Four measurements and a number, with the two things most calculators leave out: how far the estimate can be off, and where the floor is.
Measure relaxed, after breathing out, with the tape snug but not tight. Your number appears instantly.
The equation behind the number
This is the US Navy circumference method, developed by Hodgdon and Beckett at the Naval Health Research Center in 1984 and still used for body composition screening across the US services. It fits body density from the logarithm of a few tape measurements, then converts density to a fat percentage using the Siri equation. For men it uses waist, neck and height. For women it adds the hips, because the same waist measurement means something different when fat is distributed differently.
What a tape cannot do. It estimates. Against underwater weighing the method carries roughly three to four percentage points of error either way in most validation work, and more at the extremes: it tends to overestimate in very lean people and underestimate in very muscular ones, because a thick neck reads as a thinner body in the formula. Measure at the same place each time, breathe out normally rather than sucking in, and read the change across months rather than the single figure.
If you want one number for health rather than for composition, waist divided by height is the better one and needs two measurements instead of four. Check waist to height.
Every figure on this site names where it came from. How we check.
Where the number comes from
This uses the circumference equations developed for the US Navy, which estimate body fat from height, neck and waist, plus hips for women. They were chosen here because a tape measure is something most people already own and the result is close enough to a scan to follow a trend. A DEXA scan is the reference standard and will disagree with this by a few points in either direction.
What the number cannot do is tell you whether you are healthy. Blood pressure, HbA1c, cholesterol and how much you can carry up a flight of stairs all describe your health better than a body composition percentage does.
Common questions
Add your height in your profile and this sharpens: the estimate is built on height.
The commonly used ranges from the American Council on Exercise put men at roughly 14 to 17 percent for fitness and 18 to 24 percent for average, and women at roughly 21 to 24 percent for fitness and 25 to 31 percent for average. Below those sit the athletic and essential ranges. These describe populations rather than targets, and health outcomes are good across a wide span, so being in the average range is not a problem to fix.
What is the lowest safe body fat percentage?
Essential fat is about 2 to 5 percent in men and about 10 to 13 percent in women, and that is a floor rather than a goal. It is the fat the body uses for hormones, nerve tissue and the padding around organs. Living at or under it is associated with hormonal disruption, bone loss, poor immunity and, in women, periods becoming irregular or stopping. Lower is not better on this measurement.
How accurate is a tape measure body fat calculator?
The circumference equations used here typically land within three to four percentage points of a DEXA scan for most adults, which is good enough to track a trend and not good enough to argue about a decimal. They are least accurate at the extremes, in very lean and very heavy people. Consistency matters more than precision: measure the same way each time and watch the direction.
Why do bathroom scales give a different body fat number?
Scales that report body fat send a small current through the body and infer fat from the resistance, which depends heavily on how hydrated you are. The same person can read several points apart morning and evening, or before and after a drink. They are reasonable for tracking a trend at a fixed time of day and poor for a one-off number.
How fast can I reduce body fat safely?
Around 0.5 to 1 percent of body weight a week is the range where most of what you lose is fat. Faster than that and a growing share of the loss is muscle, which is self-defeating: losing muscle lowers your weight while raising your body fat percentage. Adequate protein, roughly 1.6 grams per kilogram of body weight a day, and two resistance sessions a week are what protect the lean side while a deficit does its work.
Is body fat percentage better than BMI?
It answers a different question. BMI compares weight to height and cannot tell muscle from fat, which is why athletes score badly on it. Body fat percentage addresses that, but the estimate carries real error. For predicting heart and metabolic risk, waist-to-height ratio beats both and costs nothing: a waist under half your height is the simple version.
Does body fat percentage change with age?
Yes. Body fat tends to rise gradually through adult life while muscle falls, even when weight is stable, and the ranges considered normal shift a few points upward in older adults. After about 60 the more useful target is holding on to muscle rather than lowering fat, because rapid weight loss at that age takes muscle first and costs more function than the fat was worth.