Do I Really Need 8 Cups of Water Per Day?

Kate Chury NW Calgary Dietitian

You’ve probably heard that you need to drink 8 cups (2 liters) of water each day. This isn’t exactly true. Considering the amount of times you’ve probably heard the ‘8 cup rule’, you would think that there would be some sort of science backing up this claim. Surprisingly, there is not. There is nothing that says you or I need 8 cups of water each day to keep adequately hydrated. Unfortunately, it’s a little more complicated than that.  

While it could be true that YOU need 8 cups of water each day, the person sitting next to you may need a little more or less than that. Eight cups is a totally reasonable number to aim for but it may not be exactly what you need. Fluid needs vary quite a bit and depend upon several factors including:

  • Age - children, adults and the elderly have different body water percentages and needs
  • Sex - men are, on average, about 50-60% water by weight; women are ~ 45-50%
  • Body size and muscle mass - more lean mass is means more total body water
  • Lactation - may require an extra 1 liter of fluids per day
  • Physical activity level - more intense activity requires more water
  • Climate - hot and humid environments require more water
  • Illness - fever, diarrhea, or vomiting increase fluid requirements

According to the Institute of Medicine (IOM), it estimates that the average woman needs about 2.7 liters of water per day, while the average man needs about 3.7 liters. For women, this translates to about 11 cups of fluid each day, and for men about 15 cups. This is a little more than the 8 cup per day recommendation.

But wait…

Before we start gulping down 11-15 cups of water, we need to consider all the sources from which we get fluid. It is estimated that we get about 20% of our fluid from food. That means that we can probably bring that 11 and 15 cups of water down to 9 cups for women, and 12 cups for men. Again, this is all an estimate. Some of us eat way more hydrating foods (those fruits and veggies with high water content) than others and may get away with drinking a little less actual fluid. 

Dietitian Calgary NW Chury

When it comes to fluids that count towards our total water needs, pretty much all beverages will provide some water. It was once thought that beverages like coffee or tea shouldn’t be counted towards your total water intake due to their diuretic (or dehydrating) effect but research now shows that this effect is minimal. This means that cup of coffee you so dearly enjoy in the morning counts towards your daily fluid goals (hooray!). So coffee, tea, milk, juice, water, and foods that melt at room temperature (like popsicles!) all will provide hydration. My only caution is to make the majority of your fluids from non-sugar sweetened sources. More specifically, water should be your primary source of hydration.

If there are so many factors contributing to your water needs (and these can change from day to day, or season to season), how can we ensure proper hydration? One way is to pay close attention to your urine. Yup, that’s right, take a look at your pee. What colour is it? The colour of our urine can give us a good indication of our hydration status. The darker the urine, the more concentrated it is and it probably means you aren’t drinking enough. Studies show that using urine colour as a gauge is a pretty good way of staying on top of our hydration. It should be noted that factors such as taking a multivitamin (especially with riboflavin, or vitamin B2) or eating beets will alter the colour of your urine. A urine colour chart like this one (below) can give you an idea of the urine colour which indicates adequate hydration. 

Image source:

Image source:

Another important factor to pay attention are your thirst signals. While depending on thirst sensations may not be a great way of keeping hydrated for all people, especially in the elderly and children, it is an important signal in which to be mindful.

Now, you may have heard that if you are thirsty, you are already be dehydrated. And yes, this could be true to some degree but the body’s first thirst signals are not likely indicators of any severe dehydration. Thirst is your body’s way of telling you to top up your fluids. If you feel thirsty, your body is giving you a clue that your fluid intake just isn’t enough. It doesn’t necessarily mean that your body’s water stores have been drastically depleted. That’s not to say that dehydration isn’t an a serious condition, because it absolutely is. The following are signs of dehydration and should be taken seriously:

Severe dehydration:

  • Dark yellow or orange urine
  • Decreased urine output
  • Severe dizziness
  • Orthostatic hypotension (drop in blood pressure when changing from sitting/lying down to standing position)
  • Rapid heart rate
  • Confusion
  • Lethargy
  • Coma

Mild to moderate dehydration:

  • Increased thirst
  • Decreased urine output
  • Bright yellow or dark urine
  • Headache
  • Dry skin
  • Dizziness



*If you have symptoms that concern you, please get medical attention*

Final Verdict:

To recap, we don’t necessarily need 8 cups of water a day. We may need more, or we may need less. The truth is that there are a number of factors that influence our individual water needs and there isn’t a simple recommendation out there that will be accurate for the entire population.

As a dietitian who believes in evidence-based nutrition recommendations, this non-science backed advice to drink ‘8 cups of water per day’ doesn’t really bother me too much. Here’s why. Despite it not being evidence based, I feel that for most healthy people 8 cups of water (or fluid) per day is a good starting point for determining an individual’s optimal hydration. If you aim to drink 8 cups of fluid per day, plus the water you get from food, you can then take note of things like urine colour and thirst to see if your body requires more or less hydration. For me, 8 cups is a starting point, not an absolute value (or volume) to strive for. 

Note: water requirements in this article are discussed with respect to healthy individuals. There are a number of medical condition which require either fluid restrictions or high fluid intake. As always, when medical conditions are present, please consult a doctor or Registered Dietitian for personalized advice.