Every day you lose fluids, and every day you need to replace those  fluids.  We are all very unique in the amount of fluid we lose in a day -  this depends on factors such as our age, gender, activity level, but  also the humidity and temperature of our environment.  A great fluid is  water, although your milk, juice, coffee, tea, etc. also contribute to  your hydration.
Curious about how much fluid you lose?
1.    Weigh yourself before your workout and record your weight in kilograms.
2.    Workout.
3.    Keep track of fluids consumed during your workout.
4.    Weigh yourself after your workout and record your weight in kilograms.
Your sweat loss in liters = bodyweight before - bodyweight after 
(add on any additional fluids consumed during the workout)
To get your sweat rate, divide the sweat loss by the length of your workout (minutes or hours) 
Example: Runner       Pre-workout weight 65Kg
                             - Post-workout weight 64 Kg
                             + 500ml water consumed in 1 hour run
                                    ** Note: 1 Kg = 1 L fluids 
        Sweat Rate: 65kg – 64kg = 1 + 0.5L fluids consumed = 1.5L/1 h