Ask ten adults how much water they need to drink and at least eight will say "eight glasses a day, right?" Ask them where that number comes from and the room goes quiet. That's because nobody really knows. The rule is a paraphrase of a 1945 US Food and Nutrition Board recommendation that suggested adults consume about 2.5 litres of water per day — including, crucially, the water in food. The "from food" caveat fell out of the soundbite, and the number drifted into cultural law.
So what does the actual evidence say? It says: there is no universal number. There is, however, a usable framework. Let's unpack it.
The two big benchmarks
Two health bodies publish the most-cited reference values for adult fluid intake. Both are explicitly adequate intakes — population-level averages assumed to meet most people's needs — not recommended daily prescriptions.
- The European Food Safety Authority (EFSA) sets adequate total water intake at 2.0 L/day for adult women and 2.5 L/day for adult men, in temperate climates with moderate activity.
- The US Institute of Medicine (IOM, now the National Academies) sets it at 2.7 L/day for women and 3.7 L/day for men.
The gap between those two numbers isn't a scientific disagreement; it's a methodological one. EFSA includes only fluids from drinks and "moisture in food." IOM includes a more generous food-water estimate. Both are explicitly population averages. Neither claims to be your personal number.
Food is a quiet 20–30% of your daily total
Most of us think of hydration as "stuff you drink." That's about three-quarters of the story. Roughly 20 to 30% of an average adult's daily fluid arrives in food: fruit, vegetables, dairy, soups, yoghurt, even bread and pasta. A cucumber is 96% water. A roti is around 30% water. A bowl of dal is, generously, soup.
Drimin doesn't ask you to log food because it would turn a ten-second habit into a chore. What it does is set a sensible default for your drinks goal that assumes you're eating normally. If you're on a very dry diet (lots of crackers, no produce, no soup) you may need to log a little more. The orb is forgiving.
What counts as a "drink"?
Effectively everything fluid you put in your mouth that isn't alcohol or industrial amounts of espresso. The old "coffee dehydrates you" claim is largely a myth: at normal daily doses, caffeinated drinks contribute net positive water. The American Journal of Clinical Nutrition's 2014 study on this is brutal in its clarity. Tea is water. Coffee is water with a tax. Sparkling water is water. Diet sodas are water (with other concerns of their own).
Alcohol, on the other hand, is a mild diuretic in modest doses and a meaningful one in larger doses. A pint of beer is not a 500 ml hydration win. We unpack this and seven other widely-believed half-truths in Eight hydration myths, politely debunked.
Your daily fluid need is a moving target. Stop trying to hit a fixed number and start trying to hit a sensible band.
The things that change your number
Your daily fluid need is shaped by at least seven inputs, most of which you can estimate without a lab:
- Body mass. Larger bodies hold more water and shed more through skin and breath.
- Activity. A 60-minute moderate run can cost you 500–1,000 ml of sweat depending on weather.
- Climate. An Indian summer week can double your sweat loss versus a temperate winter.
- Altitude. Above ~2,500 m, ventilation and urine output both increase.
- Pregnancy and lactation. EFSA bumps the AI to ~2.3 L and ~2.7 L respectively, on top of normal.
- Illness. Fever, vomiting, and diarrhoea all increase loss substantially.
- Diet. High-sodium days and high-protein days both raise water demand.
That's why "8 glasses" is the wrong unit. A 55 kg woman in cool Bangalore and a 90 kg man training in Chennai will not converge on the same number, no matter how well-meaning the meme.
A practical recipe
Here is the heuristic we suggest to friends who don't want to think about it again:
- Start at 30 ml per kilogram of body weight. A 70 kg adult lands at ~2.1 L. That's the "drinks" target, not the total.
- Add 500 ml for every hour of moderate exercise, more if it's hot.
- Add 200 ml for every hour at altitude above 2,500 m.
- Adjust by colour-and-frequency feedback over a week. If your urine is consistently dark yellow late afternoon, add 250–500 ml to your daily target. (Clear is not the goal; pale straw is.)
Open Drimin's Settings, pick a daily goal in that range, and let the orb take it from there. The right number isn't the one a 1945 committee picked. It's the one you can hit, most days, without thinking about it.
Don't over-hydrate either
Drinking far above your needs is uncomfortable, occasionally dangerous, and never useful. Hyponatraemia — dangerously low blood sodium from excessive water intake — is rare but real, and it shows up most often in endurance athletes who over-correct. The body asks for what it needs. Listen to it. Then top it up.
If you have heart, kidney, or liver conditions, your "right" number may be materially different. Please talk to a clinician rather than an app.
The whole point of the orb
Drimin is built to make the answer to "did I drink enough today?" a glance instead of a calculation. Set the number once, log in your usual cup size, and let the orb be the gentle ledger. If the math is right, the habit follows.
For the philosophical companion to this piece, see The quiet case for water. For the design choices that make the orb possible, see Designing for stillness.
This article is informational and is not medical advice. References — EFSA Scientific Opinion on Dietary Reference Values for Water (2010); IOM Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005); Killer et al., "No Evidence of Dehydration with Moderate Daily Coffee Intake," PLoS ONE (2014).