Calculate your hydration needs: Fluid replacement worksheet

Dehydration can hurt your performance and increase your risk of heat injury. 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 the usual water losses from breathing and urinating, you need to consider fluid loss 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 0.08–0.1 oz per pound of body weight 2–4 hours before the event to allow enough time to normalize your body’s water levels. Drinking fluids that contain sodium (such as a sports drink) or eating small amounts of salty foods helps stimulate thirst and retain the fluids you consume. If you’re hydrated, your urine should be pale yellow, and your body weight will be within 1% of your usual 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 pound lost during activity represents about 16 fluid oz of sweat loss (roughly half 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. These 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 (a 2% or greater body weight reduction from baseline).

After activity. The goal of rehydrating after your mission or activity is to fully replace any fluids or electrolytes you’ve lost. As long as you have more than 12 hours between events, you should be able to achieve normal hydration status through regular food and fluid intake. If your recovery time is limited (less than 12 hours between events), you may need to take a more intentional approach to rehydrating quickly. 

To rehydrate quickly, drink 16–24 oz of fluid for every pound of body weight lost. The additional fluid (beyond the 16 oz per pound recommended during activity) compensates for sweat and urine losses that continue after you finish the activity. If possible, drink small amounts over time, preferably with sodium and carbohydrates. Good options include sports drinks, electrolyte replacement packs, or whole foods such as 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? Weigh yourself pre- and post- activity when possible. Knowing whether, and how much, your weight changed during an event enables you to estimate your fluid needs. Medical providers, registered dietitians, and Service Members can use the Fluid Replacement Worksheet to guide this process, minimize dehydration, and maximize performance.

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