Knowing how much you need to drink

Asker Jeukendrup’s point of view

There are different theories about drinking during prolonged exercise. As we exercise, we heat up and through sweating we keep our body temperature within acceptable limits.


However, sweating means we are reducing body fluid content, and this can have effects on blood volume, heart rate and in general it increases cardiovascular strain. Dehydration is normal, but excessive dehydration can have negative effects on performance and in extreme cases health. Thus, we need to prevent excessive dehydration by drinking.

Drinking to thirst or drinking with a plan?

There are many things scientists agree on and a few things that are topic of debate.

What scientists agree on is that dehydration can have negative effects, but it is debated at what level of dehydration this occurs. Scientists also agree that drinking can prevent dehydration.

There is, however, debate about the best advice to give to athletes. Some have argued you can drink when thirsty and this solves all problems, while others, including me, have argued that it is better to be prepared, to have a plan, have an idea how much you need to drink so you don’t drink too little and you don’t drink too much.

Assuming adequate hydration at the start of training or race, the amount of fluid that needs to be consumed is dependent on 3 factors:

1. Sweat rate

2. Duration of exercise

3. Level of dehydration at the end of exercise that is acceptable (i.e. that does not cause any negative side effects).

Predicting sweat rate

We can usally predict the duration of exercise usually fairly well, but predicting sweat rate is a little more complicated. To understand the sweat rate in certain situations we must measure sweat rate regularly. Fortunately, this is relatively easy to measure and all you need to do it is a set of scales.



How to calculate sweat rate

It is important to realize that you probably need to do several measurements in different conditions to get a good idea of your sweat rate: easy training and hard training, hot conditions and cool conditions and so forth. The more you measure the better you will be able to predict your sweat rates.


Here is a brief step-by-step guide on how to figure out your sweat rate. Use the overview above to do your calculations: white boxes are for values you measure and grey boxes for values you calculate.


1. Empty your bladder and record your weight (ideally nude body weight) (A)

2. Perform your workout, race or competition and record/memorize exactly how much you drank. This is easy if you drink from a bottle as you can weigh your bottle before (X) and after (Y) and record the difference (1 gram = 1 millilitre). (Z) If you use different measurement units, you need to convert all values to litres. Make sure all units are in kg or litres.

3. After exercise: Towel dry, empty your bladder and then record your weight (nude). (B)

4. Ideally, measure total urine production (U) after pre-weight recording. If that is not possible estimate it by using 0.3L per visit to the loo.

5. Now subtract your post-exercise weight from your pre-exercise weight to get the weight you lost during exercise.

6. Also subtract the weight of the bottle or bottles before (X) and after (Y) to obtain the volume you consumed (Z).
Weight lost C = A-B
Volume consumed = X-Y

7. You can now calculate your sweat rate: (C+Z-U) / time (in hours, calculated as number of minutes divided by 60).


A = Weight before (kg)

B = Weight after (kg)

C = Weight loss (kg)

X = Weight of bottle(s) before (full bottles)

Y = Weight of bottle(s) after

Z = Weigh difference before and after

U = Urine list

From measurements to drink advice

Your sweat rate estimations will never be 100% accurate, but they will give you a much better idea how much to drink than simply relying on thirst. With the above calculation you make an assumption that all weight lost if sweat loss. This is not entirely correct and it is important to realize there are other reasons for weight loss as well (you will use some carbohydrate and fat up to around 1 lbs or 0.5kg during exercise of 2 hours or longer). During very prolonged exercise these losses may even be even greater.


The 3rd and final variable we need to calculate a drinking plan is the acceptable level of dehydration. There is a bit of debate about this topic, but the majority of studies shows measurable effects on performance around 3%. It may be 3% maybe even a bit more in cool conditions and it may be 2-3% in hot conditions.


A small percentage weight loss 2-3% (1.5-2 kg for most athletes) is unlikely to be a problem in most conditions. Knowing how much you need to replace helps to prevent larger weight losses as a result of dehydration.


In conclusion, knowing your sweat rate gives you important insights into how your body works and how different it will be in different conditions. Not everyone has access to laboratory facilities, but this is something that is easily done at home with very simple equipment. Perform these measurements in different conditions and you will start to see trends and start to be able to predict your sweat rate!


  • Baker LB. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Med. 2017; 47(Suppl 1):111-128.
  • Gonzalez RR, Cheuvront SN, Montain SJ, Goodman DA, BlanchardLA, Berglund LG, Sawka MN.  Expanded prediction equations of human sweat loss and water needs. J Appl Physiol. 2009;107(2):379-88.



Professor Asker Jeukendrup is one of the world’s leading sports nutritionists/ exercise physiologists who spent most of his career as a professor at the University of Birmingham (UK). Dr Jeukendrup authored 8 books and over 200 research papers and book chapters. His expertise stretches from exercise metabolism and sports nutrition to training and overtraining. He is currently a (visiting) professor at Loughborough University and director of his own performance consulting business “Mysportscience” and communicates science through the popular website mysportscience.com. Asker works as Performance Manager for the Dutch Olympic team and is Head performance Nutrition for the Lotto Jumbo Pro cycling team. He also works with FC Barcelona, and other elite football clubs. Asker practices what he preaches and completed 21 Ironman races including 6 time the Ironman world Championship in Hawaii.