Running Hydration Calculator
Estimate your fluid and sodium needs based on body weight, duration, and race-day conditions.
How it works
This calculator estimates your sweat rate based on body weight, exercise intensity, temperature, and humidity. It recommends replacing about 80% of sweat losses, the ACSM guideline for maintaining performance without over-hydrating.
Sodium recommendations are based on 500mg per liter of fluid consumed, the mid-range of ACSM guidelines. If you're a heavy sweater or see salt stains on your gear, consider going toward the higher end (600-700mg/L).
For runs over 2 hours, consider using a drink mix to get both hydration and carbs in one step. This reduces the number of gels you need and makes your fueling plan simpler.
Signs of dehydration during a race
A 2% loss in body weight from fluid (about 3 lbs for a 150 lb runner) is where performance starts to drop measurably. Early signs: dark urine before the race, dry mouth, and a heart rate that drifts higher than expected for your pace. By the time you feel thirsty, you're already behind on fluids. The fix is drinking early and consistently—not catching up at mile 18.
The risk on the other side: hyponatremia
Over-hydrating is just as dangerous as under-hydrating. Hyponatremia (low blood sodium from drinking too much water) causes nausea, confusion, and in severe cases, seizures. It's most common in slower runners who drink at every aid station regardless of thirst, especially in cooler conditions where sweat rates are low. The rule: drink to thirst, not to a fixed schedule. If you gain weight during a race, you drank too much.
Sweat rate varies more than you think
Individual sweat rates range from 16 oz to 64 oz per hour. Two runners side by side in the same race can have wildly different fluid needs. The best way to know your rate: weigh yourself before and after a training run (without drinking during), and measure the difference. Every pound lost is about 16 oz of sweat. Do this at different temperatures to build a real picture of your needs.
Racing in warm conditions? Read our full guide on how heat affects your pace and fueling for temperature-specific hydration and sodium strategies.
Not sure how today's conditions will affect your pace? Use the Running Heat & Dew Point Calculator to get adjusted paces and hydration targets for your exact conditions.
Frequently Asked Questions
How much water should I drink during a marathon?
Most runners need 16-32 oz (500-1000 ml) of fluid per hour during a marathon, depending on body weight, pace, temperature, and humidity. The ACSM recommends replacing about 80% of sweat losses. A 150 lb runner in 65°F conditions typically needs around 20-24 oz per hour. In hot conditions (80°F+), fluid needs can jump to 32+ oz per hour.
How much sodium do I need while running?
The ACSM recommends 300-600 mg of sodium per liter of fluid consumed during endurance exercise. For most runners, that works out to 500-700 mg per hour during a marathon. Heavy sweaters or runners who see salt stains on their gear may need more (up to 1000 mg/hr). Sodium helps maintain blood volume and prevents hyponatremia.
What is a normal sweat rate for running?
Individual sweat rates range from 16 oz to 64 oz (0.5-2.0 liters) per hour. The average runner loses about 1-1.5 liters per hour at moderate intensity in temperate conditions. Sweat rate increases with temperature, humidity, intensity, and body size. To measure yours, weigh yourself before and after a run without drinking—every pound lost equals about 16 oz of sweat.
Can you drink too much water during a marathon?
Yes. Over-hydrating causes hyponatremia (dangerously low blood sodium), which can lead to nausea, confusion, and in severe cases, seizures. It is most common in slower runners who drink at every aid station regardless of thirst, especially in cooler conditions. The rule: drink to thirst, not to a fixed schedule. If you gain weight during a race, you drank too much.