What Is Body Fat Percentage? How to Measure It, Targets by Sex and Age
Body fat percentage is the share of your body weight that is fat. This article explains what body fat percentage is, how it differs from BMI, estimation methods such as bioelectrical impedance (Tanita-style) and the Navy circumference method, targets by sex and age, how home scales work and their errors, and a healthy range. Note that this is general information, not medical advice.
You see "body fat percentage" at health checkups and on body composition scales. Even at the same weight, a different body fat percentage means a different appearance and different health risks. Yet many people feel unsure how to measure it correctly, or notice the number jumping from day to day. This article explains, step by step, what body fat percentage is, how to measure it, and what numbers to aim for.
What Is Body Fat Percentage
Body fat percentage is the share (percentage) of your total body weight that is made up of fat. For example, if a 60 kg person has 20% body fat, then 12 kg of that is fat and the remaining 48 kg is "fat-free mass" such as muscle, bone, water, and organs. At the same weight, having more fat or more muscle makes a large difference to your shape and health.
Fat is not merely "excess." It plays essential roles such as storing energy, retaining body heat, serving as raw material for hormones, and protecting organs. For that reason, body fat percentage is a measure where lower is not always better; both too much and too little can lead to health problems. To get a sense of your own number, the Body Fat Calculator makes it easy to estimate an approximate value.
How It Differs From BMI
BMI (Body Mass Index) is another common measure of body type. It is calculated as weight (kg) ÷ height (m) ÷ height (m), a simple index using only height and weight. It is quick to compute and useful for roughly gauging a tendency toward overweight.
However, BMI does not distinguish fat from muscle. A muscular athlete can be judged "obese" by BMI even with little body fat. Conversely, someone with little muscle and a lot of fat from lack of exercise, often called "hidden obesity," may have a normal BMI but a high body fat percentage.
| BMI | Body fat percentage | |
|---|---|---|
| Information needed | Height and weight | A scale or circumference measurements |
| What it shows | Weight relative to height | Share of weight that is fat |
| Fat vs. muscle | Cannot distinguish | Reflected |
| Ease | Very easy | Needs measuring tools |
In other words, BMI and body fat percentage are not a matter of "which is correct." Rather, only by looking at both can you grasp your body type in three dimensions. You can compute BMI with the BMI Calculator, and checking it together with body fat percentage gives you a more accurate picture of your condition.
How to Measure: Impedance (Tanita) and the Navy Method
Because you cannot extract and weigh "only the fat" directly, every method is some form of estimation. Let us look at the main approaches used at home.
1. Bioelectrical impedance analysis (Tanita-style and others)
Most home body composition scales use bioelectrical impedance analysis (BIA). A weak electrical current is passed through the body and its resistance (how hard it is for the current to flow) is measured. Fat conducts electricity poorly while water-rich muscle conducts it well, so the device estimates body fat percentage from the resistance plus height, weight, age, and sex. The appeal is that you simply stand on it or hold it.
2. Circumference method (Navy method)
The circumference method lets you estimate without special equipment, and because the U.S. Navy uses it, it is also called the "Navy method." You measure around the neck and waist (and hips for women) with a tape measure, then combine those with height in a formula to estimate body fat percentage. Since it uses no current, you can calculate without a scale, which pairs well with approaches like the Body Fat Calculator, where you simply enter your measurements to get an estimate.
In clinical and research settings, more precise methods such as underwater weighing and DXA (dual-energy X-ray absorptiometry) are also used. It is important to understand that numbers measured at home are only estimates compared with these.
Targets by Sex and Age
Appropriate body fat percentage differs greatly by sex. Because of pregnancy, childbirth, and hormones, it is natural for women to have a higher body fat percentage than men. Below is one example of the general ranges widely cited as home guidelines; the exact criteria vary by device and guideline.
| Category | Men (guideline) | Women (guideline) |
|---|---|---|
| Low | Up to about 10% | Up to about 20% |
| Standard | About 11-21% | About 21-34% |
| Somewhat high / high | 22% and above | 35% and above |
Also, muscle mass tends to decline with age, so at the same weight, body fat percentage tends to rise. For that reason, standard ranges are generally set slightly higher for older age groups. Rather than treating the number from your younger years as an absolute goal, it is more sustainable to keep a range appropriate for your age in mind. What matters is not reacting to a single reading, but measuring under the same conditions over time to see the trend.
How Home Scales Work and Their Errors
As noted above, most home body composition scales estimate body fat percentage from how poorly the body conducts electricity. Convenient as that is, it has a weakness: it is easily affected by the body's water content. Because water conducts electricity well, the reading rises and falls depending on whether you have more or less water in your body.
- After exercise or a bath: sweating and changes in blood flow shift water distribution, making readings unstable.
- Right after eating or drinking: body water temporarily increases and affects the result.
- Right after waking: water is lost during sleep, so it can read higher than usual.
- A woman's menstrual cycle: in phases prone to swelling, water increases and the number can rise.
The trick to limiting this fluctuation is to measure under the same conditions every time as much as possible. For example, fixing the timing to "in the morning after urinating, before eating" makes day-to-day comparison easier. We recommend using the numbers from home devices not for absolute accuracy but as a measure of the direction of change taken under the same conditions. If you do not have a scale, you can also estimate from tape-measure readings with the Body Fat Calculator.
A Healthy Range and How to Use It
A body fat percentage that is too high is associated with risks such as lifestyle-related diseases, but one that is too low also strains the body. Extremely low fat can lead to hormonal imbalance, sensitivity to cold, weakened immunity, and, in women, effects on menstruation. It is important not to forget the premise that the body stores fat because it is needed.
To adjust your numbers healthily, the proven path is not crash dieting but gradually reviewing the quality of your diet and your exercise habits. Muscle in particular supports fat-free mass and is tied to basal metabolism, so reducing fat while preserving muscle through moderate exercise tends to be good for both appearance and health. First check the balance of your weight and height with the BMI Calculator, and grasp your fat share with the Body Fat Calculator to see where you stand now.
Please note that this article is intended to provide general information and does not offer medical diagnosis or advice. The criteria for body fat percentage and its effects on health vary widely between individuals, so if you have a medical condition or are working on losing or gaining weight, check the latest official information and consult a doctor or specialist as needed before deciding.
FAQ
What is body fat percentage?
Body fat percentage is the share (percentage) of your body weight made up of fat. For a 60 kg person at 20%, 12 kg is fat and the remaining 48 kg is fat-free mass such as muscle, bone, water, and organs. Because fat plays roles like energy storage and heat retention, it is a measure where both too much and too little can cause health problems, so lower is not necessarily better.
How does body fat percentage differ from BMI?
BMI uses only height and weight and is quick, but it cannot distinguish fat from muscle. A muscular person may be judged obese, while "hidden obesity" with a normal BMI but high body fat can be missed. Body fat percentage shows the share of weight that is fat, so looking at both gives a more three-dimensional picture of your body type.
How do the Tanita (impedance) and Navy methods differ?
Bioelectrical impedance methods like Tanita pass a weak current through the body and estimate body fat from how hard it is for the current to flow; you just stand on the scale. The Navy (circumference) method measures around the neck and waist with a tape measure and estimates using a formula with your height, so it can be calculated without a scale.
Why does a home scale give different readings on different days?
Most scales estimate body fat using the body water content, so readings shift as water rises and falls. Times such as after exercise, after a bath, right after eating or drinking, and right after waking are prone to fluctuation. Measure under the same conditions each time (for example, mornings after urinating and before eating) and watch the trend rather than the absolute value.
Are the numbers I enter in the Body Fat Calculator sent to a server?
WAZA TOOLS' Body Fat Calculator and BMI Calculator run entirely in your browser, so the height, weight, and measurements you enter are never sent to or stored on a server. They work on smartphones too, and you can use them as many times as you like, for free.