There is no single right amount. Most endurance athletes lose between 0.5 and 2 litres of sweat per hour, and the best plan is one built around your own sweat rate, not a generic "drink 500 ml an hour" rule.
Why generic advice falls short
Two athletes running side by side at the same pace can lose wildly different amounts of fluid. Body size, genetics, fitness, heat acclimation, clothing and the weather all play a part. Drink too little and you risk dehydration, a rising heart rate and a drop in performance. Drink too much and you risk stomach sloshing or, in extreme cases, dangerously low blood sodium.
Measure your sweat rate in four steps
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Weigh yourself naked, after using the toilet, right before a 60-minute session.
- Train at race-like intensity and note how much you drink (in ml).
- Towel off and weigh yourself naked again straight afterwards.
- Sweat rate (ml/hour) = weight lost in grams + fluid drunk in ml.
Example: you lose 0.8 kg and drink 500 ml in an hour. Your sweat rate is about 1,300 ml per hour.
Repeat this in different conditions, such as a cool morning versus a hot afternoon. You'll quickly see how much your needs shift.
Turning numbers into a plan
You don't need to replace every drop. Most athletes perform well replacing roughly 60–80% of sweat losses during longer sessions, keeping body-weight loss under about 2–3%. For a 1,300 ml/hour sweater, that's around 800–1,000 ml per hour on a hot race day.
Some practical rules:
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Under 60–90 minutes: start well hydrated and drink to thirst.
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Over 90 minutes: follow a planned intake, sipping every 10–15 minutes.
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In the heat: expect needs to rise sharply and adjust upward.
Don't forget what's in the sweat
Sweat isn't just water. It carries sodium, the electrolyte your body needs to hold onto fluid and keep muscles and nerves firing. Replacing large volumes of fluid with plain water alone can dilute your blood sodium. That's why a good hydration plan pairs fluid with electrolytes like those in Precision Fuel & Hydration - 60g Carb 1000mg Sodium/Ltr Carb & Electrolyte Mix Only 930GM.
The takeaway
Know your sweat rate, plan your intake for sessions over 90 minutes, and replace sodium as well as water. Practice your plan in training so race day holds no surprises.
