You've heard the rule. Eight glasses. It’s basically ingrained in our collective DNA at this point, right alongside "don't swim for thirty minutes after eating" and "carrots help you see in the dark." But honestly? That eight-glass thing is mostly a myth. It’s a convenient number, sure, but it doesn't really account for the fact that a 100kg rugby player in a humid climate has vastly different needs than a 55kg office worker sitting in air conditioning all day. If you’re trying to figure out how many litres of water should you have a day, you have to look past the generic posters in your GP’s waiting room.
The reality is nuanced. It's messy.
In 1945, the Food and Nutrition Board of the National Research Council stated that adults should consume about 2.5 litres of water daily. Everyone saw that and ran with it. What most people conveniently ignored was the very next sentence: "Most of this quantity is contained in prepared foods." We’ve spent decades chugging bottled water while ignoring the fact that the cucumber in our salad is 96% water.
What the science actually says
The National Academies of Sciences, Engineering, and Medicine (NASEM) doesn't give a "one size fits all" number because, frankly, that would be bad science. Instead, they suggest an "adequate intake." For men, they're looking at about 3.7 litres of total fluid. For women, it’s closer to 2.7 litres.
Wait. Don't panic.
That doesn't mean you need to carry around a gallon jug like a gym bro. That total includes everything. Your morning coffee? It counts. That bowl of soup at lunch? Counts. The hydration you get from an apple? Yep, that counts too. About 20% of your daily fluid intake typically comes from food, and the rest comes from drinks. So, when you're calculating how many litres of water should you have a day, you’re really looking at drinking roughly 2 to 3 litres, depending on your sex and size.
The hydration hijackers
Environment changes everything. If you’re living in Phoenix, Arizona, during July, your sweat glands are doing overtime just to keep your core temperature from spiking. You’re losing electrolytes. You’re losing volume. In that scenario, 3 litres might be your bare minimum.
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Then there’s activity level.
If you go for a 45-minute run, you might lose anywhere from 0.5 to 2 litres of fluid through sweat alone. Dr. Timothy Noakes, a prominent exercise scientist and author of Waterlogged, argues that we’ve actually been over-hydrating in the sports world. He points out that the "drink before you're thirsty" advice can actually lead to hyponatremia—a dangerous condition where your blood sodium levels get too diluted. It can be fatal. It’s rare, but it happens to marathon runners who overdo the plain water.
Why the "8x8 Rule" won't die
It’s easy to remember. That’s it. That’s the whole reason.
Humans love simple rules. We love things we can track on an app with little droplets that turn blue when we hit a goal. But your body is way more sophisticated than an app. You have a built-in thirst mechanism that has evolved over millions of years to keep you alive. For the vast majority of healthy people, thirst is an incredibly reliable guide.
If you're thirsty, drink. If you aren't, you're probably fine.
Factors that actually shift the needle
Your "magic number" for how many litres of water should you have a day fluctuates based on things you might not even think about:
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- Pregnancy and Breastfeeding: Your body is literally building another human or producing milk. That requires massive amounts of fluid. The NASEM suggests pregnant women bump up to 3 litres and breastfeeding women go as high as 3.8 litres.
- Altitude: If you're hiking in the Rockies, you breathe faster and the air is drier. You're losing moisture with every exhale. You won't even notice you're dehydrating because your sweat evaporates instantly.
- Illness: Fever, vomiting, or diarrhea? You're leaking fluid. This is when you stop worrying about "litres" and start worrying about medical-grade rehydration.
- Diet: A keto diet, for example, causes the body to dump glycogen. Glycogen holds water. When you lose it, you pee constantly. You need more water (and salt) during that transition than someone eating a high-carb diet.
Does coffee count?
Let's kill this myth right now. For years, people said caffeine dehydrates you because it's a diuretic. While caffeine does have a mild diuretic effect, the water in the coffee more than compensates for it. A study published in PLOS ONE by researchers at the University of Birmingham compared the hydrating properties of water versus coffee. They found no significant differences in standard hydration markers among regular coffee drinkers.
So, yes. Your latte counts toward your litres.
Signs you're actually hitting the mark
Stop looking at the bottle and start looking at the bowl. Your urine color is the most honest coach you’ll ever have.
If it looks like lemonade or pale straw? You’re golden. Literally.
If it looks like apple juice or burnt orange? You’re behind. Go drink a glass of water.
If it’s completely clear? You might actually be overdoing it. You're flushing out minerals that your heart and muscles need to function.
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The dark side of over-hydration
We talk so much about dehydration that we forget about the opposite. Water intoxication is real. When you drink massive amounts of water in a short window, your kidneys can't keep up. They can typically process about 0.8 to 1.0 litres per hour. If you chug 3 litres in twenty minutes because you "forgot to drink all day," you’re putting yourself at risk.
Your cells start to swell. If your brain cells swell, you get headaches, confusion, and in extreme cases, seizures. This is why "gallon challenges" are genuinely stupid.
How to stay hydrated without being weird about it
You don't need a 2-litre bottle with motivational quotes on the side. Honestly, those things are just cumbersome.
Try this instead:
- Drink a glass of water right when you wake up. You’ve just spent 8 hours breathing out moisture; you're naturally dry.
- Eat your water. Watermelon, strawberries, celery, and spinach are almost entirely liquid.
- Use a glass you actually like. It sounds dumb, but if you like the weight of a specific tumbler, you'll reach for it more.
- Listen to your mouth. If it feels "sticky" or dry, you’re already slightly dehydrated.
There is no "secret" number. The quest to find exactly how many litres of water should you have a day ends with a realization that your body is a dynamic system, not a static tank. Some days you’ll need four litres because you hit the gym and sat in a sauna. Other days, one or two litres will be plenty because you spent the day eating juicy fruit and lounging in the shade.
Real-world check: The skin test
Want a quick way to see if you're hitting your goals? Pinch the skin on the back of your hand for a second and let go. This is called "skin turgor." If it snaps back instantly, your cells are plump and hydrated. If it takes a second or stays in a "tent" shape? You’re lacking fluid.
Putting it all together
Don't let the "litre" obsession stress you out. Stress actually messes with your hormones and can affect fluid retention anyway.
Focus on the big picture.
For a healthy adult in a temperate climate doing moderate activity, aiming for 2 to 2.5 litres of plain water is a solid baseline. Adjust upward if you're sweating, pregnant, or living at high altitudes. Adjust downward if you're eating a diet rich in fruits and vegetables or if you're particularly small-framed.
Actionable Hydration Steps
- Monitor your output: Aim for a pale yellow urine color throughout the day. This is the single most accurate "metric" for your specific body.
- Pre-hydrate for exercise: Don't try to "catch up" during a workout. Drink 500ml about two hours before you start.
- Don't ignore salt: If you're drinking lots of water but still feel thirsty or "foggy," you might be low on electrolytes. Add a pinch of sea salt to your water or eat something mineral-rich.
- Trust your thirst: If you are healthy and your kidneys are functioning normally, your thirst mechanism is the most precise instrument on the planet for determining your needs.
- Sip, don't chug: Spacing out your intake allows your kidneys to process the fluid efficiently without putting unnecessary pressure on your system.