How to use the body fat calculator
- Choose inches or centimeters and select your sex. Women also enter a hip measurement.
- Enter your height, weight and age. Height feeds the Navy equation; weight and age are used for fat mass, lean mass and the BMI-based comparison.
- Measure your neck and waist (and hips for women) with a flexible tape, as described below, and enter the numbers.
- Read your estimate, then compare it with the BMI-based figure to see how much the methods disagree.
How to measure for the Navy method
Good measurements matter more than anything else here, because the equations use the differences between circumferences: for the example man below, measuring the waist half an inch larger raises the estimate by 1 percentage point. Use a flexible, non-stretch tape and measure on bare skin.
- Neck (everyone): around the neck just below the voice box (larynx).
- Waist, men: around the abdomen at the level of the navel.
- Waist, women: around the natural waist at its narrowest point.
- Hips, women: around the widest part of the buttocks.
- Height: without shoes, standing straight against a wall.
Keep the tape level and flat against the skin, snug but not pressing in. Stand relaxed and breathe normally rather than holding your stomach in. Take each measurement two or three times and use the average.
The U.S. Navy body fat formulas
The equations were developed by James Hodgdon and colleagues at the Naval Health Research Center (Hodgdon and Beckett, 1984) and were adopted as a common standard by all U.S. military services. All measurements are in inches; metric entries are converted by dividing by 2.54 first.
%BF = 86.010 × log10(abdomen − neck) − 70.041 × log10(height) + 36.76
- a
- abdomen at the navel, inches
- n
- neck, inches
- h
- height, inches
%BF = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
- w
- natural waist, inches
- p
- hips, inches
- n
- neck, inches
- h
- height, inches
Fat mass is body weight × body fat percentage, and lean mass is everything else (muscle, bone, organs and water). The BMI-based estimate uses the adult equation from Deurenberg and colleagues (1991): %BF = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for men and 0 for women.
Worked examples
Man: 5′ 10″ (70 in), neck 15 in, abdomen 34 in, 180 lb, age 35.
- Abdomen − neck = 34 − 15 = 19; log10(19) = 1.2788.
- log10(70) = 1.8451.
- 86.010 × 1.2788 − 70.041 × 1.8451 + 36.76 = 17.5%.
- Fat mass = 180 × 17.5% = 31.5 lb; lean mass = 148.5 lb.
- BMI = 25.8, so the Deurenberg estimate is 1.20 × 25.83 + 0.23 × 35 − 10.8 − 5.4 = 22.8%.
Woman: 5′ 4″ (64 in), neck 13 in, waist 29 in, hips 39 in: waist + hips − neck = 55, so %BF = 163.205 × log10(55) − 97.684 × log10(64) − 78.387 = 29.2%. At 140 lb and age 40, her BMI-based estimate is 32.6%.
Body fat percentage by measurements
These tables apply the Navy equations to common measurements, so you can see how much each inch changes the estimate.
| Abdomen | Neck 14 | Neck 15 | Neck 16 | Neck 17 |
|---|---|---|---|---|
| 30 in | 11.1% | 8.7% | 6.1% | 3.3% |
| 32 in | 15.5% | 13.4% | 11.1% | 8.7% |
| 34 in | 19.4% | 17.5% | 15.5% | 13.4% |
| 36 in | 23% | 21.3% | 19.4% | 17.5% |
| 38 in | 26.2% | 24.6% | 23% | 21.3% |
| 40 in | 29.2% | 27.8% | 26.2% | 24.6% |
| 42 in | 32% | 30.6% | 29.2% | 27.8% |
| 44 in | 34.6% | 33.3% | 32% | 30.6% |
| Waist | Hips 36 | Hips 38 | Hips 40 | Hips 42 | Hips 44 |
|---|---|---|---|---|---|
| 26 in | 21% | 23.9% | 26.6% | 29.2% | 31.7% |
| 28 in | 23.9% | 26.6% | 29.2% | 31.7% | 34.2% |
| 30 in | 26.6% | 29.2% | 31.7% | 34.2% | 36.6% |
| 32 in | 29.2% | 31.7% | 34.2% | 36.6% | 38.8% |
| 34 in | 31.7% | 34.2% | 36.6% | 38.8% | 41.1% |
| 36 in | 34.2% | 36.6% | 38.8% | 41.1% | 43.2% |
| 38 in | 36.6% | 38.8% | 41.1% | 43.2% | 45.3% |
We do not label results with categories such as “fit” or “average,” because published body fat cutoffs differ between organizations, measurement methods, ages and sexes. If you want to know what a healthy range is for you, ask a doctor, who can also consider your waist size, blood pressure and lab results.
How accurate is the tape measure method?
The Navy equations were originally calibrated against underwater weighing (hydrodensitometry) and have been used by the U.S. military for decades. Between experienced measurers, repeated estimates agree within about 1 percentage point. Accuracy for any one person is a different matter:
- In a 2022 study of 430 men and 179 women in the U.S. Marine Corps, the equations underestimated men's body fat by about 2.5 percentage points on average compared with DXA scans, and overestimated women's by about 1.3 to 2.3 points; individual differences varied with a standard deviation of roughly 4 points.
- The same study found that lean people tended to be overestimated and people with more body fat underestimated, and its authors concluded the method is not suited to precise body composition assessment.
- A 2023 study of military trainees found that circumference-based estimates did not accurately capture body composition changes over eight weeks of training.
Treat your result as a rough estimate within a few percentage points. DXA, the reference method in these studies, is a clinical scan; a doctor can tell you whether one is worth it for you.
The BMI-based estimate
The calculator also shows the Deurenberg estimate, derived in 1991 from 1,229 people aged 7 to 83 whose body fat was measured by densitometry. For adults, its typical prediction error was about 4.1 percentage points, and it slightly overestimated body fat in people with obesity. Because it uses BMI, it cannot tell muscle from fat, so a muscular person may get a higher BMI-based estimate than tape estimate. Seeing two methods side by side is a useful reminder that every home estimate carries real uncertainty. For weight-for-height screening, use the BMI calculator, and for a weight range for your height, see the ideal weight calculator.
Using your result
Body fat percentage is most useful as a trend measured the same way over months. If you are working toward a change, the calorie calculator estimates your daily energy needs, and the macro calculator turns a calorie target into protein, carbohydrate and fat.
Frequently asked questions
How accurate is the Navy body fat calculator?
It is a reasonable estimate, not a measurement. In a 2022 study of 609 fit U.S. Marines, the Navy equations underestimated body fat in men by about 2.5 percentage points on average and overestimated it in women by about 1 to 2 points compared with DXA scans, with larger errors for individuals. Lean people tended to be overestimated and people with more body fat underestimated.
Where do I measure my waist for the Navy method?
It depends on sex. Men measure the abdomen at the level of the navel. Women measure the natural waist at its narrowest point, and also measure the hips at the widest part of the buttocks. In both cases keep the tape flat on bare skin, level all the way around, and snug without pressing into the skin. Breathe normally and do not pull the stomach in.
Why does the calculator need my weight and age?
The Navy equations use only height and tape measurements. Weight is used to split your body weight into fat mass and lean mass, and weight and age together give the second, BMI-based estimate from the Deurenberg equation. Comparing the two estimates helps show how much a single method can vary: for the example man on this page they are 17.5% and 22.8%.
What is the difference between body fat percentage and BMI?
BMI is weight divided by height squared, so it cannot tell fat from muscle. Body fat percentage estimates how much of your weight is fat. Two people with the same BMI can have quite different body fat. The Deurenberg equation estimates body fat from BMI, age and sex, but its typical error is about 4 percentage points, and the original study found it slightly overestimated body fat in people with obesity.
Can I use this calculator to track fat loss?
It can show broad trends if you measure the same way each time, but small changes are hard to detect. A 2023 study of military trainees found that circumference-based equations did not accurately capture changes in body composition over eight weeks. Measure at the same time of day, take each measurement two or three times, and look at changes over months rather than weeks.
Is the Navy method the same one the military uses today?
These Hodgdon-Beckett equations were used across the Department of Defense for decades. In 2023 the U.S. Army adopted a new body fat estimation method, so current service rules may differ from this calculator. This page is for personal education only and is not a substitute for an official military assessment.