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How to Calculate Hydration

By Suzanne S. Wiley ; Updated July 18, 2017

Remaining properly hydrated during a workout, race or other sport is essential but also rather tricky. Dehydration and hyponatremia--what the University of Alabama at Birmingham Health System describes as a low sodium level due to too much water diluting bodily fluids, resulting in the extra water swelling your body’s cells--are due to too little and too much water, respectively. To calculate your individual needs, you must take sweating and body weight into account. As Central Washington University professor David Gee notes, your goal is a net weight loss of zero.

Weigh yourself before you start exercising or participating in the event. Note your weight in pounds.

Track how much water, in cups, you drink during the activity. Also track any sports or electrolyte drinks.

Weigh yourself after you’ve finished the activity, but before drinking anything afterward.

Subtract the post-exercise weight from the pre-exercise weight. This is your water loss. Turn this number into a percent of body weight by dividing the water loss figure by your pre-exercise weight. This is the percent by which you’re dehydrated.

Change each pound of water loss into a pint--if you’ve lost 5 pounds, that would become 5 pints. Add the number of pints you drank during the activity. Every 2 cups or 16 ounces equals 1 pint. Convert the total number of pints to ounces by multiplying by 16. So, if you lost 5 pounds and drank 2 cups, those numbers become 5 pints and 1 pint, respectively. Added together, you have 6 pints. Multiply that by 16 oz. to get 96 oz. However, keep the pints figure handy for Step 7.

Multiply your pre-exercise body weight by 0.02. Gee of Central Washington University calls this figure your "allowable sweat loss." The initial number will be in pounds, but again, convert this to pints one for one.

Subtract the "allowable sweat loss" pints from the total pints in Step 5. Multiply by 16 to get the number of ounces.

Multiply the number of ounces from Step 7 by the number of minutes you exercised. This gives you an ounce-per-minute figure. Multiply that by 15. The resulting number is the minimum number of ounces you need to drink every 15 minutes when performing the same exercise.

Calculate the maximum amount to drink every 15 minutes by dividing the total number of ounces from Step 5 by the number of minutes you exercised. Again multiply this ounce-per-minute figure by 15. The two numbers you end up with in Steps 8 and 9 are the minimum and maximum amounts--bottom and top of the range--of liquid to drink every 15 minutes when you repeat the exercise.

Weigh yourself before and after when you repeat the exercise with similar intensity to see if drinking the recommended amount of liquid has helped you stay within your allowable water loss amount. If not, speak with your doctor and your coach to further refine the amount.

Tips

Both Rice University and Colorado State University Extension recommend beginning to drink water well before the event. Drink about 2 cups of water for every pound lost after exercising.

Warnings

If you begin to feel sick while exercising, notice parts of your body swelling or see other signs of possible heat illness, hyponatremia or severe dehydration, notify a doctor immediately.

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