Determine Your Sweat Rate to Hydrate Smarter

A girl squirts water in her mouth

Whether you glisten or look like you just got out of a pool, everyone sweats when they work out. But, whether you’re damp or soaked, you need to replace the water and electrolytes you lose so you can be ready for the next workout.

Chugging a glass of water right after your workout, though, isn’t going to cut it. With a basic understanding of your sweat (and a little bit of simple math), you can determine your sweat rate and be on your way to rehydrating smarter.

Here’s how:

A runner sweats during a summer workout

Determining Your Sweat Rate

Your sweat rate is the measurement of how much sweat you lose during a workout. Determining your sweat rate will tell you how much fluid you need to replace to maintain hydration.

Here’s the basic formula to determine you sweat rate:

  • Pre-exercise weight - post-exercise weight = sweat loss
  • Multiply sweat loss by 16 to convert your loss to ounces
  • Add in any ounces you drank during exercise
  • Divide sweat rate by 60 to get your sweat rate per hour
  • If your workout was less than one hour, multiply by duration

Here’s how that looks for a person who weighs 150 lbs, works out for 60 minutes and drinks 12 ounces of water during the workout:

  • 150 - 147.5 = 2.5lbs
  • 2.5 lbs x 16 oz = 40 oz
  • 40 + 12 oz = 52
  • 52 / 60 min = .86

In this example, the person’s sweat rate would be .86 oz per minute (or 3.25 lbs per hour, about 2.1 percent of body weight).

If your workout lasts less than one hour, though, multiply your final number by the duration of your workout. So, if you ran for 45 minutes, multiply by 45.

  • .86 x 45 min = 38.7 oz (or 2.4 lbs, about 1.6 percent of body weight)

Before you hop on the scale, there are a few things you should keep in mind when trying to calculate your sweat rate. Here are some tips to get an accurate measurement:

  • Weigh yourself without clothes on, both before and after your workout, to get the most accurate numbers
  • Drink only water during your workout, and measure the exact number of ounces you drink
  • Don’t pee before the post-workout weigh-in

Sweating and Sodium Loss

Three people wearing running shorts run together on a trail

Do crusty white salt marks appear on your face and clothing when you work out? This can help determine if you are a salty sweater.

Sweat rate isn't directly related to sodium loss. You can be a heavy sweater and not necessarily lose a lot of sodium, and vice versa. But, no matter how much you sweat, you lose nutrients as well as water.

Sweat contains electrolytes: sodium, potassium, chloride and small amounts of minerals (iron, calcium, and magnesium). Of all these, sodium takes the biggest hit from sweat loss, so it's the most important to replace.

Most athletes lose about 500mg sodium per pound of sweat loss, and heavy, salty sweaters can lose more than 1500mg sodium each hour.

Because these electrolytes are essential to proper hydration, it’s important to also replace them after a workout. It’s difficult to measure exactly how much sodium and other electrolytes you lose in sweat, so you should contact a sports dietitian to determine the ideal amount you should be replacing.

Post-Workout Hydration Tips:

Once you know your sweat rate, you can develop a rehydration plan that’s tailored to your body.

For every pound of fluid (16oz) lost in exercise, you should consume 20oz to cover obligatory urine loss. The rehydration beverage should contain both sodium and simple carbs to accelerate reabsorption.

Products like Skratch Labs Hydration Mix and nuun drink tablets pack both electrolytes and carbs to help you replenish the other stuff you lose in sweat.

By Susan Kitchen. Susan is a Sports Certified Registered Dietitian, USA Triathlon Level II Endurance Coach, IRONMAN Certified Coach, published author and founder of Race Smart, a sports nutrition and coaching company.

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