Estimate your body fat percentage from a few tape measurements using the US Navy method, and see how much of your weight is fat versus lean tissue. Includes the formula, the real accuracy, and what this number does and does not tell you.
A body fat calculator estimates what proportion of your body weight is fat tissue rather than muscle, bone and water. This one uses the US Navy circumference method, which needs only a tape measure and predicts body fat from the relationship between your height and a few key circumferences. Unlike BMI, it can distinguish a muscular build from a heavier one, which is why it is the more informative of the two — though it is still an estimate, typically within 3 to 4 percentage points of a laboratory measurement.
Measure with a flexible tape, keep it level and snug without compressing the skin, and enter the figures below.
These are estimates from circumference equations, not measurements. Typical error against laboratory methods is around 3 to 4 percentage points, and technique matters more than the arithmetic. This is general information, not medical advice.
Measurement technique matters more than the formula. A tape held one inch higher, or pulled tighter, changes the answer more than any difference between equations.
Use a flexible tape measure, not a metal builder's tape. Measure directly against skin or over very thin clothing. Keep the tape level all the way round, snug enough to stay in place but not tight enough to compress the skin. Breathe out normally and do not suck in.
| Measurement | Where | Common Mistake |
|---|---|---|
| Neck | Just below the larynx, tape sloping slightly down at the front | Flaring the neck or measuring too high |
| Waist (men) | Horizontally at the navel | Measuring at the narrowest point instead |
| Waist (women) | At the narrowest part of the torso | Measuring at the navel instead |
| Hips (women) | At the widest point of the buttocks | Tape not level at the back |
First thing in the morning, before eating or drinking, after using the bathroom. Circumferences shift through the day with food, fluid and posture. Consistency is what makes a series of readings comparable, and comparing a morning reading with an evening one can manufacture a change that did not happen.
The method was developed by the Naval Health Research Center in the 1980s as a practical way to assess body composition at scale without laboratory equipment. It works because the ratio between certain circumferences and height correlates reasonably well with body fat across a population.
Notice that weight does not appear in the body fat equation at all — only circumferences and height. This is deliberate, and it is exactly why the method can tell a muscular 90 kg person apart from a sedentary one of the same weight, which BMI cannot.
Step 1: Find the waist-minus-neck value
Step 2: Take the logarithms
Step 3: Build the denominator
Step 4: Finish the equation
Step 5: Convert to fat and lean mass
Estimated body fat 16.4%, which is 12.3 kg of fat and 62.7 kg of lean mass
These bands come from the American Council on Exercise and are widely reproduced. They describe populations, not individuals, and the boundaries are conventions rather than physiological cliffs.
Essential fat is the minimum the body needs for hormone production, organ protection and normal function. Sustained levels at or near that floor carry real health consequences, particularly for women, where very low body fat is associated with menstrual and bone health problems. These bands describe where people fall, not where anyone should aim.
Our BMI calculator page makes the point that BMI cannot see body composition. Body fat percentage is the measure that addresses exactly that gap, and comparing the two on the same person shows why it matters.
| Two men, both 180 cm and 88 kg | BMI | Body Fat (Navy) |
|---|---|---|
| Trains with weights, waist 82 cm, neck 41 cm | 27.2 — "overweight" | About 13% — athletic |
| Sedentary, waist 104 cm, neck 39 cm | 27.2 — "overweight" | About 27% — above average |
Identical BMI, materially different body composition, and only the circumference method distinguishes them. This is the single clearest illustration of why BMI is a population screening tool rather than an individual assessment.
If you only take one measurement, make it waist circumference. It reflects abdominal fat, which is more closely associated with metabolic risk than total body fat is. A frequently cited rule of thumb is to keep your waist under half your height, which needs no formula at all.
Circumference methods sit in the middle of a range of techniques that trade accuracy against cost and accessibility.
| Method | Typical Error | Practicality |
|---|---|---|
| DEXA scan | About 1–2% | Clinical, costly, considered a reference standard |
| Hydrostatic weighing | About 2% | Laboratory only |
| Bod Pod | About 2–3% | Specialist facilities |
| Skinfold calipers | About 3–5% | Cheap, but heavily technique-dependent |
| US Navy tape method | About 3–4% | Free, repeatable at home |
| Bioelectrical impedance scales | About 4–8% | Convenient, sensitive to hydration |
The practical conclusion is that no home method gives you a figure accurate to the decimal place. What they do give you, used consistently, is a reliable sense of direction, which is usually the question people actually care about.
The Navy equation was derived from a military population, which shapes where it performs well and where it drifts.
This calculator is a general reference for adults. It is not diagnostic and cannot account for medical conditions, medications or individual variation. Children and adolescents are assessed with age-and-sex-specific growth references by a clinician, because body composition changes substantially through growth.
A single body fat reading is close to meaningless. A series of readings taken the same way is genuinely useful.
Measure every two to four weeks rather than daily. Shorter intervals mostly capture noise from hydration and digestion rather than real change in tissue. Record the raw circumferences alongside the percentage, because those are the actual measurements; the percentage is a derived number that inherits every error in them.
When you compare readings, look at the trend across three or four sessions rather than the gap between the last two. Body composition changes slowly, and a method with a three to four point error band cannot resolve a one point change in a fortnight.
Body fat percentage says nothing about your blood pressure, blood glucose, lipid profile, sleep, strength or how you feel. Those measure health directly rather than by proxy. If health is the actual question, they are better things to track, and most of them need a clinician rather than a tape measure.
If tracking these figures causes distress, or you find yourself measuring compulsively, stepping away from the numbers is a reasonable and often healthier choice. Weight and body composition figures can be difficult, and a calculator has no way of knowing that. Speaking to a doctor or a registered dietitian will get you considerably further than any formula.
Studies comparing the Navy circumference method against hydrostatic weighing generally find it falls within about 3 to 4 percentage points for most people. That is reasonable for tracking change over time but not precise enough to treat a single reading as exact. Measurement technique is the largest source of error: a tape held at a slightly different height or tension can shift the result by a percentage point or more.
Commonly cited reference ranges put fitness level at roughly 14 to 17 percent for men and 21 to 24 percent for women, with average at about 18 to 24 percent and 25 to 31 percent respectively. Essential fat, the minimum the body requires, is around 2 to 5 percent for men and 10 to 13 percent for women. These are population reference bands, not individual targets, and healthy ranges vary with age, genetics and athletic background.
Women carry more essential fat for physiological reasons including hormone production and reproductive function. Essential fat is roughly 10 to 13 percent in women compared with 2 to 5 percent in men, so every reference band sits correspondingly higher. Comparing a woman's percentage against male ranges is a common and misleading error.
BMI compares weight to height and cannot distinguish muscle from fat, so a muscular person often reads as overweight despite low body fat. Body fat percentage estimates the proportion of your weight that is fat tissue, which is the more informative measure. The Navy method gets at this through circumference measurements rather than weight alone.
Every two to four weeks is enough to see a genuine trend. Measuring more often mostly captures noise: hydration, food in the digestive system and time of day all shift circumferences slightly. Measure under the same conditions each time, ideally in the morning before eating, and compare the trend rather than individual readings.
The tape method is generally more consistent, with typical error around 3 to 4 percent against roughly 4 to 8 percent for bioelectrical impedance scales, which are sensitive to hydration and time of day. Either works for tracking direction if you are consistent. Neither is accurate enough to treat a single reading as a precise measurement.
No. The equations were developed for adults and do not apply to growing bodies. Children and adolescents are assessed with age-and-sex-specific growth references by a paediatrician, because normal development involves substantial variation in body composition over short periods.
No. All calculations happen entirely in your browser. Your measurements stay on your device, nothing is sent to our servers, and no data is stored after you close the page.
This calculator provides estimates for general educational purposes and is not medical advice, diagnosis or treatment. Circumference equations carry a typical error of several percentage points and were developed for adult populations. Speak to a doctor or registered dietitian about your individual health, and seek medical advice before making changes to diet or exercise.