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Everyone needs to drink plenty of fluid, at least 64 ounces of water a day to keep hydrated, especially during the summer heat. But if you are working out or training, that average intake needs to be increased.

“The best way to really figure out if you are well hydrated is to drink to thirst, but to also add the caveat that you need to replace what you lose to sweat,” says certified sports dietician Lauren Antonucci, director of Nutrition Energy.

There's not just one formula for everyone. However, Antonucci uses something called the "Sweat Rate Test" to help calculate how much fluid you should aim to consume during exercise.

In addition to perspiration, the formula requires a little math.

To calculate your sweat rate, you need to subtract the difference between your pre-workout and post-workout weights and then multiply that figure by 16 ounces. Add that number to the amount of fluid ounces you actually consumed during exercise. Divide that number by the amount of time you actually exercised.

The final number should be how much water you should aim to drink per hour during future exercise sessions to stay well hydrated.

“For every pound of sweat you lose on the scale during an activity, in a short duration of time, it's attributed to 16 ounces of water that you've sweat out,” Antonucci explains. “If you lose a pound over a week or two, you may have had a caloric deficit and you're achieving weight loss, which may be something you are trying to do. But if it is in a short duration, over exercise, it is not a weight loss. It's not body fat loss. It's a fluid loss and we want to prevent that.

The sweat test does not need to be conducted after every workout. In fact, Antonucci suggests just doing it whenever there's a seasonal change or if there's an extreme shift in temperature or humidity.

“If you are training for a spring marathon in February or March, and now when it is really hot and humid, if you are doing runs now, you'd want to redo those sweat tests because your sweat rate will be higher and your fluids will be higher,” she says.

For athletes especially, keep in mind sports drinks are also important to replace sodium lost in sweat. Too much water without replacing electrolytes can dilute sodium and that can be dangerous. So whip out your workout gear, calculators, and drink up people!