An estimate of your body fat percentage from three tape measurements, using the US Navy circumference method. Expect it within three to four points of a lab reading.
Estimated body fat
%
Enter your measurements and this fills in.
Athletic below 13
Fit 13 to 17
Average 17 to 24
Obese 24 to 35
Method
not yet
Typically the method carries a standard error of roughly 3 to 4 percentage points
Taking the measurement
1
Measure your neck
Put the tape just below your larynx, sloping very slightly downward toward the front. Keep your shoulders down and do not flex.
2
Measure your waist
Put the tape around your navel. Stand relaxed, breathe out normally, and read it without bracing or pulling in.
3
Measure your height
Stand against a wall without shoes, heels down, and mark the top of your head.
4
Enter all three
The calculator predicts body density from the waist-minus-neck span and your height, then converts that density to a percentage.
This estimates body fat from your height, neck and waist using the US Navy circumference method, a procedure published in 1984 and calibrated against underwater weighing. It needs a tape measure and nothing else. Expect it to land within about three to four percentage points of a lab measurement.
How this is worked out
Height
Without shoes. It anchors the equation, so an error here shifts the whole result rather than one term of it.
Neck circumference
Just below the larynx, with the tape sloping slightly down at the front. This is the equation's proxy for frame rather than fat, which is why a neck measured too high reads as a leaner result than you are.
Waist circumference
At the navel, relaxed, at the end of a normal breath out. The waist term does most of the work in the formula, so this is the measurement worth taking twice.
The equation itself
Density is predicted from the waist-minus-neck span and height, then converted to a percentage by the Siri equation. Both steps are published. Neither is ours, and neither is a black box.
What each measurement is
Reference bands are this tool's own, chosen for the landmarks it uses.
Where research reports a figure it is given separately, with the source,
because the two are different claims. And each one says what actually
moves it, which for most facial measurements is the more useful half.
Estimated body fat %
Formula
density from log(waist − neck) and log(height), converted by the Siri equation
Reference
the method carries a standard error of roughly 3 to 4 percentage points
Research
the American Council on Exercise classifies adult men as 6-13% athletic, 14-17% fit, 18-24% average and 25%+ obese. Those are fitness categories, not attractiveness findings
Better for tracking your own change across months than for settling an absolute figure.
What moves it The ACE fit band, 14 to 17%, is where most men see the facial change they came for, and the 12 to 15% figure quoted for visible definition comes from bodybuilding practice rather than a study - useful, and labelled as what it is. The lever is a calorie deficit held long enough to matter; nothing else moves this. These are adult figures: if you are still growing, this is a conversation for a doctor rather than a calculator.
Method
Formula
US Navy circumference, Hodgdon & Beckett (1984)
This estimates your body fat percentage from three tape measurements using the
US Navy circumference method. It’s free, it runs in your browser, and it tells
you its margin of error, which is the part the rest of the results page for this
query tends to leave out.
Where the method comes from
Hodgdon and Beckett published it at the Naval Health Research Center in 1984.
They fitted a regression against hydrostatic weighing, which was the reference
standard at the time, and the resulting equation is still what the US Navy uses
to screen personnel.
For men it works in two steps. Body density is predicted from the span between
your waist and neck measurements and from your height. That density is then
converted into a percentage by the Siri equation.
Both steps are published. Neither is ours. You could work the same numbers by
hand with a calculator and get the same answer, which is the point.
Men’s formula only. The women’s version takes a hip measurement and a
different set of constants. Running the male equation for everyone would return
a number that looks plausible and is wrong, and quiet wrongness is worse than
no number.
How accurate it is
The honest figure is a standard error of roughly three to four percentage
points against laboratory methods.
That shapes what the number is good for. Tracking your own direction across
months, it works well, because the error is fairly consistent for the same
person measured the same way. Settling whether you’re at 11% or 14% today, it
does not work, and no tape-based method does.
You get something out of that limitation. Because the error is consistent,
the change is more reliable than the absolute figure. Someone who measures 18%
in January and 14% in May has almost certainly lost real fat, even if neither
figure is exactly right.
Taking the measurements
Neck. Just below the larynx, tape sloping slightly downward at the front.
Shoulders relaxed. This is the equation’s proxy for frame rather than fat, so a
neck measured too high on the throat reads leaner than you are.
Waist. Around the navel, standing normally, at the end of a normal breath
out. Not braced, not pulled in. The waist term does most of the work in the
formula, which makes it the one worth taking twice.
Height. Without shoes, heels against a wall.
Same time, same way, every time. Waist circumference moves a couple of
centimeters across a day with food and water. First thing in the morning removes
most of that variation.
What this is, and what it is not
Body composition is a fitness measurement. It is not an attractiveness finding,
and this is a distinction worth holding onto, because the looksmaxxing category
collapses the two constantly.
The research on appearance measures things like torso shape and facial
proportion. It does not establish a body fat figure that people find attractive,
and any page telling you to reach one is importing a number from a different
field and hoping you won’t check. We won’t publish a target here, and it isn’t
squeamishness: there isn’t one to publish.
What is fair to say is that facial fat falls as total fat falls, and that this
is one of the larger levers most men have. How far to take it is a question for
you and, past a point, for a doctor.
Where to take this next
The shoulder-to-waist ratio uses the same waist
measurement you just took, and torso shape has better evidence behind it than
any composition figure does. If you want the number a doctor
will already have, body mass index takes two measurements and
explains its own limits.
If you’re deciding what to do with the number, the community’s body composition
threads are worth more than another calculator.
It carries a standard error of roughly three to four percentage points against laboratory methods. That makes it good for tracking your own change over months and poor for settling an argument about whether you are at 11% or 14%.
What is the US Navy body fat formula?
For men it predicts body density from the log of your waist minus neck measurement and the log of your height, then converts density to a percentage using the Siri equation. Hodgdon and Beckett published it in 1984 after calibrating it against hydrostatic weighing.
Why does it only need my waist and neck?
The waist term tracks abdominal fat and the neck term acts as a proxy for frame size, so the span between them carries most of the signal. It is a regression fitted to real bodies rather than a description of where fat sits.
Does this work for women?
Not as built. The female version of the equation adds a hip measurement and uses a different set of constants, and running the male formula for everyone would return a number that is quietly wrong. Only the men's formula is implemented here, and the page says so rather than hiding it.
Is this more accurate than calipers?
Different rather than better. Calipers can be more accurate with a practiced operator and are worse with an unpracticed one, which is most people measuring themselves. The tape method has no skill component, which is why it repeats well.
Is this more accurate than a smart scale?
Bioelectrical impedance scales swing with how hydrated you are, so they can move several points between morning and evening without anything changing. The tape method holds steadier day to day.
What body fat percentage is healthy?
The American Council on Exercise classifies adult men as 6 to 13% athletic, 14 to 17% fit, 18 to 24% average, and 25% or above obese. Those are fitness categories rather than attractiveness findings, and where you should sit within them is a question for a doctor who can see the rest of your picture.
At what body fat does facial definition show?
Figures around 12 to 15% circulate widely and they come from bodybuilding practice rather than from a study, so treat them as a rule of thumb from people who do this rather than as a finding. The mechanism is solid: facial fat falls as total fat falls, and it is one of the larger levers most men have.
Why does my result change so much day to day?
Waist circumference varies by a couple of centimeters across a day depending on food and water, and the waist term drives the equation. Measure first thing in the morning, the same way each time, and compare months rather than days.
Should I measure my waist at the navel or the narrowest point?
The navel, for this equation, because that is where it was calibrated. The narrowest point is a landmark that migrates as your composition changes, which makes one month impossible to compare with the next.
Is body fat percentage an attractiveness measurement?
No, and it is worth being exact about this. Body composition is a fitness measurement. Importing it into appearance research and presenting it as one is a category error, and it is the first thing a critic would find.
What is the Siri equation?
It converts body density into a fat percentage, as 495 divided by density minus 450. It is the second half of this calculation and it is a published standard rather than anything specific to the Navy method.