Hydration Calculator
Calculate exactly how much water you need before, during, and after your run based on your weight, weather, and intensity.
Frequently Asked Questions
How much water should I drink while running?
A general guideline is 150 to 250ml (5 to 8 oz) every 15 to 20 minutes during a run. However, your actual needs depend on your weight, the temperature, humidity, and your intensity. Heavier runners, hot weather, and harder efforts all increase your fluid needs. Use this calculator to get a personalized recommendation.
Should I drink water or sports drinks while running?
For runs under 60 minutes, water is usually sufficient. For runs longer than 60 minutes, a sports drink with electrolytes and carbohydrates helps replace sodium lost through sweat and provides energy. If you are a heavy sweater or running in hot conditions, consider adding electrolytes even for shorter runs.
How do I know if I am dehydrated during a run?
Early signs of dehydration include increased thirst, dry mouth, darker urine, headache, and a noticeable drop in performance. More severe signs include dizziness, confusion, rapid heartbeat, and stopping sweating. Check your urine color before and after runs as a simple hydration gauge.
Can you drink too much water while running?
Yes. Overhydration, called hyponatremia, occurs when you drink so much water that your blood sodium levels drop dangerously low. This is especially risky during long, slow events. Symptoms include nausea, headache, confusion, and in severe cases, seizures. Drink to thirst and include sodium in your hydration plan for runs over 90 minutes.
How much should I drink before a run?
Drink about 5 to 7ml per kilogram of body weight 2 to 4 hours before running. For a 70kg runner, that is roughly 350 to 500ml. Then sip another 150 to 200ml in the 15 minutes before you start. Avoid chugging large amounts right before running as this can cause stomach discomfort.
How much fluid should I drink after running?
Aim to replace 150 percent of the fluid you lost during your run. A simple method is to weigh yourself before and after running. For every kilogram lost, drink 1.5 liters over the next 2 to 4 hours. Include sodium in your post-run fluids to help your body retain the water you drink.
Does humidity affect how much I need to drink?
Yes, high humidity significantly increases your hydration needs. When humidity is high, sweat does not evaporate as efficiently, so your body produces more sweat to try to cool down. This can increase your sweat rate by 20 percent or more. In humid conditions, start drinking earlier and drink more frequently.
What are electrolytes and why do runners need them?
Electrolytes are minerals like sodium, potassium, magnesium, and calcium that help regulate fluid balance, muscle contractions, and nerve signaling. You lose electrolytes through sweat, especially sodium. Replacing them during longer runs prevents cramping, maintains performance, and helps your body absorb and retain water more effectively.
How does temperature affect hydration needs?
Higher temperatures increase your sweat rate significantly. Running in 90F (32C) heat can increase your fluid needs by 50 percent compared to running in 60F (15C). Cold weather also has effects since dry cold air and layered clothing can cause dehydration that is easy to miss because you do not feel as thirsty.
Is this hydration calculator free?
Yes, this running hydration calculator is completely free with no sign up required. Calculate your personalized hydration plan for any run distance, weather condition, or intensity level. Bookmark this page and use it before every run.
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