Calculator · Free, no signup
Body
fat calculator
Three measurements and a tape. We show the equation, your fat and lean mass, and — more usefully — how much error to expect from it.
What this measures, and what it cannot
The Navy method infers body fat from the relationship between your waist, your neck and your height. The logic is that most people store fat viscerally and subcutaneously around the middle, while the neck stays relatively stable — so the gap between the two tracks fatness reasonably well across a population.
It is a population model applied to an individual, which is where the error comes from. If you carry fat on your legs rather than your waist it will under-read. If you have a thick midsection from heavy squatting and deadlifting rather than from fat, it will over-read. Neither is a flaw in the equation; it simply cannot see what it does not measure.
Measurement error is bigger than you think
A single centimetre on the waist tape moves the result by more than a percentage point. That is larger than most of the changes people are trying to detect month to month.
Three things reduce it. Measure at the same time of day, ideally first thing before eating. Keep the tape snug without compressing the skin — pulling tight is the most common way to flatter yourself. And take three readings and average them rather than trusting one.
Use the trend, not the number
The absolute figure has a few points of error attached. The direction of travel, measured the same way over six to eight weeks, has far less — the error is largely consistent for a given person and tape technique, so it cancels out when you compare readings.
That makes this a good tool for answering "am I losing fat?" and a poor one for answering "am I exactly 14 percent?".
What to do with the result
Lean mass is the more useful output for planning. Feed it into theTDEE calculator — entering a body fat percentage there switches it to the Katch-McArdle equation, which works from lean mass and is more accurate for lean or muscular people than the default.
If your body fat is above roughly 30 percent, theprotein calculator notes that total body weight overestimates protein needs; lean mass is the better basis there too.
Common questions
How accurate is the Navy body fat calculator?
Within roughly 3 to 4 percentage points of a DEXA scan for most people. That is worse than a scan and considerably better than guessing from a photo. Its weakness is that it only sees girth, so it under-reads on people who carry fat on their limbs and over-reads on people with a thick waist from muscle.
How do I measure my waist and neck correctly?
Waist at the navel for men and at the narrowest point for women, with the tape snug but not compressing the skin. Neck just below the larynx, sloping slightly downward at the front. Measure at the same time of day, before eating, and take the average of three readings — measurement error is the biggest source of noise here.
What is a healthy body fat percentage?
For men, roughly 10 to 20 percent covers athletic through fitness ranges, with essential fat around 3 to 5 percent. For women the whole scale sits higher because of essential fat requirements: about 18 to 28 percent, with essential fat around 10 to 13 percent. Below those essential floors is not a target, it is a medical concern.
Is the Navy method the same as the Army one?
They are close cousins using the same circumference approach, but the services use slightly different tables and standards. This calculator uses the Navy equations, which are the ones most widely published and most often reproduced online.
Why did my body fat go up when I did not gain weight?
Almost always measurement noise or bloating rather than a real change. A centimetre on the waist tape moves the result by a percentage point or more. Track the trend over a month rather than reacting to single readings.
Should I use body fat percentage or the scale?
Both, and neither on its own. Scale weight moves with water and gut contents; body fat estimates move with measurement error. Together with a waist measurement and photos in the same light, they give a picture no single number does.