Why the Scale Lies About Fat Loss — And Better Ways to Track Progress
A look at why BMI and body weight can be misleading and better ways to measure fitness and health.
- BMI and scale weight can't distinguish fat from lean mass. For anyone exercising, they're often misleading and sometimes flat-out wrong.
- Waist circumference is a simple, practical alternative that better reflects health risk and isn't skewed by the amount of muscle that you have.
- Progress photos, clothes fit, strength improvements, and blood markers tell a more complete story. Use multiple measures, not just one number.
There are more important things that can be measured beyond the number on a scale, including body composition, strength gains, and changes in how your clothing fits. This article explains practical, evidence-based methods to track progress better. You may have heard that weightlifting and building muscle are good for your health and for losing fat — that’s undeniably true.
Why the scales and BMI are unreliable measures of health
So, you’ve been going to the gym consistently for months. Your clothes fit better, you notice it in the mirror, you feel stronger, and your friends are even making comments about it.
But when you step on the scale, the number has barely moved — or worse, it's gone up. Frustrated, you wonder if all your hard work has been for nothing: “I’ve not even lost any weight”, you say to yourself. Here's the real truth: the scale is lying to you.
Therefore, so is your Body Mass Index (BMI) if your doctor bothers to look at that sort of thing. These popular metrics, while simple to calculate and widely used, are fundamentally flawed when it comes to what health and fitness progress actually look like. We’ll explain how and why these measurements fail, and what you should think about looking at instead.
Why is BMI a poor measure of fat loss?
A 200-year-old tool that never fit
BMI was developed in the 1830s by the Belgian statistician Adolphe Quetelet. It wasn't created to measure how fat or how fit a person was; it was created to study “population” trends across thousands of different people all at the same time.
The formula is deceptively simple: take your body weight in kilograms and divide it by the square of your height in meters. A BMI over 30 classifies you as obese, while a BMI over 25 means you're overweight. While BMI correlates reasonably well with disease risks across “populations” (i.e., thousands of people all analysed together), it has flaws when it is used at an individual level.
The fundamental issue?
BMI cannot distinguish between fat mass and 'fat-free mass', which also includes muscle, bone, organs, and water. It treats all weight equally, when clearly (from a health perspective) it's not. Gaining one kilogram of muscle does not lead to the same health outcomes as gaining one kilogram of fat.
In fact, in most cases, they have opposite effects on your health. Gaining muscle is nearly always good for your health. Whereas most of the time, gaining fat is bad. Consider this: if you gain 5 kilograms of healthy muscle through strength training, this is something nearly every scientist and medical expert would agree is beneficial for your health1.
However, despite the applause, your BMI will actually increase, suggesting you've become less healthy! The one exception is that if you're underweight to start with, which won't apply to most people, so this article is just for those who are looking at maintaining or losing weight, rather than gaining it.
The exercise paradox
This limitation is problematic for people who are exercising regularly, especially weightlifting, as it’s particularly efficient at stimulating muscle growth. Over the course of a resistance training programme, you can typically expect increases in your muscle mass to occur alongside decreases in fat mass — a highly desirable outcome.
Yet because changes in muscle mass and BMI are positively correlated with each other, BMI is completely inappropriate for assessing progress in people who are exercising. If you’re thinking this doesn’t make sense, you’re right. Yes, the medical community will advise you to exercise for weight loss, then, at the same time, use a flawed measure to assess your progress. Bizarre!
Let me illustrate with a real-world example. Imagine an obese person weighing 100 kg (220 pounds) who is 1.8 meters (5 feet 11 inches). Their body fat is 40% (40 kg of fat and 60 kg of fat-free mass), and they have a BMI of 31 31 kg/m². Over several months of exercise training, they go on to lose 5 kg (11 lbs) of fat while simultaneously gaining 5 kg (11 lbs) of muscle — an excellent outcome.
What happens to their BMI? Absolutely nothing. It remains at 31. They’re still classified as “obese” even though their body fat percentage has dropped from 40 to 35%, and their health will have improved dramatically.
Now, worse: imagine that they only lost 5 kg of fat, they would have reduced their BMI from 31 to 29. They would no longer be defined as obese (just overweight), despite not losing a single gram of fat, and their body fat would have only dropped from 40 to 37%.
It can technically get even worse. Imagine that person skips the gym, lies on their sofa and bed, and loses 5 kg of muscle. They’ll have been moved out of the obese category by their doctor, but their body fat would have gone up from 40 to 42%.
These contradictions reveal why BMI fails as a measure for monitoring progress for people who are exercising. Any reduction in fat or body fat percentage can be completely offset by simultaneous increases in muscle mass. This makes BMI changes misleading or even discouraging for people doing exactly what they should be doing (from a health perspective).
Beyond fat and muscle: water and carbohydrate
The problems with scale weight extend even further. Your body weight fluctuates significantly based on factors that have nothing to do with gaining or losing fat.
Even if you’re drinking the same amount of water every day, the amount of water in your body can vary by several pounds day-to-day based on your sodium intake, hormones, inflammation, menstrual cycle, stress, and medication.
After eating carbohydrate-rich meals, your body stores carbohydrate for energy (as “glycogen”) in your muscles and the liver. Each gram of glycogen stored in your muscle takes 3–4 grams of water with it. This is why low-carb diets produce rapid initial “weight loss” after a few days — it's primarily water and glycogen, not fat2.
Starting a new exercise program often causes temporary “water retention” in your muscles to repair and adapt. This means you could be burning fat while the scale stays the same or your weight even increases due to water weight — another way the scale misleads you about your actual progress.
