Calculate your sweat rate and sodium replacement needs using pre- and post-workout weight.
This is the gold-standard sweat test used by sports dietitians. Weigh yourself naked before and after exercise. The difference in grams equals milliliters of fluid lost. Add any fluid you drank during the workout. Divide by hours to get your sweat rate.
Sodium loss = sweat rate (L/hr) × sweat sodium concentration (mg/L). If you're a salty sweater (white salt stains on clothes), use the higher end. If you never see salt rings, use the lower end.
Tip: Do this test in different conditions — hot vs. cool, easy vs. hard. Your sweat rate changes with temperature and intensity. Keep a log and adjust your race-day plan accordingly.