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How much should you drink while training?

The advice most of us grew up with — drink as much as you can, replace every litre — has been quietly walked back. Overdrinking is a real risk, not just wasted effort, and the right target is lower than you probably think.

Last updated September 2026

Why "drink as much as you can" is wrong

Older guidance pushed athletes to match sweat losses litre for litre. That's since been revised, because overdrinking dilutes the sodium in your blood — a condition called hyponatremia, which at its worst is genuinely dangerous and has killed marathon runners. The athletes most at risk are not the fastest ones. They're the slower finishers who are out in the heat the longest and have the most opportunities to drink at every aid station.

Meanwhile, the performance cost of mild dehydration turned out to be smaller than assumed. Losing up to around 2% of your body mass across a session is well tolerated. For a 70kg athlete that's 1.4kg, or about 1.4 litres of sweat — which in warm conditions is a perfectly ordinary two-hour ride. You do not need to drink your way back to your starting weight mid-session, and trying to is the error that causes harm.

So what is the target?

Replace enough that you finish somewhere inside that 2% band, and drink to a plan rather than to thirst alone when the session is long or hot. Thirst is a reasonable guide for sessions up to an hour or two in mild conditions. It lags behind actual need in heat and at high intensity, which is exactly when the margin for error is smallest.

Sweat rates vary enormously

Two athletes of the same size doing the same session in the same weather can differ by more than a litre an hour. This is the reason generic hydration advice is close to useless: a plan built for the average will badly over-serve one person and badly under-serve the next.

What moves the number:

• Heat — the obvious one, and it does push sweat rate up steeply.
• Humidity — matters more than most people expect. Sweat that drips rather than evaporates cools you less, so you sweat more for the same cooling effect. A humid 24°C can be harder work than a dry 30°C.
• Intensity — more heat produced means more sweat to shed it.
• Individual physiology — some people are simply heavy sweaters, and no amount of acclimatisation changes that.
• Clothing and airflow — a headwind-free indoor trainer session is a different sport from the same effort outdoors.

This is why any app's estimate, including ours, is a starting point rather than an answer. It gets you into the right range on day one. Replacing it with your own measured number is what makes the plan actually yours.

How to measure your own sweat rate

It takes one session and a set of bathroom scales:

1. Weigh yourself before the session, ideally with minimal clothing and after using the toilet.
2. Train for a known length of time, keeping track of how much you drink.
3. Weigh yourself again straight afterwards, towelled dry.

The weight you lost, plus the fluid you drank, divided by the hours you trained, is your sweat rate per hour. Each kilogram lost is roughly a litre of sweat. So if you lost 1kg over two hours and drank 500ml along the way, you sweated about 1.5 litres in two hours — 750ml an hour.

Do it once in cool conditions and once in heat and you'll have the two numbers that cover most of your year. The sweat-test tool inside My Fuel Plan does this arithmetic for you and then uses your measured rate in every plan it builds, instead of the estimate.

Practical signs, without any equipment

• Urine colour before training. Pale yellow suggests you're starting well hydrated. Dark and concentrated suggests you're not. It's a crude signal but a free one.
• Body mass before and after. The most reliable check there is, and the basis of the test above.
• How you feel late in a session. Disproportionate fatigue, a rising heart rate at steady effort and a dry mouth in the last third are worth noting and correcting next time.

One thing to be careful of: if you finish a session heavier than you started, you drank too much. That's worth taking seriously rather than treating as thorough preparation.

Fluid alone isn't the whole picture

Water replaces volume. It doesn't replace the sodium you sweated out, and drinking plenty of plain water while under-replacing sodium is precisely the combination that leads to hyponatremia. On anything long or hot, fluid and sodium need to be planned together — which is the subject of the next guide.

Reflects current American College of Sports Medicine guidance on fluid replacement. General information for healthy adults, not individual medical or dietary advice.