How much water do you really need each day?

12 Feb

I wrote a lengthy article about a month ago all about dehydration and the importance of water and its purpose in the human body, so I won’t repeat all of that here. What I do wish to revisit however is the section about the amount of water that the human body needs each day. The issue of hydration has been bothering me quite a lot ever since I started taking a serious interest in nutrition and fitness. With all of the varying opinions and the non-scientifically based advice out there, I decided to think about the issue to see if there was a way that I could create a reasonable definition as to the amount of water that the human body actually needs each day.

Now please bear in mind, I am no nutritional expert, I am not a scientist, nor do I have any conclusive evidence or peer-reviewed studies to present to you. What I am going to describe here are my thoughts and some calculations that I made to try to help me to create a better guide for myself and for my own understanding. If you find any of this information is useful to you then I would be very pleased if you could contact me with your own observations and experience. Please however don’t take any of the information that I have written here either as medical advice, nor as being entirely appropriate to your own individual health and well-being without first seeking the advice of your doctor.

In my previous water article I mentioned that I had been aiming to consume 1 litre of water per 22 Kg of bodyweight. Now, if I am working out hard every day, and I’m not prepared to carefully hydrate before, during and after each workout, then I suspect that this guideline would be easier to follow. When I am having a comparatively inactive day however, I find it hard to consume so much water. I just don’t find myself getting thirsty enough to trigger a need to drink nearly 3.5 litres of water given that most of the food that I consume these days also has a relatively high water content. It’s different when I am working out or the weather is very hot, but not during a rest day at relatively cool temperatures. So I got to thinking about what the minimum water would be that I would need in terms of the amount of water that my body would generally lose through ordinary metabolic processes.

Water losses occur through both digestion & sweating, and you also lose a substantial amount of water simply by breathing. Now, rather than attempting to catch all of the water that I use in a day, I decided to take a really basic mathematical approach, based on several assumptions:

  1. For the sake of simplicity, assume that water losses over time are linear. I’m certain that the body’s built in survival mechanisms would mean that water losses decrease over timet, but as it will make the math much easier and I am really shoddy at math I’ll stick with what I can do.
  2. Assume the average person can survive for 3 days without consuming any water given very little activity.
  3. Assume the average person reaches a point of fatal dehydration with 15% water losses.
  4. Assume the average person has a body water content of 57%. I.e.: a Water_Mass_Multiplier of 0.57.

Given these assumptions,

  1. Divide the 15% fatal water loss by 3 days, and you get a roughly estimated 5% water loss per day.I.e.: a Loss_Multiplier of 0.05.
  2. Take your body weight in kilograms (Mass) and multiply it by the Water_Mass_Multiplier to estimate the total Water_Volume in Litres.
  3. Multiply the Water_Volume by the Loss_Multiplier to estimate the Daily_Loss in Litres.
  4. Assuming I have an error in my formula, we’ll add a 10% error_correction_multiplier of 1.10 to ensure that we don’t completely underestimate the water requirement.
  5. Assuming the average person doesn’t sit completely still for an entire day, we’ll add a 25% loading which I’ll call an activity_multiplier of 1.25, to account for all of the mild activity that would increase the metabolic usage of body water.
  6. Assuming that all estimated losses are the daily water intake requirement, convert the result to get the estimated average mare minimum water requirement in millilitres per Kilogram of bodyweight.

So to make it all look easier to follow and more math like:


No matter what body weight I put into this formula, the DailyRequirement factor is always the same, which according to my very rough formula works out to be:

39 millilitres per Kilogram of body weight

So this  means that with all the hard work already done, all I need to do is take my bodyweight in kilograms (presently 76) and multiply it by 39 to get my personal minimum daily requirement of about 2900 ml.

But what about when you exercise, or if you live at a high altitude, or if the weather is hot? Well, As I mentioned earlier, other factors can affect your daily water consumption (including the water content of the food you eat), so it’s pretty clear that this formula and the daily requirement factor that I worked out is not going to suit every single situation. I would still suggest drinking at least 600ml about an hour both before and after a workout, and to be sure that you have water on hand during the workout to avoid dehydrating yourself through your exertions. This daily requirement factor is really something that I’ve worked out to give me a rough guide to ensure I avoid chronic mild dehydration and to ensure I am adequately hydrated throughout a normal and mildly active day. As always, I will continue to allow thirst to be a guide, and to monitor the colour of my urine which should always remain a “pale straw” colour. Any darker and I’m likely to be dehydrated, and lighter means I’m probably drinking more water than I strictly need. This shouldn’t be a problem however, as the body is very good as self regulating its water levels, and as I am spreading my water consumption across an entire day, the risk of water toxicity is basically non-existent.

I have personally been sticking to this new guideline for about a week now, and in that time I’ve noticed no difference physically as compared to when I was drinking the extra half litre per day, except that I find it less of a chore staying hydrated. My post workout recovery remains good, urine colour remains the same and I don’t have any of the dehydration symptoms that I used to get about 6 months ago, so I guess that backing off on the water slightly hasn’t been a problem.  I’ll keep monitoring how this goes, and I’ll update this formula as I learn more.

The thing that disappoints me though, is that I’m not sure if this formula is as applicable for people in the obese or severely underweight body mass categories. I know that fat cells don’t contain as much water as compared to muscle, yet the fat cells themselves still need to be maintained with access to body fluids, so it would make sense that having more body fat would still require more water. What I can’t figure out is how much, and I am not sure how much water a larger person feels compelled to consume. So what I’m saying is that I’m pretty sure that this formula works for me, but I’m not sure how useful it will be for people who are not of a statistically “average” weight. Given however that medications are applied based on body mass, I feel it would be reasonable to assume that taking a prescriptive approach to hydration would mean that a larger person could safely consume proportionally larger quantities of water, and that my rough formula would probably be applicable regardless… but I can be sure until I’ve had a chance to research this puzzle further.

As it is, I’ll continue to consume water according to my math and I’ll update the formula as I learn more. Interestingly enough I’ve just learned that if I convert my weight to pounds and use the half body weight in fluid ounces method, the amount of water I would consume is almost the same give or take a few ounces. So perhaps my logic isn’t so messed up after all! 🙂


One Response to “How much water do you really need each day?”

  1. teenieyogini 12 February 2013 at 13:57 #

    Interesting. Too much to process right now. But I look forward to revisiting this soon!

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