Estimate your daily baseline fluids plus how much to drink on the run — adjusted for weight, duration, and weather.
Enter your body weight, how long you plan to run, and the expected conditions. The calculator returns your estimated daily baseline fluid needs and the additional fluid to take in during the run. Use it to plan how much to carry, where to refill, and whether you need electrolytes.
Every runner sweats differently. To personalize these estimates, weigh yourself (minimally clothed) before and after a one-hour run without drinking. Each pound lost is roughly 16 oz of sweat — that is your hourly sweat rate, and the target you want to mostly replace on longer runs. Re-test in hot weather, since your rate can climb sharply.
A common guideline is 16–24 oz (about 500–700 ml) of fluid per hour of running, but sweat rate varies widely with body size, intensity, and heat. This calculator estimates your needs from your weight, run duration, and conditions. The most accurate method is a sweat test: weigh yourself before and after a run, and replace about 16–24 oz per pound lost.
For runs under about 60 minutes, water is usually enough. For longer efforts — and especially in heat — add sodium and other electrolytes through a sports drink, electrolyte tabs, or gels with sodium. Drinking large volumes of plain water on long runs without electrolytes can dilute blood sodium (hyponatremia), which is dangerous.
Early signs include dark urine, thirst, a higher-than-usual heart rate at a given pace, and unusual fatigue. A simple check is urine color: pale straw is well hydrated, dark yellow means drink more. Significant dehydration (more than ~2% of body weight lost) measurably hurts performance.
Start the race well hydrated rather than trying to catch up mid-race — sip fluids in the hours beforehand until your urine is pale. During a half marathon, take advantage of aid stations roughly every few miles based on your plan and the weather, and practice your race-day hydration during training long runs so nothing is new on race day.