How much water you need depends mostly on your body size and activity. A widely used rule of thumb is about 35 mL per kilogram of body weight, with extra added for exercise — roughly 350–500 mL for every half hour of activity. The total counts water from all sources, including food and other drinks, so you don't have to sip it all as plain water. The honest caveat: this is a starting estimate, and your real needs shift with weather, health and lifestyle.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the 35 mL/kg hydration guideline. Not medical advice.
The hydration estimate
The 35 mL/kg baseline scales hydration to body size — larger bodies hold and turn over more water. Exercise adds fluid to replace what you lose through sweat and faster breathing, so longer or harder sessions need more. Converting the total into familiar servings (a 250 mL glass, a 500 mL bottle) makes the goal easier to hit through the day. Because roughly a fifth of fluid comes from food, the plain-water target in practice is a little lower than the headline number.
Worked example — an active adult
Scenario: 70 kg, 30 minutes of exercise a day.
So a 70 kg person training for half an hour lands around 2.81 litres a day — about 11 glasses or just under six 500 mL bottles. Without exercise the same person needs about 2.45 litres. Body size moves the figure predictably: 50 kg is around 1.75 litres and 90 kg around 3.15 litres at rest. Treat these as targets to drift toward, not strict quotas — on a hot or very active day, drink more, and let thirst and urine colour fine-tune the rest.
Frequently Asked Questions
≈ 35 mL per kg, plus exercise. 70 kg ≈ 2.45 L at rest, ≈ 2.8 L with 30 min activity.
weight × 35 + minutes × 12 mL. 70 kg + 30 min = 2810 mL ≈ 11 glasses.
Yes — ~20% comes from food; tea, milk and juice count. It's total fluid, not just plain water.
Heat, altitude, exercise, fever, pregnancy/breastfeeding. Check thirst and urine colour.
Rarely, but possible — spread intake out. If told to limit fluids, follow your doctor.