“Drink eight glasses a day” sounds scientific. But it traces to a 1945 guideline that said most fluid comes from food — the part everyone forgot. Eight glasses isn’t dangerous, but it’s not a rule either.
This is general information, not medical advice. If you have a kidney, heart, or fluid-related condition, your doctor’s guidance beats any article.
A Better Rule: Go by Body Weight
Aim for 30–35 ml of total water per kg of body weight daily — from everything, drinks plus food.
Worked Example: 70 kg Adult
- Lower: 70 × 30 = 2,100 ml (2.1 L)
- Upper: 70 × 35 = 2,450 ml (~2.5 L)
Food supplies ~0.5 L, leaving 1.5–2 L to drink — 6–8 glasses. That’s why the old rule landed there: calibrated for an average adult, never stated.
50 kg adult: 1.5–1.75 L total → ~1–1.3 L to drink (4–5 glasses).
90 kg adult: 2.7–3.15 L total → ~2–2.5 L to drink (8–10 glasses).
When You Need More
Exercise/sweat, hot weather, fever or illness, dry heated rooms, pregnancy/breastfeeding (ask your doctor). Baseline is a floor, not a ceiling.
Two Free Checks
Colour: pale straw = enough; dark yellow = drink more. Frequency: every few hours = good sign.
Can You Overdo It?
Extreme intake very quickly can dilute blood salts — rare, mostly endurance sports. Sipping normally through the day is safe. Spread it out; don’t chug litres at once.
For more body numbers, see Your Body in Numbers and What Your BMI Really Means.
Frequently Asked Questions
Does tea/coffee count? Yes — mostly water. Caffeine’s mild diuretic effect doesn’t cancel the fluid.
Cold vs room-temperature? No hydration difference. Drink whichever means you drink enough.
Kids and older adults? Needs differ; older adults feel thirst less sharply. Get professional guidance.
Just drink when thirsty? For most healthy adults, yes — thirst works, the colour check backs it up.
How this guide was made
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This guide was written by the ArmTema team, checked against standard reference formulas, and edited for plain-English clarity. The examples use round sample numbers so you can follow the math. Last reviewed: October 2026.