Sweat carries away more than water — it also removes sodium and, to a lesser extent, potassium. This calculator estimates how much fluid, sodium, and potassium a given workout costs you, based on duration, intensity, and sweat composition, so you can plan how much to replace. It's built for athletes, endurance trainees, and anyone exercising in the heat who wants a starting-point estimate rather than a lab-grade sweat test.

How the calculator works

The calculation is a simple chain: workout duration (converted to hours) is multiplied by a sweat rate to get total fluid lost in liters. That fluid volume is then multiplied by a sweat sodium concentration to get sodium lost in milligrams, and by a typical sweat potassium concentration to get potassium lost in milligrams.

Sweat rate defaults come from typical values for light, moderate, and heavy exercise intensity (roughly 0.5, 1.0, and 1.75 L/hr), and sweat sodium concentration defaults to a moderate value of 900 mg/L — both editable if you know your own numbers from a sweat test or careful weigh-ins before and after exercise.

Inputs and what they mean

Duration is the length of your exercise session in minutes — the single biggest lever on total loss, since everything scales with time. Intensity (Light, Moderate, Heavy) sets a starting sweat rate; pick the tier that best matches your effort level and the conditions (heat and humidity push sweat rate up regardless of pace).

Sweat rate (L/hr) and sweat sodium concentration (mg/L) are both editable. If you've never measured either, the defaults are reasonable population averages, but individual sweat rate and sodium concentration can each vary several-fold — some people are consistently 'salty sweaters' who lose far more sodium than the default assumes.

Limits and edge cases

This is an estimate built from typical population values, not a personalized sweat test — the only way to know your exact sweat rate and sodium concentration is to weigh yourself before and after a controlled workout and, ideally, have your sweat sodium tested. Heat, humidity, acclimatization, fitness level, and individual physiology all shift the real numbers, sometimes substantially.

This calculator is for general fitness and educational planning only. It is not a substitute for guidance from a sports medicine professional or registered dietitian, particularly for endurance athletes, people exercising in extreme heat, or anyone with a medical condition that affects fluid or electrolyte balance, or who experiences symptoms of dehydration or electrolyte imbalance during or after exercise.