a. Weigh yourself before the workout (Kg)
b. Weigh your bottles with fluids (Kg)
Do your workout
Wipe any sweat off your body with a towel
c. Weigh yourself after the workout (Kg)
d. Weigh your bottles after the workout (Kg)
e. Sweat Loss = (a-b)+(c-d)
If you’re not using Kg and grams, then you need to convert to metric; the reason is that 1L of water = 1Kg, so calculating fluid loss in Kg is also the same in Litres.
The sweat loss calculator can be downloaded, along with some other tools from here
The sweat loss is as above. This can be done any time, and should be done the same method each time. The rate will change depending on conditions, eg. indoor vs outdoor, cooler vs warmer vs extreme heat waves.
This will help you determine how much fluid you need per hour, in similar conditions.
The sodium concentration is done using a sample of blood taken from the arm, and also a urine sample. This is to help identify the amount of sodium you would lose per hour in the sweat.