How accurate are body fat percentage calculators?
Pretty much, in general. But an online body fat calculator isn’t the most accurate way to determine your body fat percentage.
There are many different methods you can use to calculate body fat percentage.
Here are the most common body fat measurement methods, in order from most accurate to least accurate.
1. DEXA scan
Dual-energy X-ray absorptiometry (DEXA) uses low-level X-rays to determine your lean mass, fat mass, and bone mineral density.
DEXA is the most accurate body fat measurement method, but a downside is that it is not readily available in every geographic area and can be relatively expensive compared to other methods.
2. Hydrostatic weighing
In this method, it is first weighed and then weighed again while being immersed in water. Because fat is less dense than water, a person with more body fat weighs less underwater.
This is also a very accurate method of measuring body fat percentage (it was the gold standard prior to DEXA), but is not commonly used outside of research settings. (It requires a deep tank that looks like a whirlpool and, amusingly, an autopsy scale.)
3. Air Displacement Plethysmography (Bod Pod)
A machine estimates body fat percentage in a similar way to hydrostatic weighing, only using air pressure instead of water.
This method is relatively accurate and more readily available (more and more gyms now have it), but is more expensive than the less accurate options below.
4. Skinfold measurements
A vernier caliper is used to measure the thickness of fat and skin in multiple areas of the body. Then these numbers are plugged into a formula to determine body fat percentage.
The accuracy of this method depends on the skill of the person taking the measurements. In order to track changes over time, it is important that the same person takes all measurements.
Also, this method may not be accurate for people with very high body fat percentage.
5. Circumference Measurements
Circumference measurements (as used in the body fat percentage calculator on this page) can be included in formulas with other information such as height, weight, age, and gender to estimate body fat percentage.
While it’s not the most accurate approach, it’s one of the easiest and doesn’t require any special equipment (other than a tape measure).
This method is particularly useful for measuring progress over time, as changes in girth can indicate changes in muscle mass and body fat.
6. Bioelectrical Impedance
Scales that measure body fat percentage use this method. Essentially, the scale sends an electrical current through your body and measures the resistance.
Fat creates more resistance than muscle and water, which helps the scale estimate your body fat percentage.
The problem: This method can be sensitive to hydration status. (So your results can fluctuate even on the same day.)
7. Body weight alone
Weight changes can reflect loss or gain of body fat, but a scale doesn’t tell you whether the weight gain or loss is from muscle or fat.
What are the three body fat formulas?
As mentioned earlier, this body fat calculator takes your input and plugs it into three scientifically validated formulas for body fat percentage:
The YMCA Body Fat Formula: Originally used by the YMCA. It uses waist size, body weight and gender. The Navy Body Fat Formula: Developed by the US Naval Health Research Center. It uses neck and waist measurements for men and neck, waist and hip measurements for women.1 The CUN BAE body fat formula: Developed at the University of Navarra in Spain. CUN BAE is an acronym for Clinica Universidad de Navarra (CUN) Body Obesity Estimator (BAE). It uses body weight, height, age and sex.2
We then average all three to give you a solid idea of where you stand. Each formula for body fat percentage has its pros and cons, so we’ll use all three to get a more realistic estimate.
“My body fat estimate doesn’t make sense!”
Okay, we’ve already established that this body fat percentage calculator is only an estimate and, depending on your specific body, may not accurately reflect your exact body fat percentage.
But let’s examine this a little more.
Suppose a million random people use the calculator. Chances are 950,000 of them will find that to be a pretty credible estimate.
At the same time, for 50,000 of those people, it could be a long way away – or totally unbelievable.
While 50,000 may seem like a lot of people, compared to 950,000 it is pocket change.
Think of it this way: In the general population, there aren’t many people – percentage-wise – who are built like an NFL linebacker.
However, you could be.
Or you might be on the other end of the body composition spectrum.
Formulas — like the one in this body fat calculator — designed to estimate body fat percentage from girth measurements simply aren’t sensitive enough to account for all body types, especially those furthest from average.
But… they are the best formulas we have based on scientific research.
You don’t define these numbers either. It’s just data you can use to measure changes over time.
Your age, gender and ethnicity will affect your results.
People vary greatly in their body shapes and sizes. We are all unique individuals.
In addition, several other factors affect your body fat percentage and distribution, including:
As you age, you tend to lose muscle mass and gain body fat. They also tend to accumulate more visceral fat.
On average, men and women have different body fat percentages. Also, this fat is often distributed differently on your body, mainly thanks to the action of hormones.3,4,5,6
Men, in general, tend to put on more fat in the middle.
Women tend to gain more fat on their lower abdomens, hips and thighs, as well as in breast tissue.
However, men, whose bodies convert testosterone to estrogen more easily, may also gain fat in the breast tissue and around their hips and thighs, similar to a typical female pattern of fat distribution.
Men and women tend to differ in how much body fat is optimal for health, function, longevity, and performance.
In general, female bodies prefer higher ranges of body fat for overall health.
And on average, men tend to have more lean mass than women over their lifetime.
What if I’m intersex or transgender?
So far there are no reliable calculations for people who are intersex or transgender. (This is due to a lack of scientific data in this area.)
Typically, the most accurate estimate comes from body composition calculations, which most closely match a person’s current hormone profile.7,8
Supplemental hormones such as estrogen and testosterone affect muscle mass and body composition over time.
Trans men who have been supplementing testosterone for at least six months and have undergone top surgery to remove breast tissue may find the male body composition equation to be the most accurate.
Trans women who have been supplementing estrogen for at least six months may find the female body composition equation to be the most accurate.
There can be significant differences between ethnic groups.9,10,11,12,13
This is done both in terms of how accurately the body fat calculation estimate reflects reality and in terms of predicting health and disease risks.
For example, people with some types of South and East Asian ancestry may have a higher risk of metabolic diseases (such as cardiovascular or type 2 diabetes) at a relatively lower body fat percentage than someone with Western European ancestry.
And people from populations that typically have heavier, denser bodies — like many South Pacific indigenous peoples — may be classified as “obese,” or in a higher-risk category, even though this body type isn’t associated with any adverse health effects for them.