How Much Should You Actually Drink During Exercise?

Measure it on yourself, because averages are useless here

Weigh yourself immediately before a session and immediately after — undressed and towelled dry both times, so wet kit doesn't confuse the reading — and note everything you drank.

Sweat rate = (weight lost + fluid drunk − urine) ÷ hours

Most people land between 0.5 and 1.5 L/h. The spread between individuals is wide enough that using an average instead of your own figure defeats the point.

⚠️ The most important thing on this page

Your sweat rate is a ceiling, not a target.

Exercise-associated hyponatremia — blood sodium diluted to a dangerously low level — causes rapid swelling of the brain and can lead to seizures, coma and death.

In athletes it is almost always the result of drinking too much fluid, and the standard advice for avoiding it is to make certain that fluid intake does not exceed sweat loss.

It has killed people at endurance events. They were typically drinking diligently, not carelessly. That's exactly what makes it worth understanding: more fluid is not automatically safer, and at the extreme it's considerably more dangerous than being mildly dehydrated.

Full replacement isn't the goal either

The American College of Sports Medicine consider 0.4 to 0.8 L/h optimal during intense endurance activity — higher for faster, heavier athletes in the heat, lower for slower, lighter ones in the cold.

Notice that's often less than a measured sweat rate. On the tool's defaults, a two-hour session gives:

	Fluid
ACSM guide	0.8 – 1.6 L
Your sweat-rate ceiling	2.6 L

Finishing slightly down on fluid is normal and well tolerated. Finishing up on fluid is the direction that causes harm.

There's a built-in check for that: if your body weight rises across a session, the tool reports a negative percentage — 70 kg going to 70.3 kg after 1.5 L of drinking shows −0.43%. That means you drank more than you sweated.

Drinking to thirst is a good default

For most people, most of the time, thirst both minimises hyponatremia risk and prevents serious dehydration. That's a rather good pair of properties for a mechanism you already have.

So why measure a sweat rate at all? To know roughly what your body does, to spot an extreme, and to plan how much fluid to carry on a long or remote session.

Not to replace thirst with a stopwatch and a quota — which is closer to the behaviour that causes trouble.

Sodium, for anything over an hour

Roughly 0.5 to 0.7 g per litre of fluid is recommended for sessions longer than an hour.

It improves how the drink tastes — which matters more than it sounds when you have hours of it ahead — helps your body hold the fluid rather than passing it straight through, and may help prevent hyponatremia in people drinking large volumes.

For a short session in cool conditions it makes very little difference.

Your sweat rate isn't a fixed constant

It changes with temperature, humidity, intensity, clothing and acclimatisation — and acclimatisation itself shifts it over a couple of weeks in a new climate.

A figure from a cool easy run tells you very little about a hot hard one, which is exactly the session where getting it wrong matters most.

The tool shows this plainly: the same rider at 70 kg losing 1.5 kg over 90 minutes in the heat has a sweat rate of 1.67 L/h against 1.30 on an easier day.

Measure in the conditions you care about, and more than once.

⚠️ Warning signs, and why you need them in advance

Confusion, headache, nausea, or noticeable swelling in the hands, feet or face during or after a long event needs urgent medical attention.

Here's why it's worth knowing beforehand: hyponatremia can look like dehydration. If you respond by drinking more, you make it substantially worse.

Don't try to work out which it is from symptoms alone mid-event. Get medical help, and tell whoever assesses you how much you have been drinking — that information changes what they do.