While many of us know that body composition is more important than the number on the scale, for many, the intricacies of body fat percentage and how we calculate it remain baffling.
Here’s our quick-start guide to tell you everything you need to know about body fat percentage and what it means for your goal.
Key Takeaways
- Body fat percentage represents all your fat mass, divided by your total body weight.
- Body fat percentage is an important benchmark for progress because we want to retain as much lean body mass as possible during a diet.
- We calculate energy requirements based on lean body mass rather than total body weight to prevent overestimating how many calories you need.
- We calculate protein based on lean body mass in order to fuel muscle tissue, not fat.
- All forms of body composition analysis come with flaws and should therefore be used as a benchmark for progress and not regarded as an absolute number.
What is body fat percentage?
Your body fat percentage is the weight of all the fat on your body (fat mass) divided by your total body weight. For example, if you weigh 80kg and have 10kg of body fat, your body fat percentage is 12.5%.
Why measure your body fat percentage?
Measuring your body fat percentage allows you to track your fat loss progress over time. You can also use it to calculate how much lean body mass (LBM) you have, which is the total weight of your body minus all your fat mass.
A common misunderstanding is that LBM only refers to muscle, when in fact it describes everything apart from body fat. So, in addition to muscle, it also includes your organs, bones, skin, body water and food weight.
When creating your meal plan, the U.P. app calculates your daily targets based on how much LBM you have, rather than total body weight.
This distinction is important because body fat and LBM do not have the same energy or nutrient requirements. Each day your body burns roughly 12 kCal per pound maintaining its current amount of LBM.
In contrast, although your fat cells are more than just passive storage sites for fat and perform several essential functions, like hormone production, they have a relatively low maintenance cost of 2 kCal per pound.
If you are relatively lean, this is not really an issue. For example, if you weigh 80kg and have 10% body fat, whether you decide to use 80kg or 72kg (your LBM) to create your meal plan will not make a big difference.
However, if you carry a lot more body fat (upwards of 30%), calculating your calorie requirements based on your total body weight will overestimate how many calories you need.
This also applies to other nutrition targets that we tend to set relative to body weight, such as protein. The primary role of protein is structural, which means that your body uses it to build and repair components of your LBM, such as muscle. As a result, it makes sense to set your protein target relative to how much LBM you have, rather than body fat, which does not require large amounts of protein.
How to measure your body fat percentage
The only way to directly measure your body fat is via cadaver analysis (dissection of a dead human body), which has some obvious practical limitations.
All other methods of body fat testing only estimate how much body fat you have. Each body fat assessment tool is based on a different set of assumptions, and you cannot compare your results between them.
Skinfold calliper testing
The skinfold calliper technique is based on the assumption that there is a relationship between the thickness of skinfolds (which includes the ‘pinchable’ subcutaneous fat beneath your skin) at various sites around your body and your total body fat.
Of all the available options, skinfold testing is the most cost-effective and practical method of body fat assessment available to personal trainers.
The primary limitation of skinfold callipers is that measurement reliability depends on the skill and experience level of the operator. Measuring just 1cm away from a defined measurement site produces significant differences in measurement values. As a result, we do not recommend using skinfold testing unless you have access to a personal trainer who has a relevant qualification and experience at taking measurements.
High-tech methods
Two examples of more high-tech options are dual energy X-ray absorptiometry (DXA) and hydrostatic weighing.
A DXA scan is a special type of X-ray that works by sending low-dose X-rays through your body and measuring the absorption rates of different body tissues. The rationale is that body fat and LBM have different absorption properties, which allows them to be isolated and measured.
Hydrostatic weighing (also known as underwater weighing) involves comparing your dry weight on land to your underwater weight. Components of LBM like bone and muscle are denser than water, whereas fat is less dense. As a result, if two people weigh the same on dry land but one has more body fat, the fatter person will weigh less underwater. Put simply, fat floats and muscle sinks!
While DXA and hydrostatic weighing may be more accurate than some low-tech options, they are expensive and impractical (require you to visit a research laboratory or clinic) for the purposes of creating a meal plan and monitoring short-term changes in your body composition. However, DXA scans can be useful at the start and end of your transformation, or annually, if you want to learn more about your body composition.
Bathroom scales
Bioelectrical impedance (BIA) is the technical name for the technology found in body weight scales that also estimate your body fat percentage.
It works by sending a small electrical signal through your body and measuring the resistance of different body tissues to the signal. The scale then estimates your body fat percentage based on the fact that the current flows more easily through parts of your body that have a high water content (e.g. muscle and blood) than it does those with low water content (e.g. bone or fat).
A big problem with BIA devices is that they are incredibly sensitive to changes in your body’s water balance. Exercise, food, drink and whether or not you have been to the toilet can all affect the reliability of your results. Overall, there is limited research to support the accuracy of consumer-grade devices, so we recommend only using your scales to weigh yourself.
Tape measurements
The tape measurement method involves measuring the circumference of various sites around your body and inputting the results into one of several prediction equations.
One of the most popular equations was developed by the U.S. Navy who needed a quick, inexpensive and reasonably accurate method of assessing the body composition of their personnel.
The Navy developed individual equations for men and women, which take into account gender differences in body fat distribution. The major advantages of the tape measurement method are that it is simple to perform and only requires a tape measure.
