Eight glasses of water a day—it's a myth

This discussion occasionally comes up on MPU:

And here’s an update:

Research confirms that you don’t need eight glasses of water a day. You probably need much less than that. And you can get half your water needs from food, if you eat a varied diet. By Hannah Devlin at The Guardian.

Most scientists don’t even know where the “eight glasses of water” thing comes from.

This has been a pet peeve of mine for years. It’s junk science. There are even apps to remind you to drink water. It’s all nonsense. Just drink water when you’re thirsty.

Most of my liquid intake is coffee and decaf. A year or two ago, a friend tipped me off that drinking hot water is nice, and it is, so I often have a cup of hot water in the afternoon.

Clean drinking water costs energy to produce, so if you drink more than you need, that’s wasteful. And much of the world is going through drought, and that’s going to get worse.

Despite the energy cost and drought, drink as much water as you want anyway—just don’t pound down water just because some stupid app told you to.

Journalist Emma Bedding, who hates drinking water, celebrates the research:

Having grown up before Big Hydration got its unpleasantly moist claws into the world, I don’t get on with water, never have. Days go by without me feeling compelled to drink any: I find my thirst is adequately quenched by imagining how unpleasant it would be to drink a cold, flavourless glass of nothing.

I can’t, in good conscience, claim it’s never done me any harm. A nutritionist recently told me that fatigue, brain fog and headaches – all regular events – are usually signs of dehydration; I always assumed they were just facets of my delightful personality. My optician told me I had the driest eyeballs she had ever seen, a fact I have been relating with misplaced pride ever since: I imagine them like little bundles of tumbleweed, rolling dustily around my sockets (no wonder they itch).



That 100% does not work for me, as also mentioned by Emma Bedding. One other “fact” I have heard, whether it has basis in science or not, is that feeling thirsty is the ambulance at the bottom of the cliff.

Also, the apps I have used do not specify 8 glasses a day (and what even is “a glass”?) but ask for some basic information like weight, sex, and activity level, which seem like a good basis for how much water your body needs.

I track my liquid intake daily based on the above metrics. I keep it higher if I have any of several illnesses that doctors say “drink plenty of fluids” for, like a recent cold (it helps keep the mucus loose). Why do I do this? Because as I said above, “thirst” is not a reliable indicator for me. A much stronger indicator that I should have drunk more is a dehyrdation headache which I used to get frequently until I started tracking my fluid intake and kept it roughly where my app has recommended.

So yes, “8 glasses a day” may have no basis in science, but you should drink a decent amount of fluids daily, in my opinion. It does not need to be water, but you should be aware of diuretic effect and many other nutritional interactions.

Finally… as someone who has never drunk tea or coffee, the standard epithet of “just drink water” is the bane of my life. Like Emma, I’m not really a fan. I drink about a litre a day of plain water, but the rest has flavours of some kind as after a certain point, water just tastes… bleugh!


Did they say anything about eight glasses of wine?




My “requirements” did not list a daily goal… :wink:

In fact, I was more worried about drinking too much (water, coffee, green tea), which can also be unhealthy or a symptom of an underlying disease.

(I’m really happy with WaterMinder for tracking both water and caffeine consumption for quite some time and thus no longer looking for an alternative)

I still don’t have a goal for water, but I try to drink at most 4 cups of coffee per day and at least one cup of green tea.

You are correct, it’s not rocket science. The advice I was given 50+ years ago when I was training in south Louisiana still works: If your urine is clear or slightly yellow you are probably well hydrated. If not drink more.


There’s a saying amongst sign language interpreters and lipspeakers to determine one is properly hydrated: “pee pale”.

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I think having a target to make sure you’re hydrated makes sense for people to remember it.

I imagine 8 cups came from something like that, much like 8 hours of sleep and 2500 (/2000) calories.

I also agree that just trying to have clear urine is an optimal guide!

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I love that you’ve posted this. I’m imagining you irritably imagining a man seated at a table fastidiously drinking a glass of water, smacking his lips, updating his daily tally on one device and making sure the other four surrounding him sync correctly. :stuck_out_tongue:

In all seriousness I think we should be able to have an app that advises drinking one glass of water when it detects probable sluggishness with health/activity data. If it didn’t bug you for the day because you did fine, no problem. Also, I’d probably pay for BrothMinder.


This was the bane of my mother’s existence the last years of her life.
Her physician propagated the myth, and she abided by his instruction.

For most people, drink when you’re thirsty. It’s worked for thousands and thousands of years. If thirst meant you were already dehydrated, none of us would be here. This also works for every other animal on the planet.

If you do the Four Deserts Ultramarathon, have a different physiology, etc. adjust as needed.

While we’re on the subject, drinking too much water can be lethal, as has been seen in radio station contests, child abuse, IV fluids, etc.

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I’ve had medical advice that clear is not the goal. There should be some colour, just not a lot.

Dehydration doesn’t kill in moderation. I said it above; I used to get regular headaches because I don’t feel thirsty until it’s too late to stave off the headaches (and I’m not the only one as, even only as evidenced by this thread so far).

The myth is “8 glasses for everyone.” It is good to drink water — just how much varies from person to person based on physique, metabolism, diet, activity, age, and environment. (At least.)

I certainly didn’t worry about any of this stuff in my 20s but now in my 50s I have to. I also used to eat a lot of junk and be just fine. Can’t do that now either. If your thirst reflex works for you, then great. But it’s no more universal than 8 glasses a day.

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Yes. It’s a good start :smiley:

Which is why I said:


My intention was to be descriptive, rather than prescriptive. I probably didn’t do that well.

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Right, it’s not a goal. It’s a way for people to gauge their risk of dehydration.

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Clear has never made sense to me. It would be a sign to me that my kidneys weren’t doing their thing or that I was overloading them with too much water. Or that something was wrong. Avoid fads. Moderation in all things.


As somebody that has had a kidney stone, what I can say is the amount of urine you produce affects the dilution of certain chemicals and substances in there that, when concentrated and in close proximity, can contribute to things like kidney stones.

It’s not uncommon at all for a kidney stone former to go in, do a 24 hour urine collection, and discover that they have 1 L or so of urine per day. Which is not particularly healthy.

Advice from the kidney stone side is to drink enough fluid to produce 2 1/2 L per day of urine. That’s advice obviously for the typical adult. More urine volume is slightly better, although you don’t need excessive either. The “mostly clear” guidance will get you there typically.

Oddly enough, the number one advice for preventing future kidney stones, apart from medication that’s sometimes used, is quite literally “drink more water”.

Obviously, that’s only a single lens, but one in 10 adults will have a kidney stone – so it may be worth considering. :slight_smile:


If you have a medical condition that brings you to a doctor, please follow that doctor’s advice, which will be tailored to your condition, and ignore casual advice given on forums like this one. :slightly_smiling_face:

Absolutely. :slight_smile:

Like most things, the “8 glasses of water per day” thing is over-generalized advice, but the idea that it’s baseless and “nobody knows where it came from” seems downright silly to me when one does any more reading on the topic. For example:


It’s a rule of thumb that’s necessarily derived from data. That assessment generally agrees with the article OP linked above regarding how much water somebody needs. You need X amount of water per day, and that X varies based on activity level, biological sex, age, etc. The question is how much of that water comes from drinking liquids, and how much comes from food. And that’s where the articles differ.

The article linked by OP has researchers assuming that you’ll get maybe 50% of your water from food sources, rather than drinking liquids, and that therefore 8 glasses per day is over-consuming. The Institute of Medicine of the National Academies indicates that the average American gets about 20% of their water from food sources. And that 30% discrepancy is the whole point of contention.

OP’s article also uses a bizarre number for the upper end of their research - 90-year-olds. People in their 20s need much more water, and people in their 90s need much less. But somebody in their 90s (the lowest adult figure for water intake) still turns over 2.5 liters of water per day. If they’re getting 80% of that from food (US survey data), that’s still 2 liters they’d have to drink - about…wait for it…8 cups. :slight_smile:

When I’m talking about kidney stone issues, all I’m saying is that getting it wrong on the low side can have incredibly painful, expensive consequences for a significant portion of the population. That’s the gist of the article I linked. Getting it wrong on the low side isn’t good, and causes problems.

Basically, no matter how you arrive at the right solution, getting it right is important. :slight_smile:


In the UK the traditional guidance is 2 litres, not 8 glasses. I wonder if the US have adopted the UK rule (or at least the generally accepted rule is around the same volume) and converted to a measure that is easy to remember (rather than half a gallon or 3.5 pints).

I think it is worth noting that healthcare communicating with the public on best practice needs to be simple and easy to remember. 2 litres a day ticks both of those boxes. This is much easier than a urine colour chart, and is repeatable and works for most people without causing anyone harm.

I think calling it a myth is harsh - it’s just a generalisation that you can teach kids and they understand it. If you make it more complicated then people will get confused.


Please correct me if I’m wrong, but I’m of the impression that the liter is the primary measurement in the UK (i.e. people think of something as “half a liter”, “1/4 of a liter”, etc.) That has the advantage of being a nice, precise measurement.

Where I live it’s typically ounces or cups - even if those measurements make things less comprehensible/correct, not more. Random examples:

  • We have half-liter plastic bottles for water and they’re frequently referred to as 16 oz water bottles.
  • If somebody tells you they have a cup of coffee every day, you can rest assured that’s probably about 10-11 ounces - not an 8-oz cup.
  • …unless you’re measuring based on coffee maker capacity, in which case a 12 cup coffee maker will give you 12 6 oz servings - because that’s a “cup” of coffee to them.
  • “Glass” usually means “whatever drinking glass I grabbed from the cupboard”, which could be anything from 8-12 ounces (or more) - giving you a range of 64 oz or 96 oz for 8 glasses.

“2L” or “64 oz” is much more precise…but usually requires more thinking on the part of the person following the rule. And as you so aptly noted…

Exactly. The easier the thing is to remember, the more likely it will be to stick. One of my kidney stone doctors said preventing kidney stones was “as easy as 3-2-1 - 3 liters of fluid per day, 2g (or 2000mg) of sodium or less per day, and about 1g (or 1000mg) of calcium per day”. That’s a MASSIVE oversimplification, and even leaves certain important things out - but compliance is usually a gradient, not a binary. Memory devices are huge. :slight_smile:

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Hm. Maybe someone should sell a urine tracker on the App Store.

Then again, never mind. :flushed: