Most runners need roughly 400 to 800 milliliters (about 14 to 27 ounces) of fluid per hour during a run, but the real answer depends on your body size, pace, and the weather. Sweat rates among runners range from 0.5 to 3.0 liters per hour, which means a one-size-fits-all number will leave some people dehydrated and others waterlogged. The good news is that figuring out your personal needs is straightforward once you know a few key principles.
Before You Run
Start hydrating two to four hours before your run by drinking about 2 to 4 milliliters per pound of body weight. For a 150-pound runner, that works out to 300 to 600 milliliters, or roughly 10 to 20 ounces. Spreading this out over a couple of hours gives your body time to absorb the fluid and lets you urinate any excess before you head out the door. Your urine should be pale yellow at the start of your run. If it’s clear, you may have overshot. If it’s dark, you need more.
During Short Runs (Under 60 Minutes)
For easy runs lasting less than an hour, drinking to thirst is usually enough. Research comparing thirst-based drinking to scheduled intake during shorter, lower-intensity exercise found no meaningful differences in performance, heart rate, or core temperature regulation. Plain water is fine for these efforts. Many runners don’t need to carry a bottle at all for a 30- to 45-minute jog, especially in mild weather, as long as they started well-hydrated.
During Longer Runs (90 Minutes and Beyond)
Once you’re out for more than about 90 minutes, a more deliberate hydration plan pays off. Losing more than 2% of your body weight in fluid (roughly 3 pounds for a 150-pound person) measurably reduces endurance performance, even in moderate temperatures around 68 to 70°F. A general guideline is to consume 200 to 300 milliliters every 10 to 20 minutes, adjusting based on how much you’re sweating.
Planned drinking becomes especially important in hot conditions, at higher intensities, and during races where you want to perform your best. One study comparing scheduled versus thirst-based drinking during a 30-kilometer cycling effort in the heat found that matching fluid intake to sweat losses provided a performance advantage through lower body temperature and better sweating responses. The same principle applies to long runs and marathons in warm weather.
When to Add Electrolytes
Plain water works for most runs under 90 minutes. Beyond that, you start losing enough sodium through sweat that water alone can actually work against you. Sodium helps your body hold onto fluid and keeps the concentration of your blood in a healthy range. Without it, drinking large volumes of plain water can dilute your blood sodium to dangerously low levels.
The American College of Sports Medicine recommends 300 to 600 milligrams of sodium per hour during prolonged exercise. Most commercial sports drinks contain sodium in the range of 230 to 690 milligrams per liter, which falls in the sweet spot for absorption and preventing low blood sodium. If you prefer water over sports drinks, pairing it with salted snacks, salt tablets, or electrolyte tablets achieves the same goal. Sodium replacement becomes especially critical for runs longer than two hours, in hot weather, or if you’re a naturally heavy or salty sweater (you’ll notice white residue on your clothes or skin).
The Danger of Drinking Too Much
Overhydrating is a real risk, not just an academic concern. Exercise-associated hyponatremia, a condition where blood sodium drops below safe levels, develops when a runner takes in more fluid than the body can excrete through sweat, urine, and breathing. This can happen when intake exceeds losses by more than about 1.5 liters over the course of a run.
The risk is highest during marathons and ultramarathons, particularly among slower runners who have more time and more aid station stops to drink. Physical exertion itself can trigger your body to retain extra water by releasing a hormone that reduces urine production. Symptoms range from nausea and headache to confusion and, in severe cases, seizures. The practical takeaway: never force fluids beyond what thirst and your hydration plan call for. Gaining weight during a run is a red flag that you’re drinking too much.
How to Calculate Your Sweat Rate
The single most useful thing you can do is figure out your personal sweat rate. The CDC recommends a simple formula: weigh yourself before your run (without clothes, ideally), run for a set time, then weigh yourself again. Add back whatever you drank during the run and subtract any urine volume.
The formula looks like this: pre-run weight minus post-run weight, plus fluid consumed, minus urine volume, divided by exercise time in hours. The result is your hourly sweat rate. For example, if you lose 1.5 pounds during a one-hour run and drank 16 ounces, your total fluid loss was about 40 ounces per hour (since one pound of body weight equals roughly 16 ounces of fluid). That gives you a target to aim for next time.
Run this test a few times in different conditions. Your sweat rate on a cool fall morning will be very different from a humid July afternoon. While overall humidity doesn’t always increase the total amount you sweat, heat raises your core temperature faster and demands more aggressive cooling, so your fluid needs shift with the season.
After Your Run
Post-run rehydration should replace 100% to 150% of whatever fluid you lost. The range goes above 100% because your body continues to produce urine after you stop running, so some of what you drink passes right through. If you lost two pounds during a run, aim to drink 32 to 48 ounces over the next few hours. The higher end of that range matters most when you have less than four hours before your next workout or a second run the same day.
Including sodium in your recovery fluids or meal helps your body retain what you’re drinking rather than flushing it out. A meal with normal salt content, paired with water, handles this naturally for most runners. If you’re recovering quickly between sessions, a sports drink or adding an electrolyte mix to your water speeds the process.
Quick Hydration Check
Urine color remains the simplest day-to-day hydration monitor. Pale straw-colored urine suggests you’re well-hydrated. Dark yellow or amber means you need more fluid. Completely clear urine, especially in large volumes, can signal overhydration. Checking your urine first thing in the morning gives you the most reliable reading, since food, supplements, and medications can temporarily shift the color later in the day.
Pairing the urine check with occasional weigh-ins before and after runs gives you a complete picture. If you’re consistently losing more than 2% of your body weight during runs, drink more. If you’re gaining weight, drink less. Over time, you’ll develop an intuitive sense of what your body needs at different distances, paces, and temperatures.