How to Calculate Your Sweat Rate

Drinking to match your sweat rate is a key part of a smart hydration plan that helps to ensure you get all the performance benefits of proper hydration and avoid the risk of over-hydration. If fluid and electrolytes lost through sweat are not replaced, a person will become dehydrated. Dehydration compromises athletic performance and is a risk factor for both heat exhaustion and heat stroke. On the other hand, consuming more fluid than is lost through sweat is the primary factor in exercised-induced hyponatremia, a serious condition in which the sodium level in the blood becomes too low.

What is a Sweat Rate?

Your sweat rate is the amount of fluid you lose, primarily through sweat, during each hour you exercise under your normal exercise conditions. It is also the amount of fluid you should consume each hour while exercising to stay well hydrated and perform your best. Because sweat rates differ extensively from one person to the next, even under similar exercise conditions, it’s important to monitor your sweat rate.

Sweat Rate Calculation

Sweat Rate = (A + B) ÷ C, where
A = Pre-exercise body weight – Post-exercise body weight, recorded in ounces. (1 lb. = 16 oz.)
B = Fluid Consumed During Exercise, recorded in ounces. (1 cup = 8 oz; 1 gulp = about 1 oz)
C = Exercise Duration, recorded in hours. (40 min = .66 hr)

Example:

During her regular 90-minute outdoor run, Sue drank 22 ounces of fluid. Her pre-exercise weight was 125 lbs; post-exercise weight was 124.5 lbs.

Sue’s sweat rate is = (A + B) ÷ C where:

A = Weight Change During Exercise = 125.0 lb – 124.5 lb = 0.5 lb = 8 ounces
B = Fluid Consumed During Exercise = 22 ounces
C = Exercise Duration = 90 min = 1.5 hours

Sweat Rate = (8 oz + 22 oz) ÷ 1.5 hr = 20 oz/hr

To meet ACSM hydration recommendations, Sue should plan to drink 20 ounces per hour of exercise, or 30 ounces total for 90 minutes (~ 5 ounces every 15 minutes)* when she runs under similar conditions.

*For greatest accuracy, calculate sweat rate under normal workout conditions, weigh without clothing or shoes and avoid using the restroom prior to post-exercise weigh-in. Since factors like outdoor conditions, fitness level and exercise intensity affect sweat rates, recalculate when environment or fitness levels change, and/or engaging in a different type or intensity of activity.

Learn more about ACSM’s Fluid Replacement Recommendations.