Sweat is one of the obvious things that happens during a workout; it’s the way our bodies cool down when they start to heat up. It follows that the more a person sweats, the more water they should drink to replenish. Sweat rates vary by individual and are further influenced by factors including exercise intensity, exercise duration, the climate (including temperature and humidity), and the body’s pre-exercise hydration state. It’s straightforward to check your individual sweat rate by weighing yourself directly before and after exercise and accounting for exactly how much water you consumed during the workout……you’ll also have to either hold your pee or take your pee-volume into account. You can google “calculating perspiration rate” for more details on how to compute your rate or use a handy sweat rate calculator on the Gatorade Sports Science Institute’s website. Another quick metric of proper hydration is to note how often you’re urinating. Ideal hydration leads to urination every 1 – 2 hours, even during exercise.
Depending on your gender, age, size and perspiration rate, you lose about four cups (approximately one liter) of water per hour of exercise. If you’re working out in a hot climate, you can easily lose up to two litres of water per hour during a cardio session.
Adequate water intake before, during and after exercise does more than replenish water lost from perspiration, it also plays a key function in maintaining blood volume and electrolyte balance. Electrolytes such as sodium, magnesium and potassium, are lost from the body with sweat. However, except in extreme cases, losses are small and replacement during exercise is not a priority. ‘Sports drinks’ often contain electrolytes, particularly sodium. These have the effect of stimulating water absorption from the small intestine, which is beneficial during exercise. In addition, after exercise, replacing lost sodium is essential for full recovery and rehydration. Extreme athletes will require fluids from a variety of sources, not just water. .
The graphic below is from a post on greatist.com and includes recommendations on how much water to drink while exercising as well as a lot of other great information.