Calculate your hydration needs: Fluid replacement worksheet

Dehydration can hurt your performance and increase your risk of heat injury. So maintaining good hydration levels is important for tactical athletes and Service Members, whether you’re executing a mission or getting in your daily workout. In addition to usual water losses through breathing and urinating, you also need to consider the loss of fluids from sweating during activity. To optimize your performance, remember to hydrate before, during, and after activity.

Before activity: The goal of prehydrating is to start your mission or activity with normal body fluid and electrolyte levels. To do so, slowly drink roughly 2–4 ml/lb of your body weight, 2–4 hours before the event, to allow enough time to normalize your body’s water levels. Drinking fluids with sodium (such as a sports drink) or eating small amounts of salty foods will help stimulate thirst and retain the fluids you consume. If you’re hydrated, your urine should be a pale yellow color and your body weight will be within 1% of your normal body weight.

During activity: The goal of drinking during your mission or activity is to limit dehydration and its negative effects on exercise and cognitive performance. Your fluid needs during activity will vary depending on individual sweat rate, exercise type and duration, environmental conditions, clothing or equipment worn, and opportunities to drink. Every ~1 lb lost during activity represents about 16 fluid oz of sweat loss (~½ of a standard Nalgene bottle). If you’re active for more than 2 hours or in a hot or humid environment where you sweat a lot, have a sports drink. Sports drinks contain sodium and potassium to help replace electrolytes lost in sweat and carbohydrates to fuel continued activity. Ideally, you should drink enough during activity to prevent excessive dehydration (2% or more body weight reduction from baseline).

After activity: The goal of drinking after your mission or activity is to fully replace any fluids or electrolytes you’ve lost. As long as you’ve got enough time to recover (more than 12 hours between events), you should be able to achieve normal hydration status through regular food and fluid intake. But if recovery time before your next mission or activity is limited (less than 12 hours between events) you might need to take a more intentional and strategic approach to rehydrating quickly. To rehydrate quickly, drink 20–24 oz of fluid for every lb of body weight lost. The additional fluid (beyond the 16 oz per lb recommended during activity) is to compensate for sweat and urine losses that continue after you’re done with the activity. If possible, drink small amounts over time, preferably with some sodium and carbs. Options can include sports drinks, electrolyte replacement packs, or whole foods like a peanut butter and jelly sandwich.  

With so many variables to consider, how can you figure out your unique hydration needs as a military athlete? When you can, weigh yourself to measure your body weight pre- and post- activity. After knowing whether, and how much, your weight changed during the event, you can estimate your fluid needs. Medical providers, registered dietitians, and Service Members can use the Fluid Replacement Worksheet to help guide you through this process to determine your individualized fluid needs, minimize dehydration, and maximize performance.

This worksheet is not meant to be used for every event. Consider using it occasionally or when conditions change, such as the season, environmental location, or time of day. For example, you’re likely to lose more sweat during a 10-mile ruck in the summer than in the winter, even if  you’re wearing the same clothing and equipment. Similarly, you’re likely to lose more sweat completing an hour-long gym session at a humid, location downrange than you might at your home duty station. So it can be helpful to routinely assess your fluid loss and needs under varying conditions so you can customize your fluid replacement. Use this worksheet as a reference over time to estimate fluid needs based on time of year, time of day, location, type of activity, and duration of event.

Related content:
Basic hydration strategies
How to tell if you’re dehydrated
Hydration guidelines for Warfighters
Performance hydration messages for leaders
Use nutrition to lower your risk of exertion-related events
Work/rest and hydration tips