This article is written by Amy from The Running Dietitian.
Why "just drink more water" isn't good advice
You’ve heard it a thousand times: stay hydrated and drink more fluids. It’s in every coach’s pre-race reminder, running through your head every time you fill up a cup. What almost nobody tells you is how much you actually need.
A short, easy run doesn’t require the same needs as a long run in the summer heat. This is where “stay hydrated” falls short, it doesn’t give you a concrete number or even range to aim for. You need a way to figure out what your body actually needs, on easy days, hard days, and everything in between.
In this guide you’ll learn the framework for determining how much fluid you actually need.
The Science in Simple Terms
Hydration does a few things for you when you run.
It keeps you cool: your muscles produce heat and sweat is your body’s main way of getting rid of it. As sweat evaporates off your skin, it carries heat away with it. If you lose too much fluid without replacing it, your cooling system can start to fail, causing your core temperature to climb higher, which can decrease performance.
Sweat rates can be wildly different from one runner to the next. Sweat rate depends on body size, fitness level, heat-acclimation status, and even genetics. One runner may lose half a liter an hour while their running partner, running the same pace and distance could lose two liters an hour.
Another big factor in sweat rate is the weather: a runner will sweat more in hot and humid conditions than doing the same workout in cooler temperatures.
Fluid also plays a role in keeping your blood pumping efficiently. Fluid loss shrinks the volume of blood moving through your body, so your heart has to work harder to push the same amount of oxygen to your working muscles. Fluid is also involved in nearly every process your muscles use to contract, including how they interact with electrolytes like sodium and potassium.
Bottom line: there’s no single fluid number that fits every runner. Below we’ll break down different types of runs and the fluid ranges to aim for on each.
Everyday Hydration (non-training days and easy runs)
Long runs and race day get all the attention, but your daily habits are the foundation to being a well hydrated runner. Show up to a run, no matter the distance, dehydrated and your performance will suffer.
A commonly cited baseline for daily fluid intake is around 2.7 liters or 91 ounces a day for women and 3.7 liters or 125 ounces a day for men. This includes fluid intake from all sources, not just water intake. Runners who are regularly training will need a bit more than that, since they’re losing more fluid through sweat than the average person.
About 20% of most people’s daily fluid intake comes from food, especially fruits, vegetables, and soups, so you don’t have to get your fluid intake from drinks alone. Although coffee is typically talked about as dehydrating, both coffee and tea count towards fluid needs as well.
Easy runs, lasting under an hour and in mild weather, usually don’t need a specific hydration plan. Hitting your baseline or slightly above it should be enough to keep you hydrated. An everyday gauge is urine color: pale yellow usually means your hydration status is good, while dark yellow means you are dehydrated and need to drink more fluids.
Scaling It Up: long runs, heat, and race day
The right amount of fluid during a run depends on: how long the run will last, the intensity of the run, your hydration status going into the run, and what the weather conditions are. On runs lasting >60 minutes, during the run fluid intake becomes essential.
The single most useful thing you can know about your own body as a runner is your sweat rate.
To test it: weigh yourself without your kit and shoes on before a run, then again at the end of a 60 minute run after towel drying off, without drinking or going to the bathroom during the run. Take your pre-run weight, subtract your post-run weight, and that total is your approximate sweat loss. Divide by the run duration in hours to get your estimated hourly sweat rate. Test it a few times in different conditions, since it shifts with heat and effort.
You don’t need to replace 100% of the fluid that you are losing on a run. Replace a sensible proportion of your losses, scaling up as event duration and heat increase. A good rule of thumb is to aim for 10-27oz/fluid per hour. If your sweat rate is closer to the higher end, around 2.5L per hour, aim for the higher end of that range.
Applying a Hydration Strategy to Race Day
Race day hydration starts before the race even begins. Toeing the start line properly hydrated has as much to do with your habits in the days before as what you drink that morning.
Taper week is a good window to really focus on hydration. As training volume decreases, fluid needs may ease off slightly too, but it’s also a good stretch to dial in your fluid habits and make sure you’re not starting race week in a deficit.
Check out the Race Day Hydration Strategy in this article for specific guidelines on what hydration and sodium intake should look like depending on the race distance.
The Other Side: Can You Drink Too Much?
Most hydration advice focuses on drinking too little. But drinking too much can put you at risk too.
When you take in more fluid than you're losing through sweat, especially without enough sodium, blood sodium can drop to dangerous levels – a condition called hyponatremia. This is why more water is not always better. Early symptoms of hyponatremia like nausea, headache, and bloating can look like mild dehydration, which sometimes leads runners to drink even more, making things worse.
Fixed hydration schedules often ignore how different runners are, and how even one runner can lose different amounts depending on conditions. A runner losing half a liter an hour and one losing 2 liters an hour following the same schedule end up in very different places. Matching intake to your own sweat rate is essential.
Common Hydration Mistakes
Drinking a fixed amount: this won’t account for how much you are losing that day since sweat rate can vary by both weather conditions and intensity of the run.
Basing needs on thirst alone: thirst often lags behind what you actually need.
Treating every run the same: a 30-minute easy run and a 3-hour long run in the heat call for different strategies.
Not repeating a sweat rate test throughout the year: since sweat rate varies by conditions, testing a few times a year gives you an accurate range to aim for.
Key Takeaways
There’s no single fluid number that works for every runner. Daily hydration is the foundation to staying hydrated on your runs. Easy runs under an hour, in mild weather, usually don't need extra fluid. For runs over an hour, aim for 10-27oz of fluid per hour. For exact race-day fluid and sodium targets, check our Electrolytes and Race Day Hydration Guide.