The Standard Advice Isn't Enough for Active People
The commonly cited "8 glasses a day" (about 2 litres) is a general population baseline — it doesn't account for body weight, exercise intensity, sweat rate, or climate. If you exercise regularly, your actual needs are likely 30–80% higher than this figure.
The Weight-Based Formula
A more accurate starting point is body weight. Research supports roughly 35 mL per kilogram for men and 31 mL per kg for women as a resting baseline. From there, you add for activity and environment:
| Factor | Adjustment |
|---|---|
| Resting baseline (men) | 35 mL × body weight in kg |
| Resting baseline (women) | 31 mL × body weight in kg |
| Moderate exercise (30–60 min/day) | +500 mL |
| Active exercise (60+ min/day) | +1,000 mL |
| Hot climate | +500 mL |
| Cold climate | −200 mL |
| Pregnant | +300 mL |
| Breastfeeding | +700 mL |
Worked Example: 75 kg Man, Moderate Exercise, Mild Climate
Base: 75 × 35 = 2,625 mL. Add moderate exercise: +500 mL. Total: 3,125 mL per day — that's about 12–13 glasses of 250 mL each. The standard "8 glasses" would leave this person around 1 litre short every day.
When to Drink — Timing Matters
- Before exercise: 400–600 mL in the 2 hours before a workout.
- During exercise: 150–250 mL every 15–20 minutes, more in heat.
- After exercise: 500 mL for every 0.5 kg of body weight lost during the session (weigh yourself before and after for precision).
- Morning: A 500 mL glass immediately on waking replaces overnight losses and kick-starts digestion.
The Simplest Hydration Check
Urine colour is the most reliable real-time indicator. Pale straw yellow means well-hydrated. Dark yellow or amber means you're behind. Clear water means you may be overhydrating, which has its own risks (hyponatremia in endurance athletes). Aim for pale straw consistently throughout the day.
Your exact target depends on your weight, sex, activity level, and climate. The calculator below factors all of these in to give you a personalised daily recommendation in litres, glasses, and fluid ounces.