Sweat rate: how much to drink and how much sodium during exercise?
Weigh yourself before and after a session to calculate your sweat rate, sodium loss and how much to drink during and after exercise.
Short answer
Your sweat rate = (weight before − weight after + water drunk) ÷ duration. It often ranges from 0.5 to more than 2 litres per hour, depending on the person and the heat. The goal is not to replace everything during exercise, but to avoid losing more than 2% of your body weight, without drinking beyond your thirst.
Calculator
Validated inputsYour measurements
Take your measurements under similar conditions from one session to the next.
Saltiness affects how much sodium needs replacing. If you often have white marks on your clothes, choose "Very salty".
Tip: weigh yourself just before and just after training, without heavy clothing, and try to keep measurement conditions identical. A small variation of 100 to 200 g can already change your estimate.
Sweat rate
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L / hour
Total loss
--
Estimated L
Sodium lost
--
mg of sodium
Sodium / hour
--
mg/h
Salt equivalent
--
g of total salt
Target drink
--
L / hour
Recovery plan
What you can drink after your session to replenish losses.
Practical tips
- • If your sweat rate is high, spread out your drink intake rather than drinking it all at once.
- • Sodium-rich drinks are often more effective than simply drinking excess water.
- • Significant weight loss after a session indicates an inadequate hydration strategy.
- • For prolonged efforts, sodium is generally more important than potassium during exercise.
What the tool measures
Water alone is not always enough
Sweat mainly contains water and sodium. Its sodium concentration varies enormously between people, from about 200 to more than 2,000 mg per litre. That is why the same drinking plan does not suit everyone.
The American College of Sports Medicine's position stand recommends avoiding water loss exceeding 2% of body weight during exercise, and also avoiding drinking more than you lose. Drinking large amounts of water without sodium during prolonged exercise can dangerously lower blood sodium: this is hyponatraemia.
Weighing yourself before and after a session is the simplest way to determine your sweat rate. For everyday water needs, use the hydration calculator.
Interpreting the result
Interpret without overpromising
- Sweat rate: below 0.5 L/h is low, above 1 L/h is high.
- Sodium lost: estimated from a concentration of 500, 800 or 1,200 mg per liter based on your answer about salty sweat.
- Fluid target: around 75% of your sweat loss, between 0.35 and 1.35 L/h, for your next workout in similar conditions.
- After the session: around 1.25 to 1.5 liters per kilo lost, with sodium, spread over a few hours.
Calculation and assumptions
Methodology
- Sweat loss (L) = weight before − weight after + water consumed, assuming 1 kg = 1 L.
- Sweat rate = loss ÷ duration in hours.
- Sodium lost = loss × estimated concentration (500, 800 or 1,200 mg/L).
- Salt equivalent = sodium ÷ 393, because 1 g of salt contains 393 mg of sodium.
- Fluid and sodium per hour = 75% and 70% of losses, adjusted by −10 to +15% depending on conditions.
Sources consulted
Sources
- American College of Sports Medicine (2007) : Exercise and fluid replacement
Reference position statement: avoid losing more than 2% of body weight through water loss, without drinking more than you lose.
- Baker (2017) : Sweating rate and sweat sodium concentration in athletes, a review of methodology and variability
Review of sweat rate measurement and the wide variability in sweat sodium content.
Scope of validity
Limitations to know
- Your sweat sodium concentration is estimated, not measured. Only a laboratory test can measure it.
- Weigh yourself without wet clothing and account for any bathroom visit.
- Headache, nausea, confusion or swollen hands during prolonged exercise may indicate hyponatremia: this is an emergency.
- If you have high blood pressure, heart failure or renal impairment, seek medical advice before adding salt.
Frequently asked questions
Before using the result
How do you calculate your sweat rate?
Weigh yourself naked before and after a session lasting around an hour. Add the water you drank during the session to the weight difference, then divide by the duration in hours. One kilo lost corresponds to around one litre.
Do you need electrolytes during exercise?
For a session lasting less than an hour, water is usually enough. Beyond that, especially in the heat or if you sweat heavily, a drink containing sodium becomes useful.
How much should you drink during exercise?
Enough to avoid losing more than 2% of your weight, without drinking more than you sweat. For many people, this means 0.4 to 0.8 liters per hour.
What is hyponatraemia?
A drop in blood sodium, often caused by drinking large amounts of water without salt during prolonged exercise. It can cause nausea, headaches and confusion, and severe cases are serious.