Calculate Your Rehydration Needs
“I drank when I was thirsty—isn’t that enough?”
As a coach, I hear this all the time. But here is the reality: by the time you actually feel thirsty, you might already be 1–2% dehydrated. In the world of cross country and distance running, even mild dehydration can kill your performance, slow your recovery, and increase your risk of injury.
Thirst is a delayed signal. The most accurate way to know exactly what your body needs is to measure what you lost.
The Math of Recovery When you run, you sweat. That weight change on the scale isn’t fat loss; it is water loss. To recover fully, you can’t just replace what you lost 1-for-1. Your body continues to burn energy and produce urine even after you stop running.
According to the American College of Sports Medicine, you need to replace 125–150% of the fluid you lost to return to a fully hydrated state.
Use the calculator below to take the guesswork out of your recovery.
Post-Run Hydration Calculator
Weigh yourself before and after your run to see how much you need to rehydrate.
Note: This calculator uses the ACSM recommendation of replacing 125-150% of fluid lost. This is for informational purposes only and is not medical advice.
Frequently Asked Questions
💧 Water vs. Electrolytes: Which do I need?
This is the most common question I get from parents and athletes.
- For runs under 60 minutes: Plain water is usually sufficient. Your electrolyte stores likely haven’t been depleted enough to require a sports drink.
- For runs over 60–90 minutes (or in high heat): You need electrolytes (Sodium, Potassium, Magnesium).
Why? Sweat isn’t just water; it’s salt. If you only drink massive amounts of plain water after a heavy sweat session, you risk diluting the remaining sodium in your blood (a condition called hyponatremia). If you see salt stains on your skin or clothes after a run or feel the gritty crystals on your skin, that is a clear sign you need electrolytes, not just water. Some of us just sweat more salt than others.
📉 Why do I need to drink more than I lost?
If you lose 1 lb of sweat (16 oz), drinking exactly 16 oz won’t get you back to baseline. Your body is a biological machine, not a sealed tank. You will lose some of that new water through urine and continued sweating as your body temperature cools down. The “150% Rule” ensures you actually absorb enough fluid to reach full hydration.
⏱️ How quickly should I drink this?
Don’t chug it all at once! If the calculator says you need 40 oz, try to sip that steadily over the next 2–3 hours. Chugging forces the fluid through your system too fast, often resulting in it going straight to your bladder rather than being absorbed by your muscles.
⚠️ What are the signs of dehydration?
Beyond thirst, look out for:
- Dark yellow urine (aim for a light lemonade color).
- Headache or dizziness (especially when standing up).
- Higher than normal heart rate during easy efforts.
- “Heavy” legs or unusual fatigue early in a run.