Race day hydration calculator
Drinking too little slows you down, and drinking too much is dangerous. Here is a sensible range for your race, and a simple sweat test for a personal figure.
Your details
A drinking range for your race
400 to 600
ml an hour, to thirst
- Over the whole race1.6 to 2.4 litres
- Per 20 minutes133 to 200 ml
Drink to thirst within this range. Drinking far more than you sweat can cause hyponatraemia (low blood sodium), which is dangerous; if you weigh more after a race than before, you drank too much. On long hot races, salty snacks or electrolyte drinks help.
Confusion, a bad headache, vomiting or collapse during or after a race needs urgent help: tell a marshal or call 999.
The working
Four hours, mild day
The ACSM range is 0.4 to 0.8 litres an hour. On a mild day we suggest 400 to 600 ml an hour, so a 4 hour marathon is 1.6 to 2.4 litres in total, about 130 to 200 ml every 20 minutes.
Sweat test
Make it personal
Weigh yourself before and after a 1 hour training run, in the same kit. Each kilogram lost is a litre of sweat; add what you drank. If you lose 0.8 kg and drank 300 ml, your sweat rate is 1.1 litres an hour. Aim to lose no more than 2% of your weight, and never gain.
Overdrinking
The quiet danger
Hyponatraemia, low blood sodium from drinking far more than you sweat, is a real risk for slower marathon runners. Signs include headache, nausea, confusion and swelling. Drink to thirst and do not drink at every station just because it is there.
Read with care
What this cannot tell you
- Medical conditionsHeart or kidney conditions change fluid advice; ask your GP.
- Salt needsVery salty sweaters may need electrolytes.
- Heat illnessHot races need slower pacing, not just more water.
Questions
Common questions
How much water should I drink during a marathon?
Typically 400 to 800 ml an hour, to thirst.
Can you drink too much water in a race?
Yes. It can cause hyponatraemia, which is dangerous.
How do I work out my sweat rate?
Weight lost plus fluid drunk, divided by the hours run.
Sources
- Sawka MN et al. ACSM position stand: exercise and fluid replacement. Med Sci Sports Exerc 2007
- Thomas DT et al. Nutrition and athletic performance: joint position statement. Med Sci Sports Exerc 2016
Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.
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