Water Intake Calculator: Your Personalised Daily Hydration Target
Eight glasses a day is a starting point. But your actual hydration needs depend on your body weight and how much you exercise. This calculator gives you a precise daily target based on both factors — not a generic guideline designed for someone who may be very different from you.
The Formula
Base Need = Body Weight (kg) × 35 ml
Exercise Addition = Duration (min) × 12 ml
Daily Target = (Base + Exercise) ÷ 1000 = Litres
Worked Example
70 kg, 45 minutes of exercise:
Base = 70 × 35 = 2450 ml
Exercise = 45 × 12 = 540 ml
Total = 2990 ml = 3.0 litres (12 glasses)
Find your personalised target with our Water Intake Calculator.
Benefits of Adequate Hydration
- Cognitive performance. Even 2% dehydration reduces concentration, short-term memory, and reaction time measurably.
- Physical performance. Dehydration of 3% reduces endurance exercise performance by 10–15%.
- Kidney health. Adequate fluids reduce kidney stone risk by 50% in people with a history of stones.
- Bowel function. Insufficient fluid intake is a primary cause of constipation.
- Skin health. Chronic mild dehydration increases skin dryness and reduces elasticity over time.
Signs of Mild Dehydration
| Symptom | Hydration Level |
|---|
| Pale yellow urine | Well hydrated |
| Dark yellow urine | Mildly dehydrated (−2%) |
| Headache, fatigue | Moderately dehydrated (−2–3%) |
| Reduced performance | Significantly dehydrated (−3%+) |
Practical Tips
Start every morning with water. After 7–8 hours without fluids, even slight dehydration from overnight is common.
Carry a marked water bottle. Tracking is far easier with a visual reference than mentally counting glasses throughout the day.
Hydrate before exercise, not just during. Starting exercise well-hydrated prevents the rapid dehydration that impairs performance in the first 30 minutes.
Include fluid-rich foods. Fruits, vegetables, soups, and yoghurt contribute meaningfully to daily fluid intake — typically 20–30% of your total needs.
Adjust for heat and humidity. On hot days or in warm climates, add an extra 500–1000 ml to your baseline target.
How Often to Recalculate
When your weight changes significantly, your exercise routine changes, or the season shifts. Summer needs are typically 500–1000 ml/day higher than winter for active people in temperate climates.
Final Thoughts
Personalised hydration beats a generic rule every time. Small, consistent improvements in daily fluid intake produce measurable benefits in energy, focus, and long-term health.
Find your personalised daily target with our free Water Intake Calculator.