Electrolyte Replacement Calculator: Sweat Sodium, Fluid and Salt to Replace
Estimate the fluid, sodium and potassium you lose in sweat during exercise, from your sweat rate or a before-and-after weigh-in, and get a per-hour drinking and sodium plan, salt, sports drink and tablet equivalents, and how much to drink afterwards.
How it works
Sweat rate (weigh-in) = (weight before − weight after + fluid drunk) ÷ hours, with 1 kg ≈ 1 L · Fluid lost = sweat rate × hours · Sodium lost = fluid lost × sweat sodium (mg/L; 1 mmol/L = 22.99 mg/L) · Salt (g) = sodium (mg) ÷ 1000 ÷ 0.3934 · Plan per hour = sweat rate × fluid share, sodium per hour × sodium share · After exercise drink 1.5 × the remaining fluid deficit
Sweat carries water and dissolved electrolytes, mainly sodium and chloride with a little potassium. Sweat rates usually run 0.5–2 L per hour, and sweat sodium ranges from about 230 to 2,000 mg per litre (10–90 mmol/L), so two people doing the same run can lose very different amounts. Multiplying your sweat rate by the duration gives the fluid lost, and multiplying that by your sweat sodium gives the sodium lost. If you do not know your sweat rate, weigh yourself before and after a typical session: each kilogram lost, plus whatever you drank, is about a litre of sweat. The plan does not try to replace everything during exercise. Sports-nutrition guidance is to drink enough to keep weight loss under about 2% of body weight, but no more than about 80% of sweat loss, because drinking more than you sweat can dilute blood sodium (hyponatremia). The calculator uses 75% for fluid and sodium by default; you can change both under More options. It shows the sodium concentration your drink would need, how much extra sodium to take on top of your sports drink, and tablets per hour. Afterwards, drinking about 1.5 times the remaining deficit, with salty food or an electrolyte drink, restores fluid balance because some of what you drink is passed as urine. Potassium is estimated from an average sweat level of 5 mmol/L. The figures are estimates: a sweat test, heat acclimatisation (which lowers sweat sodium) and how you feel should guide your own plan.
Worked example
Sweating 1.0 L/hr for 1.5 hours loses 1.5 L of fluid. At 900 mg of sodium per litre that is 1,350 mg of sodium (900 mg per hour), equal to 1,350 ÷ 1000 ÷ 0.3934 = 3.4 g of salt (0.57 tsp), or 2.93 L of a 460 mg/L sports drink. Potassium loss is about 1.5 × 195.5 = 293 mg. Replacing 75% during the session means drinking 0.75 L per hour with 675 mg of sodium per hour, a drink of about 900 mg/L. A 460 mg/L sports drink gives 345 mg per hour at that volume, so add about 330 mg per hour, or take 2.3 tablets of 300 mg per hour. Afterwards, 0.375 L is still missing, so drink about 0.56 L over the next few hours.
Sources: Baker LB. Physiology of sweat gland function: the roles of sweating and sweat composition in human health. Temperature 2019 · DGE Sports Nutrition Working Group. Fluid replacement in sports. Dtsch Z Sportmed 2020 · American College of Sports Medicine position stand: Exercise and fluid replacement (2007) · LMNT Science: Sweat sodium concentration · Rundida: Electrolyte and sodium loss calculator
Frequently asked questions
How do I find my own sweat rate?
Weigh yourself with little or no clothing before and after an hour of typical exercise, and note what you drank. Weight lost plus fluid drunk is your sweat loss (1 kg ≈ 1 L); divide by the hours. Choose the weigh-in option to let the calculator do it.
How much sodium do I lose per hour?
Multiply your sweat rate by your sweat sodium. At an average 1 L per hour and 900 mg/L that is about 900 mg per hour, but it can range from under 300 mg to over 2,000 mg per hour between people.
Do I need to replace all of it during exercise?
No. For sessions under about an hour water is usually enough. For longer or hot sessions, replace part of the loss as you go (the default here is 75%) and make up the rest with meals and drinks afterwards.
How much should I drink after exercise?
About 1.5 litres for every kilogram of body weight still lost, spread over the next few hours, ideally with some sodium from food or an electrolyte drink so you retain more of it.
What is a salty sweater?
Someone whose sweat holds a lot of sodium, roughly 1,200 mg/L or more. Signs include white salt marks on clothing and stinging sweat. A sweat-patch test gives a measured value.
Can too much sodium or water be harmful?
Yes. Drinking much more than you sweat can cause hyponatremia, which is dangerous. Large amounts of extra salt are not advised for people with high blood pressure, heart or kidney disease. Replace only what you actually lose.
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Educational estimate only, not medical advice. Sweat losses vary widely between people and conditions. If you have high blood pressure, heart or kidney disease, or take medicines that affect fluid or sodium, ask your doctor before changing your salt or fluid intake.