Is 8 glasses a day a rule?

It's a rough guide, not evidence-based. Individual needs vary.

Yes — the diuretic effect at normal intake is small.

Can I drink too much?

Rarely — extreme overconsumption can cause hyponatremia.

Dark urine, headache, fatigue, thirst.

Estimate daily water needs based on body weight, climate, and activity level.

Reviewed & updated: January 1970

Weight (kg)Vigorous exercise (hours/day)

Daily water

2.70 L

11.4 cups

Litres2.70 L

Fluid ounces91 fl oz

Cups (8 oz)11.4

Results are estimates for general information only and are not professional financial, medical, tax, or legal advice. Read the full disclaimer.

Rule of thumb: 30–40 mL per kg. Add 500–1000 mL per hour of vigorous exercise.

Example

70 kg adult, moderately active → ~2.5–3 L/day.

Is 8 glasses a day a rule?: It's a rough guide, not evidence-based. Individual needs vary.
Does coffee count?: Yes — the diuretic effect at normal intake is small.
Can I drink too much?: Rarely — extreme overconsumption can cause hyponatremia.
What are signs of dehydration?: Dark urine, headache, fatigue, thirst.