Water Intake Calculator for Runners
How much water a runner should drink per day by weekly mileage
Not medical advice: This is an estimate based on the published formula cited below. Individual needs vary — consult an appropriate professional for personal guidance.
How This Is Calculated
Runners lose fluid through sweat at rates that vary widely by pace, heat, humidity, and individual physiology — commonly cited ranges are 0.4–1.8 L per hour of running. Because both underhydration and overhydration carry performance and safety risks, most sports nutrition bodies now recommend individualised fluid plans over rigid one-size-fits-all rules like the old "8 glasses a day" or fixed per-mile targets.
The American College of Sports Medicine (ACSM) fluid replacement guidelines recommend runners aim to limit sweat-related body mass loss to under 2% during a run and avoid gaining weight during exercise (a sign of overdrinking, associated with hyponatraemia risk during marathons and other endurance events). Pre-run hydration of roughly 400–600 mL 2 hours beforehand, combined with 150–250 mL every 15–20 minutes during runs over 60 minutes, is a commonly cited practical starting protocol, ideally fine-tuned using individual sweat-rate testing.
Frequently Asked Questions
How much water should a runner drink per day?
A starting point is 35–45 mL per kg of body weight on running days, adjusted upward for longer distances and heat. Use the calculator above, then refine with individual sweat-rate testing (weighing before/after a typical run) for a more precise number.
How much water should I drink before a run?
ACSM guidance suggests roughly 400–600 mL (13–20 oz) about 2 hours before running, which allows time for absorption and pre-run urination so you are not starting in a fluid-overloaded state.
How much water should I drink during a long run?
For runs over 60 minutes, aim for roughly 150–250 mL every 15–20 minutes, adjusted for heat and personal sweat rate. For runs over 90 minutes, include electrolytes (particularly sodium) rather than relying on plain water alone.
Can drinking too much water during a marathon be dangerous?
Yes — this is a well-documented risk called exercise-associated hyponatraemia, caused by drinking more fluid than is lost through sweat, diluting blood sodium to dangerous levels. Runners should drink to a personalised plan based on thirst and sweat rate, not force maximum fluid intake.
How do I calculate my sweat rate as a runner?
Weigh yourself nude immediately before and after a run of known duration (ideally 60+ minutes), accounting for any fluid drunk and urine passed during the run. Each kilogram of weight lost represents approximately 1 L of sweat loss for that time period.