Electrolyte Replacement Calculator
Sweat contains not just water but also essential electrolytes that must be replaced after exercise, particularly during prolonged or intense activity in hot conditions. The dominant electrolyte in sweat is sodium, with average concentrations of approximately 900 mg per liter of sweat (range 200-1,800 mg/L per ACSM data). Potassium is present at roughly 200 mg/L and magnesium at approximately 8 mg/L. This calculator multiplies your estimated sweat volume by default electrolyte concentrations to estimate the sodium, potassium, and magnesium you lost. You can adjust the sweat sodium concentration if you know you are a salty or light sweater. Enter your total sweat loss in liters (body weight lost in kg equals liters of sweat) and exercise duration.
Electrolyte replacement formula
Sodium (mg) = Sweat (L) * Na concentration (mg/L)
Potassium (mg) = Sweat (L) * 200
Magnesium (mg) = Sweat (L) * 8
Fluid to replace (mL) = Sweat (L) * 1,000
Default ACSM average electrolyte concentrations in sweat: sodium 900 mg/L, potassium 200 mg/L, magnesium 8 mg/L. Adjust sodium concentration for individual variation.
Electrolyte replacement strategies
- Weigh yourself before and after exercise (without drinking during exercise) to measure sweat rate: 1 kg lost = 1 L sweat.
- Aim to replace 100-150% of fluid lost over the hours following exercise; do not drink more than needed as over-hydration can cause hyponatremia.
- Sports drinks (440-660 mg Na per liter) are suitable for exercise over 60-90 minutes; water is sufficient for shorter sessions.
- High-potassium foods after exercise (banana, potato, dairy) help restore potassium without supplementation for most athletes.
- Individuals on diuretics, with heart disease, or kidney disease should consult their doctor before aggressive electrolyte replacement.
Electrolyte replacement calculator: frequently asked questions
What electrolytes are lost in sweat?
Sweat primarily contains sodium (the dominant electrolyte), along with potassium, chloride, magnesium, and calcium in smaller amounts. Sodium loss averages approximately 900 mg per liter of sweat, though individual sweat sodium concentration varies widely (200-1,800 mg/L).
How much sweat do I lose during exercise?
Sweat rates vary from 0.5 to 2.5 liters per hour depending on exercise intensity, ambient temperature, humidity, and individual variation. A practical method is to weigh yourself before and after exercise; every kilogram of body weight lost equals approximately 1 liter of sweat.
Do I need to replace electrolytes during exercise?
For exercise lasting less than 60 minutes at moderate intensity, water is generally sufficient for most people. For exercise lasting over 60-90 minutes, or in hot/humid conditions, replacing sodium and other electrolytes alongside fluid helps maintain performance and prevent hyponatremia (dangerously low blood sodium from drinking too much plain water).
What is the sodium concentration in sweat?
Average sweat sodium concentration is approximately 900 mg per liter (ranging from 200 to 1,800 mg/L). This calculator uses 900 mg/L as the default. Salty sweaters (those who see white salt deposits on skin or clothing) may need to use higher values.
Should I use sports drinks or electrolyte tablets?
Commercial sports drinks typically contain 440-660 mg of sodium per liter and 30-60 g of carbohydrate per liter. Electrolyte tablets or dissolvable packets allow more precise control. For most recreational exercise, a balanced diet with adequate sodium restores electrolytes; only endurance athletes need targeted electrolyte supplementation during exercise.
Official sources
- NIH Office of Dietary Supplements: Sodium Fact Sheet for Health Professionals.
- CDC: NIOSH Heat Stress Water and Electrolytes.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.