Follow these 5 simple steps to calculate your sweat rate. Learn how much fluid you lose and how much to drink during exercise.
By Rachel Longo (Reviewed by Natalie Rizzo, MS, RD)
When you think about fueling your body for athletic performance, the first thing that probably comes to mind is food. But an often overlooked factor is hydration.
Staying hydrated before, during, and after exercise not only helps performance but also aids in recovery. A way to find out whether you are staying hydrated while exercising is to calculate your sweat rate.
Learn more about hydration for runners here!
What is sweat rate?
Quite simply, sweat rate is the amount of fluid or sweat you lose during exercise. It depends on the intensity and duration of the exercise, as well as the environment in which you are exercising.
Sweat consists of fluid and electrolytes, including sodium, potassium, chloride, magnesium, and calcium. Electrolytes are minerals that maintain the body’s water balance.
Sodium and chloride are the primary minerals lost in sweat, but the amount varies greatly from person to person. Some individuals are very salty sweaters– they may have cakes of white salt on their skin after exercising–while others may not lose as much sodium.
Therefore, each individual sweats differently. It is important to understand your own sweat rate to figure out how to hydrate yourself properly before, during, and after exercise.
Follow these 5 easy steps to calculate your own sweat rate
How to calculate sweat rate
- Weigh yourself before exercise.
Take your weight after urinating & wearing as minimal amount of clothing as possible. Multiply your weight in pounds x 16 = weight in ounces.
The first step in calculating sweat rate is stepping on the scale. It is best to weigh yourself after urinating in order to get a better sweat rate estimation. Also, since you will most likely be calculating sweat rate from the comfort of your own home, it is suggested that you weigh yourself nude to eliminate added weight from clothes and other accessories. That said, it’s fine if you keep your clothes on, but know that it might alter the calculation slightly.
Once you have recorded your weight in pounds, multiply by 16 to get ounces. Write down your starting weight.
- Measure the fluid (ounces) in your water bottle or hydration packet
In order to calculate your sweat rate, you will need to know how much fluid (in ounces) you consumed during exercise. Measure the amount of water or fluid (ounces) you put in your water bottle or hydration packet. After exercise, you will take a look at how much fluid was consumed (step 4).
- Take off any sweaty clothing and weigh yourself after exercising (before urinating)
As soon as you finish your workout and before urinating, take off any sweaty clothing and weigh yourself. In a clinical setting, a professional would measure your urine output. However, as an everyday athlete, it is best to weigh yourself before urinating. Similar to step 1, multiply your weight after exercise by 16 to get ounces.
- Subtract the amount of remaining fluid in your water bottle or hydration pack from the original pre-exercise amount
After your workout, measure out the fluid left in your water bottle or hydration pack. Subtract the remaining amount of fluid from the original amount. Make sure to measure the fluid in ounces. Now you are ready to calculate your sweat rate.
- Calculate your sweat rate
((Post-exercise weight (ounces) – (Starting weight (ounces))+ fluid consumed(ounces) / (Exercise duration in hours)
A 150-pound person who lost 2 pounds and drank 16 ounces during a 1-hour workout:
150 pounds x 16 = 2400
148 pounds x 16 = 2368
((2368) – (2400)) +(16) / (1) = -16 ounces/hr
This individual’s sweat rate was a loss of 16 ounces per hour. (When the number is negative it means a loss, and when it comes out to be positive it is a surplus.)
Based on this calculation, this individual would benefit from drinking an additional 16 ounces of water while exercising, or a total of 32 ounces.
How to hydrate before exercise
Now that you know how to calculate your sweat rate, let’s talk about how you can properly hydrate before, during, and after exercise. The goal is to start a workout fully hydrated. It’s recommended to consume 2-4 milliliters per pound of bodyweight around 4 hours before exercise.
For instance, a 140-pound individual would want to consume 280-560 milliliters of water 4 hours before exercise. This equates to about 9.5-19 ounces of fluid.
If you need to stimulate your thirst, consuming a drink with some sodium can help you drink more and retain more fluid.
How much to drink during exercise
Now that you know how to calculate your sweat rate, you can use that to help you properly hydrate during exercise. You want to make sure you consume fluid at regular intervals while exercising. It also is beneficial to calculate your sweat rate in different exercise conditions if possible.
For example, measure sweat rate:
- In different temperatures (hot, humid, cold)
- Outside vs. Inside
- In a gym setting
When exercising for less than an hour, water or electrolyte waters are all you will need.
When exercising more than 1 hour, take in 30-60 grams of carbs per hour to fuel your body throughout the exercise and aid in performance.
While exercising for more than 2 and a half hours, aim to take in 60-90 grams of carbs per hour.
Along with the carbs, you also want to make sure you are consuming some sodium and other electrolytes in your drink of choice. Sodium will both stimulate thirst and help to retain fluids, along with aiding in the absorption of carbs for fuel.
How to rehydrate after exercise
The method of rehydration status will depend on your next training session. If you are doing two-a-days, make sure you rapidly replenish lost fluids. To rehydrate quickly, consume about 50% more fluids than those lost in sweat. That said, it’s best to take small sips rather than chug fluids, so you retain more.
Adding salt to your food can help replenish the sodium lost while sweating, stimulate you to drink more, and help you to regain your water balance over time. If you have ample time to rehydrate before your next exercise session, eat food and drink fluids throughout the day to aid in recovery.
Rachel Longo is a Master’s candidate and currently going through her Dietetic Internship at Teachers College, Columbia University. She studied Nutrition and Exercise Physiology and has a passion for sports nutrition. Her dream is to work with collegiate and professional athletes after finishing her internship and becoming a Registered Dietitian Nutritionist.