Are dehydration calculations based on total body weight OR on body water weight?

Dehydration calculations are based on your current body weight, and not on your current water weight or body water mass percentage.

Why is this?

The person’s overall body mass is used in the design of scientific studies given that it provides a more accurate result as it takes into account the person’s overall homeostatic status. For example, body water is held not just in muscles, but also in organs and even bones – so very difficult to access such water resources during training, and everyone is different too; some people hold more water in bones and organs than others.

The scientific consensus is that performance is affected from water losses (percentages) that are relative to your current total body weight or total body mass (not water mass), in conjunction with other factors mentioned in Module 7, Unit 1, such as your individual biochemistry, how hard you are exercising, how long you are exercising for, the temperature and humidity of your surroundings, your athletic status, etc.

The data on slides 8 and 9 (Module 7, Unit 1) is based on the scientific consensus (adopted by the American College of Sports Nutrition and the International Olympic Committee) based on the popular dogma: Amstrong et al, 1985, Montain and Coyle, 1992, Below et al., 1995, McConnell et al. 1997, and Noakes, 1993.

Here’s a visual representations of this (click on the image to zoom in): (Image: 2015-01-10_1349)

Having said that, your athletic status also matters. Studies suggest that dehydration up to 3% to 4% body weight loss can be well-tolerated and does not affect performance in elite athletes (Goulet, 2011; Noakes, 2012; Beis et al., 2011, Laursen et al., 2006, Zouhal et al. 2011).

To conclude, your current body weight (total body mass) is the common denominator to calculate dehydration levels.

The best way to monitor dehydration is through body weight (before and after training) as well as performance metrics (previous vs current), impaired performance indicators (e.g. does exercise become harder?; does your maximal aerobic capacity falls?; is your breathing more laboured?; do you feel weaker or confused?) and biomarkers such as urine colour.

AUTHOR: Alex Ruani, Research Director

Updated on: 15/10/2026