Your fluid need is not printed on a bottle, it is on your scales. The weigh-in method measures your sweat rate in a single session — plus the 2 per cent mark.
The honest answer: there is no fluid intake that is right for everyone. How much you need per hour depends on your own sweat rate — and depending on the person, the weather and the intensity, that runs from half a litre to well over a litre per hour. You will not get your number from an article. You get it from a set of bathroom scales.
What is documented is the mark at which it starts costing you performance: a fluid loss of more than 2 % of your body mass impairs endurance performance — particularly in the heat — and can also reduce cognitive performance; even 1 % raises the strain on your cardiovascular system [1].
So you know what we are talking about: at 75 kg, 2 % is exactly 1.5 kg. That is one large bottle plus half a glass. That little. On a warm day you get there in ninety minutes without noticing.
The good news: the position stand those numbers come from names the method in the same breath — determine sweat rate individually by weighing before and after the session [1]. It takes five minutes and needs nothing but scales. No lab, no sweat test, no subscription.
"Two litres a day" is a rule of thumb for everyday life — and even there more of a rough guide than a law of nature. For sport it is about as useful as one shoe size for everybody.
Your sweat rate depends on things that change daily: intensity, temperature, humidity, clothing, training status and your body size. Two people, same running group, same pace — and a factor of two to three between their sweat rates. So you measure. Once. And then you know.
This is how you determine your sweat rate, exactly as the position stand describes [1]:
Example: 1.0 kg lighter, 500 ml drunk, 90 minutes out. That is 1000 + 500 = 1500 ml of sweat in 1.5 hours — your sweat rate is around 1 litre per hour.
That number is worth more than any recommendation from the internet, because it is yours.
You do not have to replace what you lose — you only have to stay under the 2 per cent mark [1]. Work out once how many kilos that is for you and you have a budget instead of a rule.
That interval advice is practice, not a study result. The documented part is the limit, not the rhythm.
Your body does report in, just late. The most usable everyday signs are experience values, not measurements:
The part almost nobody mentions: you can also drink too much, and that is more dangerous than a bit of thirst. Take on more fluid than you lose over several hours and you dilute the sodium in your blood. The result is exercise-associated hyponatraemia — a medical emergency, not a training error.
The people affected are typically those in long events who drink at every station "to be on the safe side" even though they barely sweat. The warning signs are in the safety notes below. Take just the principle from here: more is not better. Matching is better.
On longer sessions two things join in: carbohydrate and sodium. Sodium has two documented jobs — it supports thirst and fluid retention, so it keeps more of what you drink inside you [1]. And the carbohydrate in the drink counts towards your fuelling: with a single type of sugar, the absorption ceiling is reached at around 60 g per hour [2].
None of that has to be bought. How to mix your own for a few cents is in the next guide.
This guide explains training practice and does not replace medical or dietetic advice. Anyone with heart, kidney or liver conditions, on diuretic medication, or pregnant should discuss fluid and sodium intake with a doctor — general sports recommendations do not transfer to those situations unchecked.
Exercise-associated hyponatraemia develops when more is drunk than lost over several hours and the sodium in the blood becomes too dilute. Warning signs: **headache, nausea and vomiting, confusion, puffy fingers or face, weight gain during the effort.** That is an emergency. Do not drink more, do not carry on, get medical help immediately — call the emergency number if in doubt.
An emergency in the other direction too: confusion, sweating that stops, hot dry skin or clouded consciousness in the heat. Again that means calling for help, not "one more lap".
Measure twice a year — once in February, once at the height of summer. Most people are surprised how much their sweat rate differs between 5 and 28 degrees. With two values you have a range instead of a number, and that carries you through the whole year.
And the trick for out on the road: mark your bottle with a waterproof pen in quarters and assign one mark per lap or per quarter of an hour. Then you no longer have to calculate, only look. People who do not have to calculate drink more reliably.
As a rough everyday rule of thumb it works; as a sports recommendation it does not. Sport adds an amount on top that depends purely on your sweat rate — and that runs from half a litre to over a litre per hour. Instead of a daily total you need your own rate: weigh yourself once before and after a session [1].
By dark urine, by a heart rate drifting up at the same pace, by heavy legs and by concentration slipping. What is documented is that a loss of more than 2 % of body mass impairs endurance performance and can also reduce cognitive performance [1].
Yes, and it is the more dangerous mistake. Drink more than you lose over several hours and you dilute the sodium in your blood — exercise-associated hyponatraemia is a medical emergency. Headache, nausea, confusion or weight gain during the effort are alarm signals. Get medical help immediately.
Usually not much. At moderate intensity under an hour, drinking to thirst is generally enough as long as you start well hydrated. It gets critical in heat, at high intensity, or when the previous session already cost you fluid.
Over the hours after the session, not in five minutes. Drink in portions alongside a meal rather than a litre in one go — sodium from your food helps here, because it supports thirst and fluid retention [1]. The scales next morning tell you whether you are back.
Mix Your Own Sports Drink: 30 Cents Instead of 3 Euros