The 8 glasses a day rule — why it is wrong
No major health authority recommends a fixed 8-glass daily intake for all adults. Individual hydration requirements vary significantly based on body weight, activity, and climate.
How your actual water needs are calculated
Body weight: The standard baseline is approximately 33ml per kilogram of body weight per day. A 60kg person needs approximately 2.0L. A 90kg person needs approximately 3.0L.
Activity level: Moderate exercise 3-5 times per week adds approximately 0.5L. Athletes training twice daily may need an additional 1.0-1.5L.
Climate: In temperatures above 30°C, add approximately 0.5L above baseline.
Pregnancy: Approximately 0.3L additional daily fluid intake.
Breastfeeding: Approximately 0.7L additional fluid above non-pregnant baseline.
Practical example
A 70kg moderately active person in a temperate climate needs approximately:
- Base: 70 × 33ml = 2,310ml
- Activity: +500ml
- Climate: +200ml
- Total: approximately 3.0 litres
Does coffee count toward your daily water intake?
Yes. Despite their reputation as diuretics, caffeinated beverages still contribute meaningfully to daily fluid intake. Count roughly 80% of caffeinated beverage volume toward your daily total.
Signs you are not drinking enough
- Urine colour: Pale straw yellow = well hydrated. Dark yellow or amber = dehydrated.
- Frequency: Urinating fewer than 4-5 times per day is a sign of insufficient intake.
- Headaches: Mild dehydration is one of the most common causes of afternoon headaches.