Sweat Rate & Sodium Loss Calculator
Calculate your hourly sweat rate and estimated sodium loss from pre/post-exercise weight, fluid intake, and workout duration.
Total fluid lost
1.90 L
Sweat rate
1.27 L/hr
Estimated sodium lost
1,330 mg
This uses the standard sports-science formula: fluid lost = weight lost + fluid drunk − urine output (1 kg of body weight loss during exercise is treated as approximately 1 L of fluid). Sweat sodium concentration varies widely between individuals — typically 200 to 2,000 mg per litre — so 700 mg/L is only a rough average starting point; a sweat-sodium test gives you your own actual number, which will usually be more useful than this default.
About this tool
How to use it
- Weigh yourself (in as little clothing as practical) immediately before and immediately after a workout, in kilograms.
- Enter how much fluid you drank during the workout and how much you urinated, both in litres (0 is fine for either if that didn't apply).
- Enter the workout duration in hours and your sweat sodium concentration in mg/L — 700 is a rough average if you don't know your own.
- Press Calculate Sweat Rate, or just keep typing — the results update live.
Why it matters
Sweat rate varies enormously between individuals and conditions — a heavy sweater in hot, humid weather can lose several litres an hour, while someone else barely notices fluid loss during the same workout. Knowing your own actual sweat rate (rather than a generic guideline) lets you plan how much fluid to drink during future sessions of similar length and intensity, and knowing roughly how much sodium you're losing helps decide whether plain water or an electrolyte drink is more appropriate.
When to use it
Useful after a hard training session in conditions you want to repeat, when planning fluid and electrolyte intake for a race or long event, or when investigating recurring cramping or fatigue during exercise that might be linked to fluid or sodium loss.
Good to know
This uses the standard sports-science approach: fluid lost = weight lost during exercise + fluid drunk during exercise − urine output during exercise, treating 1 kg of body weight loss as approximately 1 litre of fluid (sweat is mostly water, so this is a standard practical approximation, not an exact physical equivalence for every person). Sweat sodium concentration varies widely between individuals, roughly 200 to 2,000 mg per litre depending on genetics, heat acclimatization, and diet — 700 mg/L is only a population-average starting point, not your actual number. A sweat-sodium test (available from some sports science labs and increasingly some consumer patches) gives your own real figure.
Common questions
Why do I need to weigh myself before and after exercise?
The change in body weight during a workout is almost entirely fluid loss through sweat, once you account for what you drank and urinated — it's the most direct, practical way to measure your own real sweat rate without lab equipment.
Is the 700 mg/L sodium figure accurate for me?
Not necessarily — sweat sodium concentration varies roughly 3 to 10-fold between individuals (from about 200 to 2,000 mg/L), depending on genetics, heat acclimatization, and diet. 700 mg/L is a commonly cited population average, useful as a rough starting point, but a sweat-sodium test gives your actual number if you want a precise figure.
How much should I drink during exercise based on my sweat rate?
A common guideline is to aim to replace roughly 80% of your measured sweat rate as fluid intake during similar future exercise, rather than 100% — over-drinking to fully match sweat losses can risk overhydration (hyponatremia) in some situations. This is general guidance, not personalized medical advice.
Why does urine output matter for the calculation?
If you urinated during the exercise session, that fluid left your body through a different route than sweat, so it needs to be added back to avoid overestimating how much you actually sweated. Most short workouts won't involve this, which is why it defaults to a small or zero value.
Should I use this to guide serious hydration decisions?
Use it as a general planning estimate. Real hydration needs depend on many individual factors (heat acclimatization, clothing, altitude, health conditions) this simple calculation can't capture — for competitive or medically relevant hydration planning, consult a sports dietitian or physician.