Taking the measurements this calculator needs
1. Use a soft tape, and do not pull it tight
A cloth or fibreglass tailor’s tape, not a metal builder’s one. The tape should sit flat against the skin and be snug enough not to slide, but not tight enough to compress anything. Compressing the tissue is the most common way people accidentally take two centimetres off their own waist, and two centimetres is worth roughly one and a half percentage points of body fat in this method.
2. Measure your neck below the larynx
Just below the Adam’s apple, with the tape sloping very slightly downward at the front. Keep your shoulders down and do not flex. The neck is the smallest of the three numbers and therefore the one where a one-centimetre slip does the most proportional damage.
3. Measure your waist at the navel
For men, at the level of the navel. For women, at the narrowest point of the torso, which is usually a little higher. Measure at the end of a normal exhale — not sucked in, not pushed out. Standing relaxed with arms at your sides is the position the equations were validated in.
4. Add hips only if you want the extra reading
The women’s equation requires a hip measurement; the men’s does not use one. It is offered to either sex now as an optional extra, because a waist-to-hip ratio is a useful reading in its own right. Leave it blank and nothing derived from it will appear — the tool will not invent a value, which is exactly what the previous version did.
5. Measure twice, and take the second one
Almost everyone’s first measurement of the day is slightly off while they work out how the tape sits. If the two readings differ by more than a centimetre, take a third. Consistency of technique matters far more here than precision on any single occasion, because what you will actually use this for is the change over weeks.
6. Read both estimates, and mind the gap
The large number is the U.S. Navy estimate. Beside it you will see the YMCA figure. When the two agree closely, you can have reasonable confidence in the range. When they disagree by several points, that is information — it usually means your build sits away from the average the equations were fitted to, and the honest answer is a wider range rather than a single confident number.
Why the big number is the Navy estimate and not an average
The previous version of this page computed (Navy + YMCA) / 2 and presented the result as the answer, then assigned your fitness category from it. That is not a defensible thing to do with two prediction equations. They were fitted to different populations, they take different inputs, and they have different error characteristics. Nothing in the literature endorses averaging them, and the composite has no published accuracy at all — you cannot cite an error margin for a number nobody has ever validated.
The headline is now the U.S. Navy circumference method, which is the one with a documented derivation and published validation, developed at the Naval Health Research Center by Hodgdon and Beckett in 1984 for body-composition assessment across a very large service population. For men it uses neck, waist and height; for women, neck, waist, hip and height.
The YMCA equation is still shown, but as what it is: a second opinion. The distance between the two estimates is a genuinely useful piece of information that averaging throws away. Two methods that agree within a point or so are telling you the estimate is probably sound. Two methods four points apart are telling you your proportions differ from the average build the equations assume, and that no single figure deserves your confidence.
The arithmetic, written out
For men, body fat percentage = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450.
For women, body fat percentage = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450, with all measurements in centimetres.
Two things are worth noticing. The first is that the equation predicts body density and then converts it to a fat percentage using the Siri equation, which is where the 495 and the 450 come from — the method is a chain of two estimates, not one. The second is that for men the entire abdominal term is waist minus neck. That single difference is carrying most of the prediction, which is why an error on either measurement moves the answer so much, and why the tool now asks you to check your tape rather than quietly producing a plausible-looking figure.
Reading your category without over-reading it
The bands shown are the ones published by the American Council on Exercise, and they are sex-specific because women carry substantially more essential fat — the fat required for normal physiological function, including reproductive tissue — than men do.
- Essential fat — men 2 to 5%, women 10 to 13%. This is the floor for survival, not a target. Sustained levels here are associated with hormonal disruption, loss of menstruation, bone loss and immune impairment.
- Athletes — men 6 to 13%, women 14 to 20%. Typical of people training seriously and eating to support it.
- Fitness — men 14 to 17%, women 21 to 24%. Lean and unremarkable to a doctor.
- Acceptable — men 18 to 24%, women 25 to 31%. The range most healthy adults live in.
- Obese — men 25% and above, women 32% and above.
These are categories, not cliffs. The difference between 24% and 25% for a man is one label and essentially no biology. What actually matters is where you are heading over months, and the band boundaries are useful mostly as landmarks for noticing that.
Waist-to-hip ratio, and why it is now optional
If you enter a hip measurement, the tool also shows your waist-to-hip ratio. This is a separate reading from body fat percentage and it answers a different question: not how much fat you carry, but where. Fat stored around the abdomen sits alongside the internal organs and is far more strongly associated with type 2 diabetes and cardiovascular disease than the same amount of fat carried around the hips and thighs.
The World Health Organization’s expert consultation on waist circumference and waist-hip ratio, held in Geneva in December 2008, set the widely used thresholds for substantially increased risk at above 0.90 for men and above 0.85 for women. A raised ratio in someone with an otherwise ordinary body fat percentage is a meaningful finding, which is exactly why it should never be manufactured.
The previous build hid the hip input from men and then read it anyway, where the markup carried a default of 98 cm. Every male visitor was therefore shown a ratio, a body-shape description and a sentence about metabolic risk, all calculated from a number he had never entered and could not see. That is the kind of defect that is invisible precisely because the output looks reasonable. Hip is now blank by default for everyone, and if you leave it blank the ratio, the shape and the risk sentence simply do not appear.
How close is a tape measure to the truth?
Closer than you might expect, and further than the decimal place suggests. Against dual-energy X-ray absorptiometry, the reference method in most research settings, circumference equations of this type typically land within about three to four percentage points for most people, with larger errors at the extremes of build.
The ladder of methods runs roughly like this. A four-compartment model, which combines several measurements to account for water, protein, mineral and fat separately, is the research gold standard. DEXA, hydrostatic (underwater) weighing and air-displacement plethysmography — the Bod Pod — sit close behind. Skinfold calipers in trained hands come next; the generalized equations behind them were published by Jackson and Pollock in the British Journal of Nutrition in November 1978, still the foundation of caliper work almost fifty years later. Circumference methods like this one sit below that. Bioelectrical impedance scales, the kind sold for home use, sit lower again and are notoriously sensitive to how hydrated you are when you step on them.
None of this makes a tape measure useless. It makes it a good instrument for tracking change and a poor one for a single authoritative number. If your estimate reads 22%, the honest interpretation is “probably somewhere between 18 and 26, and I will know much more after eight weeks of measuring the same way.”
When the tool refuses to give you a number
The previous version clamped any implausible result into the range 2 to 60% and printed it. A mistyped neck of 3 cm produced a wrong but entirely believable answer, and nothing on the page suggested anything had gone wrong.
Now, when the inputs produce a result outside the plausible range, the tool says so and names the measurement most likely responsible — usually the neck, because it is the smallest number and the one people most often measure in the wrong place. A calculator that says “check this measurement” is more useful than one that silently rounds a mistake into the normal range.
Where this measurement is genuinely misleading
Circumference equations assume an average relationship between the girth of certain body parts and total fat. Several groups sit away from that average.
Very muscular people often have thick necks, which in the men’s equation shrinks the waist-minus-neck term and pushes the estimate down — sometimes flatteringly so. People who are pregnant cannot use this method at all; abdominal circumference is measuring something else entirely. People with significant abdominal bloating, ascites or a hernia will see the waist term inflated for reasons unrelated to fat. Older adults tend to have their fat redistributed toward the abdomen with age, which this method reads as an increase even where total fat has not changed much — though in that case the reading is arguably telling you something true. Body builders during a cut and people who have lost a great deal of weight often sit outside the fitted range in both directions.
What this page is not
This is an estimate from three or four tape measurements, and estimates are not diagnoses. Body fat percentage on its own does not establish that you are healthy or unhealthy; it is one number among blood pressure, resting heart rate, blood lipids, glucose, sleep, fitness and family history. Nothing on this page is medical advice, and it is not a substitute for an assessment by a doctor or a registered dietitian — particularly if you are considering a substantial change to how you eat or train.
If your reading is very low and you have not been training deliberately toward it, or if it has moved a long way without any change on your part, that is worth mentioning to a doctor rather than acting on here.
Your measurements stay on your device
Everything is calculated in your browser. Neck, waist, hip, height and weight are never transmitted, never stored and never logged. There is no account and no upload. Once the page has loaded, it works with the connection off.
Questions people ask about measuring body fat
What is a good body fat percentage?
For men, roughly 14 to 24% covers fitness through acceptable; for women, roughly 21 to 31%. Athletes sit lower by design and by training load. “Good” depends heavily on what you are training for, and chasing the essential-fat floor is a health risk rather than an achievement.
Why is the women’s number so much higher than the men’s?
Because essential fat is genuinely higher in women — roughly 10 to 13% against 2 to 5% — owing to reproductive and hormonal tissue. A woman at 22% and a man at 14% are in comparable condition. Comparing the raw percentages between sexes is a category error.
How is this different from my bathroom scale’s reading?
Those scales use bioelectrical impedance: a small current, with fat and lean tissue conducting differently. It is quick, and it is strongly affected by hydration, recent food, recent exercise and even foot moisture. Readings can swing several points across a single day. A tape measure is less convenient and more stable.
Can I use this to track fat loss week by week?
Track it, yes; read too much into any single week, no. Measure at the same time of day, in the same conditions, ideally the same day of the week. Look at the trend over eight to twelve weeks rather than the difference between Tuesday and Friday.
My BMI says overweight but this says my fat is fine. Which is right?
Both may be. BMI cannot distinguish muscle from fat, and a heavier, more muscular build reads high on it. That is the classic case where a circumference estimate is the more informative of the two. Our BMI calculator is explicit about the same limitation.
Why does the tool ask for my height if it is measuring my waist?
Because the Navy equation predicts body density from the ratio of girth to stature, not from girth alone. The same 90 cm waist means something very different on someone 155 cm tall and someone 190 cm tall.
Should the two estimates always agree?
No, and it would be suspicious if they did. They are different equations fitted to different data. A gap of a point or two is ordinary. A gap of four or more suggests your proportions sit away from the average build both assume, and that you should treat the result as a range.
Is a low body fat percentage always good?
No. Below the essential range, both sexes see hormonal disruption, reduced bone density and impaired immune function; in women, loss of menstruation is a common early sign and a reason to seek medical advice rather than to continue.
Do you store my measurements?
No. Nothing is saved or transmitted. The calculation happens in your browser and disappears when you close the tab.
Sources behind these numbers
- Hodgdon JA and Beckett MB, Naval Health Research Center, 1984 — the derivation of the circumference equations used for the headline estimate.
- Jackson AS, Pollock ML, “Generalized equations for predicting body density of men”, British Journal of Nutrition, November 1978 (PMID 718832) — the basis of skinfold assessment, cited here for the comparison of methods.
- WHO expert consultation on waist circumference and waist-hip ratio, Geneva, December 2008 — the 0.90 and 0.85 ratio thresholds.
- WHO — obesity and overweight fact sheet, updated 8 December 2025.
- NHLBI, National Institutes of Health — blood cholesterol, on why fat distribution matters more than total mass.
- CDC — Adult BMI Calculator, for the screening measure this one is usually read alongside.