Running hydration: how much to drink
Estimate sweat rate with a worked example and plan access to drinks while running. Understand individual needs and the risks of overdrinking.
In this article
There is no single hourly water intake that works for every runner. Hydration planning should consider sweat loss, duration, conditions and opportunities to drink. ACSM recommends individualising replacement because fluid and electrolyte losses vary considerably. Exercise and fluid replacement.
Start with your context rather than a particular drink. Does the route have water? Will you run for one hour or three? Have you rehearsed the strategy in similar conditions?
Estimating your sweat rate
A field estimate compares body mass before and after exercise, adds what you drank and subtracts urine passed during that period:
Estimated sweat loss in litres ≈ pre-run mass − post-run mass + fluid consumed − urine.
Divide by duration in hours. For this practical calculation, one kilogram of mass change is approximated to one litre of water. It is an estimate, not a laboratory test or a measurement of sweat sodium concentration.
ACSM describes using body mass changes to estimate individual losses. Weigh under comparable conditions, wearing little clothing and drying sweat, and measure what you drink. Sawka and colleagues.
A complete worked example
Imagine a 75-minute run with these measurements:
| Measurement | Example value |
|---|---|
| Pre-run body mass | 68.4 kg |
| Post-run body mass | 67.8 kg |
| Fluid consumed | 0.35 L |
| Urine during the period | 0 L |
| Duration | 1.25 hours |
The mass difference is 0.6 kg. Adding 0.35 L gives estimated sweat loss of 0.95 L. Dividing by 1.25 hours gives approximately 0.76 L/hour.
That describes this session. It does not instruct you to drink 0.76 litres every hour on every run, or to replace exactly all sweat loss during exercise. It helps inform a plan and identify whether a copied intake is far from your observed losses.
If you repeat the estimate, record date, approximate temperature, duration, route and sensations. Compare similar situations before drawing conclusions.
More drinking is not always better
Exercise-associated hyponatraemia involves disruption of water and sodium balance; excessive fluid consumption is a central factor to avoid. The relevant consensus recommends using thirst to limit overdrinking. Adding sodium does not cancel out excessive water intake. International exercise-associated hyponatraemia consensus.
For a healthy recreational runner, forcing a bottle at every aid station makes little sense if it exceeds their needs. During long efforts, heat or particular circumstances, combine bodily signals with an individual strategy practised beforehand.
Do not aim to finish heavier as proof of good hydration. Nor does a particular mass loss automatically mean it is appropriate to continue despite symptoms.
Water, sports drink or electrolytes?
ACSM notes that carbohydrate-electrolyte drinks can be useful in some circumstances. They are not compulsory for every easy run. Fluid replacement guidance.
Separate three decisions:
- Fluid: access, volume and tolerance.
- Energy: whether the effort needs carbohydrate during exercise.
- Sodium: whether individual losses and conditions justify a specific strategy.
A product may serve several purposes, but read its label. An electrolyte capsule may contain no carbohydrate, while a sugary drink may provide little sodium.
Organise the route before leaving
For a long run, identify water access and an option to shorten the route. Check that fountains work; carrying an empty bottle to a closed fountain does not solve the problem.
For races, check what organisers provide. Do not calculate a drink’s carbohydrate contribution if you do not yet know which product it is. If your strategy relies on a specific product, verify availability and rehearse it.
Scope of this article
The worked example is original. Field estimates contain error and do not replace nutritional or medical assessment, particularly with conditions or medication affecting fluid balance.