The scale gives you one number, your weight. It cannot tell you how much of that weight is fat and how much is muscle, bone and water. Body fat percentage can. The calculator above estimates it from your height and two or three tape measurements, using the formula the US Navy developed to screen its own personnel. This guide explains what the number measures, how the formula works, how to take the measurements so the estimate holds up, and how to read the result against the standard categories.
What body fat percentage measures
Your weight is the sum of two parts. Fat mass is the fat stored under the skin and around the organs. Lean mass is everything else: muscle, bone, organs, blood and water. Body fat percentage is fat mass divided by total weight. A 180-pound man at 21 percent body fat is carrying about 38 pounds of fat and about 142 pounds of everything else.
BMI works from height and weight alone, so it cannot see that split. Two people of the same height and weight share a BMI whether one of them lifts weights and the other does not. That is the limit of BMI, and you can see it for yourself with the BMI calculator: it places you on a population chart, which it does well, but it says nothing about composition. Body fat percentage does. It is also the number that moves in the right direction when a weight-loss plan is working, because the goal is to lose fat, not muscle.
Some fat is essential. The body needs a baseline amount to insulate nerves, build hormones and cushion organs, and women need more of it than men for reproductive health. That is why the category tables below have a different floor for men and for women, and why a very low number is not a goal.
How the US Navy method works
The Naval Health Research Center published the method in 1984 as a way to estimate body fat for large groups of people with nothing but a tape measure. Its researchers measured sailors by underwater weighing, the laboratory standard at the time, then found the combination of circumferences that predicted those results best.
For men, the formula uses three numbers: height, neck circumference and waist circumference. For women it uses four: height, neck, waist and hip. The idea is simple. A larger waist relative to the neck points to more stored fat, because the neck tracks frame size and the waist tracks fat. The hip is added for women because women store proportionally more fat there. Height scales the result so a tall frame and a short frame are read on the same terms. The arithmetic uses logarithms, which is why the result does not move in a straight line with each inch, but you do not need the equation to use it.
Take the calculator’s default: a man 5 feet 10 inches tall, with a 15-inch neck and a 36-inch waist. The waist minus the neck is 21 inches. The formula turns those inputs into an estimate of 21.3 percent body fat, which lands in the Average category for men. Add a weight of 180 pounds and the calculator splits it: 38.3 pounds of fat mass and 141.7 pounds of lean mass. Change the waist to 37 inches with everything else the same and the estimate rises to 23.0 percent. One inch at the waist is worth nearly two points, which is why the measurements matter more than the math.
How to measure
Measure on bare skin with a flexible tape, standing relaxed. The tape should be snug, lying flat against the skin all the way around, without pressing into it. Keep it level with the floor; a tape that dips at the back reads short.
- Neck: just below the larynx (the Adam’s apple), with the tape sloping slightly downward toward the front. Do not tense your neck or lift your shoulders.
- Waist, men: at the level of the navel, at the end of a normal breath out, without pulling your stomach in.
- Waist, women: at the narrowest point of the torso, usually a little above the navel, also after a normal breath out.
- Hip, women only: around the widest part of the buttocks, with your feet together.
- Height: without shoes, standing straight.
Take each measurement two or three times, let the tape go slack between readings, and average them. If two readings disagree by more than half an inch, measure again. For tracking over time, measure under the same conditions each time, ideally in the morning before eating. Water and food in the gut change the waist by an inch over a day without any change in fat, and an inch is enough to move the estimate.
How to read your number
The categories the calculator shows come from the American Council on Exercise. They describe where a number sits among adults, not a target to chase. Essential fat is a floor, not a goal. Athletic describes competitive athletes in season. Fitness and Average cover most active and sedentary adults. Above average is where the health risks associated with excess fat begin to rise.
Men
| Category | Body fat |
|---|---|
| Essential fat | 2–5% |
| Athletic | 6–13% |
| Fitness | 14–17% |
| Average | 18–24% |
| Above average | 25%+ |
Women
| Category | Body fat |
|---|---|
| Essential fat | 10–13% |
| Athletic | 14–20% |
| Fitness | 21–24% |
| Average | 25–31% |
| Above average | 32%+ |
The bands are wide on purpose. A woman at 26 percent and a woman at 30 percent are both Average. Treat the boundary lines as rough. With a method that can be a few points off, a reading of 24.6 percent for a man is not clearly Average rather than Fitness. The useful question is which direction the number moves over months, not which side of a line it sits on today.
How accurate it is
For most adults the US Navy method typically lands within about 3 to 4 percentage points of a DEXA scan, the X-ray method clinics use to measure body composition. That is close enough to tell you your general category and to track change over time. It is not close enough to settle a difference of a point or two, so do not read much into small moves.
It is least accurate at the extremes. Very lean people with a lot of muscle tend to read high, because a thick neck and a muscular waist do not fit the pattern the formula was built on. People with a high body fat percentage tend to read low. The formula was also built on a sample of US Navy personnel, so people whose proportions differ from that group, for example a long torso with narrow hips, can land further off.
The right use of the tool is as a trend line. Measure the same way each time, write the number down, and compare it with the number from a month ago. Compared with itself, the estimate is far more reliable than compared with a laboratory scan taken once. If you need a precise number for a medical reason, a DEXA scan or an air-displacement test is the standard. This calculator is not a diagnosis of anything.
Body fat while losing weight
When weight comes off, some of it is fat and some is lean tissue. The aim of any weight-loss plan is to keep that second share as small as possible, because muscle is what keeps you strong, supports your metabolism and makes the weight easier to keep off. Watching body fat percentage alongside the scale shows which kind of weight you are losing. If the scale drops and the percentage drops with it, you are losing mostly fat. If the scale drops and the percentage barely moves, you are losing lean mass too.
Two things protect lean mass in a calorie deficit: enough protein and resistance training. Protein gives your muscles the material to maintain themselves while you eat less, and lifting, or any exercise that loads your muscles, gives them a reason to stay. The protein calculator sets a daily target from your weight and goal. Two or three sessions of resistance work a week is a common starting point.
If you are considering prescription weight-loss treatment, this number is background, not a gate. It does not decide eligibility; a US-licensed clinician reviews your health history and decides whether treatment is appropriate for you. The estimate is still useful during treatment, for the same reason it is useful without it: it shows whether the weight you are losing is the weight you want to lose.
How to use this calculator
Choose your sex and your units, enter your height, then your neck and waist measurements, and for women your hip. The estimate updates as you type, with your category and the full set of bands beside it. Weight is optional. Add it and the card also shows your fat mass and lean mass, in pounds or kilograms. If the card shows no result, a required measurement is blank or the waist is no larger than the neck, which the formula cannot work with; check the tape. Nothing you enter is stored.
A tape measure and five minutes give you a number the scale cannot: how much of you is fat. Take the measurements carefully, read the result as a range rather than a point, and watch its direction over months. That is what the number is for, and it is enough to tell you whether a change in your weight is the change you were after.