How much should you drink during a marathon?

The most common hydration advice for marathon runners is to drink 400–600ml of fluid per hour. It's printed in training plans, shouted at expos, and repeated on every running website you'll find.

It's also almost certainly wrong for you.

Not because the researchers made an error. But because they averaged data across dozens of runners — and averages mask the differences that matter. A 58kg runner at a comfortable 5:30/km pace in 12°C weather can lose as little as 600ml per hour. A 90kg runner pushing hard in 22°C humidity can lose 2.4 litres in the same time.

Telling them both to drink 500ml per hour is the problem.

Why sweat rate varies so much

Your sweat rate is determined by a combination of factors:

• Body mass — larger bodies produce more heat and sweat more
• Fitness level — well-trained athletes sweat more efficiently, and earlier
• Ambient temperature and humidity — both increase sweat rate significantly
• Exercise intensity — sweat rate scales with effort, not just time
• Heat acclimatisation — regular heat exposure increases sweat rate and efficiency

These factors interact. A trained runner in hot, humid conditions can sweat at 3x the rate of a less-trained runner in cool conditions. The difference in fluid needs is enormous.

The consequences of getting it wrong

Drinking too little leads to dehydration. Performance degrades at around 2% body-weight fluid deficit — for a 75kg runner, that's 1.5 litres. By 3–4%, you'll feel it in your legs and decision-making.

But drinking too much carries its own risk. Hyponatremia — dangerously low blood sodium caused by excessive fluid intake without electrolyte replacement — is rare but serious, and has killed marathon runners. It's more common in slower runners who have more time to drink at aid stations.

The goal isn't to drink as much as possible. It's to drink the right amount for your sweat rate, and to replace the electrolytes your sweat takes with it.

How to measure your personal sweat rate

What you need: A scale accurate to at least 100g, a water bottle with a known volume, 60 minutes of exercise at consistent effort.

The method:

1. Weigh yourself naked before exercise
2. Note exactly how much you drink during the session
3. Weigh yourself again naked immediately after
4. Calculate: fluid consumed (ml) + weight lost (g) = total fluid lost

For example: if you weigh 750g less after a 60-minute run, and drank 300ml during, your sweat rate was approximately 1.05 litres per hour.

Do this across several sessions in different conditions. Your sweat rate in winter will be meaningfully different from your sweat rate on a warm race day — and your race plan should reflect the conditions you'll actually be running in.

What sweat composition tells you

Sweat isn't pure water. It contains electrolytes — primarily sodium, but also potassium, magnesium, and chloride. Concentration varies: some people are "salty sweaters" who lose large amounts of sodium per litre, others lose relatively little.

White residue on your skin and kit after a long session indicates high sodium loss. If you're regularly cramping, sodium depletion is a likely contributor — even if you're drinking enough fluid.

Putting it together for race day

Your race-day hydration plan should account for your measured sweat rate (adjusted for expected race conditions), your race duration, aid station locations on the course, and the sodium content of what you'll be drinking.

On a warm day, your sweat rate will be higher than in training. On a cold day, it may be lower — but fluid needs don't disappear entirely.

Sweatr calculates your race-day fluid needs from your actual sweat rate data, your wearable activity metrics, and the predicted race conditions — and builds your plan around the aid stations on your specific course. The number it gives you isn't an average. It's yours.

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The information in this article is for educational purposes. Individual physiology varies significantly. For medical advice, consult a sports medicine professional.