The short answer
- The US Navy method estimates body fat from your height and your neck and waist measurements, plus your hips if you are a woman.
- The BMI-based estimate uses only height, weight, age and sex, so it cannot tell muscle from fat.
- The American Council on Exercise puts the average range at 18% to 24% for men and 25% to 31% for women.
- Home methods are estimates, usually within a few points of a laboratory scan.
What body fat percentage means
Your body is made of fat mass and everything else, called lean mass: muscle, bone, organs, skin and water. Body fat percentage is fat mass divided by total weight. Two people with the same height and weight can have very different body fat, which is why it adds something that weight and BMI cannot show on their own.
Some fat is essential: it stores energy, cushions organs, insulates the body and helps make hormones. Too much, especially around the middle, raises the risk of type 2 diabetes, heart disease and some cancers.
Worked example: a man
- In inches178 ÷ 2.54, 38 ÷ 2.54, 90 ÷ 2.5470.1, 15.0, 35.4
- US Navy body fat20.3%
- CategoryAverage (18% to 24%)
- Fat mass80 kg × 20.3%16.2 kg
- Lean mass80 − 16.263.8 kg
Worked example: a woman
- Waist + hips − neck76 + 100 − 33143 cm
- US Navy body fat30.2%
- CategoryAverage (25% to 31%)
- Fat mass20.6 kg
- Lean mass47.4 kg
Her BMI is 25.0 and her waist-to-height ratio is 0.46, below the 0.5 that NICE recommends staying under.
How to measure properly
- Use a soft tape measure on bare skin, snug but not pressing in.
- Neck: just below the Adam’s apple (larynx), with the tape sloping slightly downwards at the front.
- Waist, men: level with the belly button, at the end of a normal breath out.
- Waist, women: at the narrowest point between the ribs and hips, usually just above the belly button.
- Hips, women: around the widest part of the bottom, with feet together.
- Stand up straight, relax and do not hold your stomach in. Measure each site twice and use the average.
Why small errors matter
The result depends on the difference between your waist and your neck, so a centimetre or two changes it noticeably. For the man in the worked example:
Each extra centimetre of waist adds about 0.7 points. A neck 1 cm larger lowers his result from 20.3% to 19.5%. For the woman, 4 cm more on either waist or hips moves her from 30.2% to 32.2%.
The BMI-based estimate
In 1991, Paul Deurenberg and colleagues published an equation linking body fat to BMI, age and sex, based on measurements of over 1,200 people:
Body fat % = 1.2 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for men and 0 for women.
It is handy when you have no tape measure, but it treats all weight above a healthy BMI as fat. Muscular people get results that are too high, and people who have lost muscle with age can get results that are too low. You can check your BMI with our BMI calculator.
Body fat and age
Body fat tends to rise with age, even at the same weight, as muscle is slowly lost. The Deurenberg equation reflects this: at the same BMI of 25, its estimate rises with age.
| Age | Men | Women |
|---|---|---|
| 25 | 19.5% | 30.4% |
| 45 | 24.1% | 35.0% |
| 65 | 28.8% | 39.6% |
Body fat categories
There is no official NHS range for body fat percentage. The calculator uses the widely quoted categories from the American Council on Exercise:
| Category | Women | Men |
|---|---|---|
| Essential fat | 10% to 13% | 2% to 5% |
| Athletes | 14% to 20% | 6% to 13% |
| Fitness | 21% to 24% | 14% to 17% |
| Average | 25% to 31% | 18% to 24% |
| Obese | 32% or more | 25% or more |
These are guides for adults, not diagnoses. Older adults tend to sit higher in each range.
Essential fat and going too low
Essential fat is the minimum the body needs to work normally: about 10% to 13% for women and 2% to 5% for men. Women need more for hormones and reproduction. Going very low, as some athletes and bodybuilders do for short periods, can stop periods, weaken bones and lower immunity. If you have a result under these levels, check your measurements and talk to your GP if you are concerned.
Fat mass and lean mass
The calculator splits your weight into fat mass (weight × body fat %) and lean mass (the rest). Lean mass drives how much energy you burn at rest, so it matters for weight loss. The Katch-McArdle formula in our calorie calculator uses it to estimate your daily calorie needs.
Lose fat, keep muscle
When you lose weight, some of the loss is usually lean mass. Strength exercise at least twice a week and enough protein help keep more of it.
Weight at a target body fat
If you know your lean mass, you can work out what you would weigh at a different body fat percentage, assuming your lean mass stays the same: lean mass ÷ (1 − target %).
- At 18% body fat63.8 ÷ 0.8277.8 kg
- At 15% body fat63.8 ÷ 0.8575.0 kg
Set a target under More options to see this for your own figures.
Body fat, BMI and waist size
- Needs
- Height and weight
- Good for
- Quick screening
- Misses
- Muscle versus fat
- Needs
- Waist and height
- Good for
- Fat around the middle
- Misses
- Total body fat
- Needs
- Tape or scan
- Good for
- Body composition
- Misses
- Where the fat sits
The NHS uses BMI and waist size to judge weight. NICE recommends keeping your waist to less than half your height. Together with body fat, they give a fuller picture than any one alone.
Where you carry fat
Fat stored deep in the abdomen around the organs (visceral fat) is more harmful than fat under the skin on the hips and thighs. That is why waist size matters so much, and why a person with a modest body fat percentage but a large waist can still have raised health risks. The calculator shows your waist-to-height ratio alongside your body fat.
Other ways to measure body fat
| Method | How it works | Notes |
|---|---|---|
| DEXA scan | Low-dose X-ray | Very accurate; clinics and research |
| Underwater weighing | Body density | Accurate but rarely available |
| Air displacement (Bod Pod) | Body volume | Accurate; specialist centres |
| Skinfold callipers | Fat under the skin | Depends on the tester's skill |
| Smart scales | Electrical impedance | Varies with hydration |
| Tape measure | Circumferences | Free and repeatable |
Lowering body fat
Body fat falls when you take in less energy than you use over time. Our calorie calculator estimates how much to eat. The NHS recommends at least 150 minutes of moderate activity a week and strength exercises on two days. Aim to lose 0.5 to 1 kg a week; slower loss keeps more muscle.
Tracking changes
Measure every four to six weeks, at the same time of day, with the same tape. A change of a point or less can be measuring error, so look at the trend over several months. If your weight is shown in different units on your scales, our stone to kg converter will help, and the feet to cm converter does the same for height.
Who these methods do not suit
- Children and teenagers under 18, whose bodies are still growing.
- Pregnant women.
- Very muscular people, especially with the BMI-based estimate.
- People with a very large waist, where tape methods are less accurate.
- People with fluid retention or after an amputation.
When to talk to a doctor
Talk to your GP if you are worried about your weight or body fat, if you have lost weight without trying, or if your result is below the essential fat range. In England, adults aged 40 to 74 are invited for a free NHS Health Check every 5 years, which includes BMI and waist checks. In the US, a yearly preventive visit is covered by most health plans.