About the Body Fat Calculator
Body fat percentage tells you something BMI cannot: how much of your weight is actually fat.
This body fat calculator uses the US Navy circumference method — waist, neck, height, and hips for women. It works because abdominal circumference tracks body fat reasonably well, and it responds to where weight is carried rather than only how much there is.
It also does something most versions leave out. It tells you how much a measurement error is worth, which on this formula turns out to be the most important thing to know.
How to Use the Body Fat Calculator
Measure waist at the navel, relaxed, breathing out normally, tape snug but not compressing.
Measure neck just below the larynx, with the tape sloping slightly downward at the front.
Women also measure hips at the widest point.
Weight and age are optional — weight splits the result into fat and lean mass, age enables the comparison against the BMI-based estimate.
Step-by-Step Example
Male, 180cm, 88cm waist, 38cm neck, 80kg.
Girth: 88 − 38 = 50 cm
495 ÷ (1.0324 − 0.19077·log₁₀(50) + 0.15456·log₁₀(180)) − 450 = 18.4%18.4% body fat — average — which on 80kg is 14.7kg of fat and 65.3kg of lean mass.
The Measurement Error Is Bigger Than Your Progress
Here is the part that matters most, and it follows from the formula being a logarithmic regression: the girth term dominates everything.
One centimetre of waist error moves the answer by 0.45 to 0.85 percentage points, depending on build.
And it bites hardest where it hurts most:
| Waist | One centimetre is worth |
|---|---|
| 80cm | 0.84 points |
| 88cm | 0.73 points |
| 100cm | 0.61 points |
| 115cm | 0.51 points |
The leaner you are, the more a centimetre matters — and lean people are precisely the ones tracking small changes closely.
Now consider what a centimetre actually is. Tape tension. Tape position. Whether you measured before or after breakfast. Whether you were fully relaxed. Any of those can move the waist by a centimetre, which means any of them can manufacture a three-quarter-point "change" that is entirely noise.
Most people do not lose three quarters of a point of body fat in a fortnight.
The practical answer: measure the same spot, the same way, at the same time of day, relaxed and unfasted, and judge by the trend over months rather than the reading this morning. The calculator computes this sensitivity for your own figures so the error bar is a number rather than a vague warning.
Why It Disagrees With the BMI-Based Estimate
The calculator also runs the Deurenberg equation, which works from BMI, age and sex.
The two often differ by several points, and the direction tells you something:
If you are muscular, the BMI-based figure reads high. It cannot tell muscle from fat, because BMI is the only body information going into it.
If you carry weight centrally, the Navy figure reads higher. It is picking up abdominal circumference, which is exactly what it is for.
Neither is measuring you. But when they disagree sharply, your build differs from average in a way one of them cannot see, and knowing which one is useful.
The Floor Is Real
Essential fat — structural fat in organs, bone marrow and the central nervous system — is around 3% for men and 12% for women.
That is not a target. It is the level below which the body stops working properly, with consequences for hormones, bone density and immune function.
So if this calculator returns a figure below it, the calculator says so rather than reporting it as an achievement. In practice, such a result almost always means a measurement is wrong — a unit mix-up, or a tape pulled tight — rather than a genuinely extraordinary body. It is worth checking the inputs before anything else.
What This Is Not
A measurement.
A DEXA scan or hydrostatic weighing measures body composition. This estimates it from a regression fitted to a population, with an error of several percentage points either way.
That makes it a poor tool for knowing your exact level and a reasonable one for tracking direction, provided you measure consistently. Those are different jobs, and the second is usually the one people actually need.
Understanding Your Result
Body fat gives the percentage and a descriptive band.
Fat and lean mass splits your weight, which is often the more useful form — "65kg of lean mass" is something you can track through a diet.
How much measurement error matters is the sensitivity figure for your own build.
Against the BMI-based estimate shows the second method and the gap.
Worth knowing flags an implausible figure, a near-floor reading, or a large disagreement.
When Should You Use This Calculator?
To track direction over months. Its real strength.
Alongside BMI. Body fat answers the question BMI cannot.
Before and after a training block. Weight can stay flat while composition changes.
Not for a precise level. Use a scan if the exact number matters.
Common Mistakes
Measuring inconsistently. A centimetre is worth more than a fortnight of progress.
Pulling the tape tight. Snug, not compressing — and the same every time.
Measuring after a large meal. Waist circumference moves.
Treating the figure as exact. It carries several points of error.
Chasing a very low figure. Essential fat is a floor, not a goal.
Comparing a Navy figure to a scan figure. Different methods, different answers, no contradiction.
This is an estimate from a population formula, not a measurement, and it can be well out for any individual. Nothing here is medical advice — discuss any concern about your body composition or weight with a doctor.