Who this calculator is for
Most people never need to think about electrolytes. If your workouts are under an hour and you eat normal meals, your sodium balance takes care of itself. This tool is for the other cases: marathons, long rides, hikes and race days in the heat, physical work outdoors, and anyone who has learned they sweat unusually salty. Our electrolytes explainer covers when they matter and when they're marketing.
How the estimate works
Sweat sodium is remarkably variable, from around 300 mg to over 2,000 mg per liter between individuals. The calculator multiplies your sweat rate by duration to get total sweat volume, then by your sweat sodium concentration to get milligrams lost. It suggests replacing none of it for short sessions, about half for sessions of 90 minutes to three hours, and around 70% beyond that. The remainder comes from food afterwards. If you don't know your sweat rate, measure it first with the sweat rate calculator.
Turning milligrams into something you can drink
Four hundred milligrams of sodium is about one gram of table salt, or a quarter teaspoon. A typical sports drink carries 400–500 mg per liter, which is enough for moderate sessions but too dilute for high losses, because you can only absorb around a liter of fluid an hour. When the calculator shows a required drink strength well above 1,000 mg/L, that's the signal to use salt capsules or high-sodium tabs alongside your fluids.
Cramps, and the limits of sodium
Sodium replacement is often sold as cramp prevention, but the research says cramps are mostly about fatigue. Replace sodium to keep fluid balance and performance steady in long events, not as a magic fix. And don't overshoot: taking far more salt than you lose just makes you thirstier and can upset your stomach.
