
workout intensity you expect to face for an upcoming run, game or special
Step 3: After your workout, remove your perspiration-filled workout clothes.
Dry yourself thoroughly. Weigh yourself again. For example, let's say you weigh
150 pounds after your workout.
Step 4: To calculate your sweat rate per hour of exercise, subtract your
ending exercise weight of 150 pounds from your beginning weight of 154 pounds.
The difference of 4 pounds represents your fluid loss during exercise.Since you
should drink 16 ounces of fluid for every pound of fluid you sweat out during
exercise, multiply the number of pounds lost by 16. In this example, you should
drink 64 ounces of fluid to counteract the symptoms of dehydration and return
your body weight to normal.
Step 5: If you drank any fluid during your 60 minutes of exercise, you'll
need to add that number to the total amount of fluid lost during exercise that
you calculated in Step 4.
Here's another way you can use the information. Once you know your personal
sweat rate per hour, you can incorporate fluid breaks in your training so that
every 15 or 20 minutes you consume 10 to 12 ounces of fluid.