💧 Water Intake Calculator
Enter your body weight, today's activity, and the climate for a rough daily water estimate in liters, milliliters, and glasses. Drink to thirst, and never so much that you gain weight while exercising.
🚰 Estimate Your Daily Water Needs
What is a Water Intake Calculator?
It gives you a quick, rough hydration estimate instead of relying on a generic "eight glasses a day" rule. Enter your weight, your activity minutes, and the climate you're in, and it scales your baseline water need up for exertion and heat, then breaks the total down into liters, milliliters, and standard glasses so it's easy to track through the day.
It's especially useful for sailors and anyone else spending long hours on deck: sun, wind, and humidity all increase fluid loss in ways that are easy to underestimate when a constant breeze keeps you feeling cool. Use it before a long day on the water to plan how much to bring, and treat the result as a starting point — not a substitute for listening to your body or medical guidance.
Drinking too much is dangerous too
- Drinking more than you sweat out is dangerous. Overdrinking during long exercise can dilute the sodium in your blood (exercise-associated hyponatremia), which can be fatal. If you weigh more after a session than before it, you drank too much.
- Drink to thirst, and aim to replace roughly what you lose. The total is an estimate, not a target to hit: on a cool day or an easy sail you may lose far less.
- Learn your own sweat rate: weigh yourself before and after a session (no clothes, towel dry) and add what you drank. Each 1 kg lost is about 1 liter of fluid.
- Stop, get to shade and cool down if you feel dizziness, nausea, headache, confusion or fainting. Confusion, collapse or a very high body temperature is heat stroke: call emergency services.
- If you have a heart or kidney condition, take medication that affects fluid balance, or have been told to limit fluids, ask your doctor what is right for you.
❓ Frequently Asked Questions
How does the water intake calculator work?
Enter your body weight, how many minutes of activity you've done today, and the climate you're in. It starts from roughly 33ml of water per kilogram of body weight, adds around 350ml for every 30 minutes of activity, then applies a bonus for hot or humid conditions before converting the total into liters, milliliters, and standard 250ml glasses. The activity figure is an assumption: weigh yourself before and after a session to learn your own sweat loss, and drink to thirst rather than to the total.
Why do sailors need more water than people on land?
Sun, wind, and salt spray all accelerate fluid loss, and the constant breeze on deck can mask how much you're actually sweating — so it's easy to underestimate how dehydrated you're getting. Time spent hauling lines, trimming sails, or simply standing watch in direct sun adds up fast, which is why activity minutes and climate both push the estimate higher. Replace roughly what you lose, not more: gaining weight over a day on the water means you drank too much.
Does hot or humid weather really change how much I should drink?
Yes. In hot conditions your body sweats more to cool itself, and in humid conditions that sweat evaporates less efficiently, so you sweat even more to compensate. This calculator adds roughly a 10% bonus for hot climates and 15% for hot, humid ones on top of your base and activity needs.
Is this medical advice?
No. This tool gives a general, informational estimate only — actual hydration needs vary by individual (sweat rate, body composition, medications, and underlying health conditions). Drink to thirst, watch for signs of dehydration or heat illness, and speak to a doctor if you have a condition that affects fluid intake.