Body Fat Calculator — U.S. Navy Tape Method
Measure your neck and waist with a tape, type the numbers in, and this calculator returns your body fat percentage using the U.S. Navy circumference formula — the same method the Navy uses for its own fitness assessments. It also prints the Deurenberg BMI-based estimate side by side, so you can see how far apart the two methods land, along with your fat mass, lean mass, and the Jackson & Pollock reference range for your age. Nothing is uploaded; the arithmetic happens in your browser.
Body fat categories
These bands come from the American Council on Exercise. They're descriptive, not prescriptive — plenty of healthy people sit in the "average" column and stay there for decades.
| Category | Men | Women | What it means |
|---|---|---|---|
| Essential fat | 2–5% | 10–13% | The minimum the body needs to run. Not a target. |
| Athletic | 6–13% | 14–20% | Typical of competitive athletes in lean sports. |
| Fitness | 14–17% | 21–24% | Visibly fit, sustainable year-round. |
| Average | 18–24% | 25–31% | Where most adults who exercise occasionally land. |
| Obese | 25%+ | 32%+ | Associated with higher cardiometabolic risk. |
Where to put the tape
Measurement error dominates this method. A centimetre off at the waist shifts the result by about half a percentage point, and the neck is even more sensitive — so the habits in the right-hand column matter more than the formula does.
| Site | Where | Common mistake |
|---|---|---|
| Neck | Just below the larynx, tape sloping slightly down at the front | Don't flex or tuck the chin — both shrink the reading and inflate your result |
| Waist (men) | Horizontally at the navel | Measure at the end of a normal exhale, not while holding your stomach in |
| Waist (women) | At the narrowest point of the torso, usually an inch above the navel | If there's no obvious narrow point, use the midpoint between ribs and hip bone |
| Hips (women) | Horizontally at the widest part of the buttocks | Feet together — a wide stance adds a centimetre or two |
A worked example
Take a 35-year-old man, 180 cm and 85 kg, neck 39 cm, waist 94 cm at the navel. The Navy formula subtracts neck from waist to get 55 cm of girth, then scales it by height:
495 / (1.0324 − 0.19077 × log10(55) + 0.15456 × log10(180)) − 450 = 21.9%That puts him in the "average" band, with 18.6 kg of fat and 66.4 kg of lean mass. His BMI is 26.2, which on its own reads as overweight, and the Deurenberg BMI formula estimates 23.3% — about a point and a half higher than the tape. The age-35 reference is 13.7%, so reaching it would mean losing roughly 8 kg of fat while keeping the lean mass he has. Our BMI Calculator and Calorie Calculator cover the other two numbers in that sentence.
How it works
- 1 Pick units and enter your basics Switch between metric and imperial, then enter your sex, age, height, and weight. Results appear as you type — there's no calculate button.
- 2 Measure your neck and waist Use a flexible tape, pulled snug but not tight enough to compress the skin. Men need neck and waist at the navel; women also need hip girth at the widest point.
- 3 Read the two estimates The headline number is the Navy tape result. Below it sits the BMI-based estimate — if they disagree by more than a few points, the tape figure is usually the one to trust.
- 4 Compare against your age reference The calculator shows the Jackson & Pollock reference percentage for your age and how many kilograms of fat separate you from it, holding lean mass constant.
Your data stays private
All processing happens entirely in your browser. No files, text, or data are ever sent to our servers. You can disconnect from the internet and this tool will still work.
Frequently asked questions
- How accurate is the U.S. Navy body fat formula?
- Validation studies put the standard error at roughly 3–4 percentage points against hydrostatic weighing, so a reading of 20% means 'somewhere between 16% and 24%'. That's worse than a DEXA scan and considerably better than eyeballing a mirror. Its real strength is repeatability: measure the same way each month and the trend is reliable even if the absolute number is off.
- Why does the tape method need my neck measurement?
- Neck girth works as a proxy for frame size and lean tissue, while the waist tracks fat storage. The formula reads the ratio between the two. That's why someone with a thick neck from training gets a lower result at the same waist — and why tucking your chin while measuring the neck will quietly inflate your body fat by two or three points.
- Which is better, the tape method or the BMI method?
- The tape method, in almost every case. The Deurenberg BMI formula knows only your height, weight, age, and sex — it cannot tell muscle from fat, so it overstates a lifter and understates someone sedentary at the same BMI. We show it because the comparison is informative: a large gap between the two numbers usually means your body composition is unusual for your BMI in one direction or the other.
- What's a healthy body fat percentage?
- The American Council on Exercise puts the fitness range at 14–17% for men and 21–24% for women, with 18–24% and 25–31% described as average. Below roughly 5% for men and 12% for women you're into essential fat, which the body needs for hormone production and organ padding — in women, sustained very low body fat commonly disrupts the menstrual cycle.
- Why do women's numbers run higher than men's?
- Not a measurement artefact — it's physiology. Women carry more essential fat, mainly in the breasts, hips, and thighs, tied to reproductive function. A woman at 22% body fat is roughly as lean as a man at 15%, which is why the formula uses separate equations and the category table has separate columns.
- How often should I re-measure?
- Every two to four weeks. Body fat moves slowly, and day-to-day tape readings bounce around by a point or so from hydration, bloating, and where exactly you put the tape. Measure first thing in the morning before eating, and write the numbers down — the series tells you more than any single reading.
- Can I use this if I'm pregnant or under 18?
- No. The Navy formula was fitted on non-pregnant adults, and body composition in children and teenagers follows age- and sex-specific growth curves that this equation doesn't model. For children, a paediatrician's percentile charts are the right tool. During pregnancy, body-fat estimates aren't clinically meaningful.
- Is my data sent anywhere?
- No. The calculator is plain JavaScript running in your browser — your measurements never leave the page, aren't logged, and aren't stored. Close the tab and they're gone. There's no account and no upload step.