Electrolyte Replacement Calculator

How it is calculated

First the total sweat volume: sweat (L) = sweat rate (L/h) × duration (h). Sodium is then sweat × sodium per litre, where the sweat type sets a fixed concentration: 500 mg/L for light, 1000 mg/L for average (the default) and 1500 mg/L for salty sweat. Potassium is always sweat × 200 mg/L, because the tool treats it as constant. Sodium and potassium are shown in mg, total sweat in litres to two decimals. Duration must be above zero, and the sweat rate must be above zero and no more than 4 L/h; anything else gives no result. A blank sweat type counts as average, but a sweat type other than light, average or salty also gives no result. You do not type a sodium concentration; you only choose one of the three levels, so a measured sweat test will be more accurate than these averages. The result is what you lose, not a prescription for what to drink.

Common mistakes

• Entering the sweat rate in ml per hour. The unit is litres per hour, so 1.2, not 1200. A rate above 4 L/h is treated as impossible and shows no result.
• Entering the duration in minutes. The field is hours, so a 90-minute run is 1.5. The tool has no upper limit on duration, so typing 90 gives a result of 108 L of sweat without any warning.
• Believing the result is how much sodium to drink. It is the estimated loss, and the right replacement depends on how much you also take in and on the event, which is not modelled.
• Leaving sweat type on average when you often see white salt streaks on clothes. Choosing salty raises sodium by half, while potassium stays the same in every case.
• Using one hot-weather sweat rate for every workout. Rate changes with heat, intensity and fitness, and the totals scale directly with it.