Most people treat hydration as a daily number to hit. For anyone training, the useful question is narrower: how much fluid and sodium do you lose during a session, and are you replacing enough of it to finish strong.
Find your sweat rate, one session, no equipment
- Weigh yourself, minimally clothed, immediately before training
- Train for an hour, noting how much you drink (in liters)
- Towel off and weigh yourself again straight after
- Sweat rate = (weight before − weight after) + fluid drunk
One kilogram of weight lost is roughly one liter of sweat. Do this once in cool conditions and once in heat, for most people the two numbers are startlingly different, and that difference is the whole point.
What to do with the number
| Session length | Fluid | Sodium |
|---|---|---|
| Under 60 min | Water as you want it | Not needed |
| 60–90 min | About 400–800 ml/hour | Useful if you're a salty sweater |
| 90 min – 3 hr | Guided by your sweat rate | 300–600 mg per hour |
| Over 3 hr | Sweat rate, adjusted for heat | 600–1000 mg per hour |
These are starting points to test in training, not prescriptions. Aiming to replace roughly 70–80% of what you lose during a session is a reasonable target, full replacement while exercising is uncomfortable for most people and rarely necessary.
Are you a salty sweater?
White marks on dark kit after a session, stinging eyes, a salty taste, those suggest higher sodium losses, and they're the people who cramp. Sodium concentration in sweat varies several-fold between individuals, which is why generic advice fails for so many.
The daily side
Consistent intake through the day beats large volumes at once. Urine color is a decent rough guide, pale straw is the target, and dark means you're behind. Thirst lags dehydration, so it's a late signal rather than an early one.
When more water is the wrong answer
Drinking heavily without sodium during long efforts can dilute blood sodium, which is dangerous. If you're out for several hours, sodium is not optional. And if you're persistently thirsty, urinating far more than usual, or cramping despite getting this right, see your doctor, those can point at something coaching shouldn't be managing.
Working on this properly? This is ground we cover in PATH Endurance, with someone looking at your actual week rather than a general guide.
General education for healthy adults. Not medical advice, and not a substitute for your own doctor or provider. Print this page if it's useful, it's formatted for it.