A tape measure and a scale can tell you more about your body composition than the bathroom scale alone. This body fat calculator uses the U.S. Navy circumference method or a BMI based method to estimate your body fat percentage, then breaks that number down into fat mass, lean mass, and a fitness range based on your sex and age.
How to use this body fat calculator
- Choose your calculation method: the U.S. Navy tab (neck, waist, and hip measurements) or the BMI tab (based on your body mass index).
- Select your sex, Male or Female, since the formula and category ranges differ between the two.
- Pick your unit system: Metric (cm/kg) or Imperial (in/lb).
- Enter your height and weight.
- If you’re using the Navy method, enter your neck measurement, taken just below the larynx.
- Enter your waist measurement, at the navel for men or at the narrowest point of the torso for women.
- Enter your hip measurement at the widest point (this field is used for the female calculation and may not be required for men, depending on the tab).
- Enter your age in years.
- Click the button to calculate your body fat percentage.
- Review the results: body fat percentage, fat mass, lean mass, FFMI (normalized), an alternate method estimate, your ACE fitness range, and how much fat you’d need to gain or lose to reach that range. A visual slider running from 5% to 50% body fat shows where your result falls.
- Optionally, generate a PDF report, share the result, copy the report, or save it to your calculation history, which can be exported as Excel, PDF, or CSV.
What this body fat calculator does
Unlike a scale, which only reports total weight, this body fat calculator tries to separate that weight into fat mass and lean mass (muscle, bone, organs, water). It does this two ways: the U.S. Navy circumference method, which uses neck, waist, hip, height, and sex in a logarithmic formula, and a BMI based estimate, which uses your body mass index along with age and sex as a cross check. Having both numbers side by side is useful because no single at home method is perfectly accurate, and seeing where the two estimates agree (or don’t) gives you a better sense of how much to trust the result.
The U.S. Navy method formula
The Navy method was developed for military fitness screening and relies on the idea that circumference measurements at the neck, waist, and (for women) hips correlate with body fat percentage when adjusted for height. It’s quick, doesn’t require calipers or a scan, and is reasonably consistent as long as you measure carefully with a flexible, non-stretch tape.
Men: Body Fat % = 495 / (1.0324 – 0.19077 x log10(waist – neck) + 0.15456 x log10(height)) – 450
Women: Body Fat % = 495 / (1.29579 – 0.35004 x log10(waist + hip – neck) + 0.22100 x log10(height)) – 450
All measurements go into the formula in centimeters. If you enter values in inches, the calculator converts them behind the scenes before running the math.
Getting an accurate measurement
- Neck: measure just below the larynx (Adam’s apple), with the tape sloping slightly downward at the front.
- Waist: for men, measure at the navel; for women, measure at the narrowest point of the waist, usually just above the belly button.
- Hip: measure at the widest point of the hips and buttocks.
- Keep the tape snug but not compressing the skin, and measure at the end of a normal exhale for consistency.
The BMI method
The BMI tab estimates body fat percentage from your body mass index, age, and sex rather than from tape measurements. It’s a faster option if you don’t have a tape measure handy, or a useful cross reference against the Navy result. Because BMI doesn’t distinguish muscle from fat, this estimate tends to run less accurate for very muscular or very lean people, which is exactly why the calculator labels its result as an “alt. method estimate” alongside the primary body fat number.
ACE body fat categories
Once your body fat calculator result comes back, it’s compared against the American Council on Exercise (ACE) classification, which is broken out by sex since women naturally carry more essential fat than men.
| Category |
Men |
Women |
| Essential fat |
2-5% |
10-13% |
| Athletes |
6-13% |
14-20% |
| Fitness |
14-17% |
21-24% |
| Average |
18-24% |
25-31% |
| Obese |
25% and up |
32% and up |
The calculator also shows a “fitness range” and the fat change needed to reach it, so instead of just a raw percentage you get a sense of how many pounds or kilograms of fat separate you from the Fitness or Average bracket.
Fat mass, lean mass, and FFMI explained
Once body fat percentage is known, the calculator multiplies it by your total weight to get fat mass, and subtracts that from your weight to get lean mass (sometimes called fat free mass). FFMI, or Fat Free Mass Index, normalizes your lean mass against height, similar to how BMI normalizes total weight against height, which makes it easier to compare muscle mass across people of different heights.
Fat Mass = Weight x (Body Fat % / 100)
Lean Mass = Weight – Fat Mass
FFMI = Lean Mass (kg) / Height (m)^2, normalized to a 1.8 m reference height
This body fat calculator gives useful estimates, not a lab-grade body composition scan. Tape measure and BMI based methods carry a typical error of a few percentage points, so treat the numbers as a screening tool for tracking trends over time rather than a medical diagnosis, and check with a healthcare provider for anything more precise.
Frequently asked questions
How accurate is the U.S. Navy method compared to a DEXA scan?
The Navy method is generally within a few percentage points of more precise methods like DEXA or hydrostatic weighing for most body types, though it can be less reliable for people who are very muscular or carry fat in unusual distributions. It’s best used as a way to track change over weeks or months rather than a single precise reading.
Why does the calculator ask for hip measurement only in some cases?
The hip measurement feeds into the women’s formula because female body fat distribution and the Navy equation for women both factor in hip circumference alongside waist and neck. Men’s results are calculated from neck, waist, and height, so hip measurement isn’t part of that equation.
What’s the difference between the Navy result and the BMI method estimate?
The Navy method uses your actual body measurements, while the BMI method infers body fat from your height, weight, age, and sex without measuring your shape. The two numbers can diverge noticeably for very lean or very muscular people, since BMI alone can’t tell muscle weight apart from fat weight.
How should I measure my neck and waist for the most consistent results?
Use a flexible tape, keep it snug against the skin without pulling it tight, and take the reading at the end of a normal exhale. Measuring at the same time of day and in the same conditions each time (for example, first thing in the morning) makes repeat results more comparable when you’re tracking progress in your calculation history.
