FitDexBody fat calculator
The scale tells you your weight but not how much of it is fat and how much is muscle and water. Body fat percentage tries to make that split — which is exactly why how it's estimated matters as much as how well.
Body fat percentage
Lean mass: 48.6 kg
- Body mass index24.2 kg/m²
- Your healthy weight range53.5 – 72.0 kg
This number comes from the very same formula the FitDex app uses. Install the app and you won't be handed a different target.
How is it calculated?
The equation is Deurenberg: body fat (%) = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for men and 0 for women. For a 30-year-old woman at 170 cm and 70 kg, BMI comes to 24.2 and the equation gives about 30.5%. No tape. Four inputs.
Lean mass follows from it: weight × (100 − body fat) ÷ 100. In the same example, 70 × 0.695 ≈ 48.7 kilos. That figure is muscle, bone, organs and water combined — precisely what you're trying to hold onto while losing weight.
Why we don't ask for a tape measure: the most common alternative is the US Navy equation, which needs waist and neck (plus hips for women). It's a good equation when the measurement is taken properly, but for someone measuring alone at home the tape's placement and tension easily shift by a centimetre or two. A 2 cm error at the waist moves the result by roughly 1.5 points — giving back the accuracy it was supposed to add. Rather than ask for an input that doesn't repeat well, we stayed with the ones that do.
The known limit of the Deurenberg equation is this: it takes BMI as an input, so it inherits BMI's blind spot. It overestimates body fat in people with high muscle mass and underestimates it in people with very little. We say so rather than hide it; an estimate with a known limit is more useful than one without.
How to read your result
The reference bands differ by sex, and the difference is biological: essential fat is around 10–13% in women and 2–5% in men. Roughly 21–32% for women and 8–20% for men is generally considered healthy; athletes sit below that band, while above 32% for women and 25% for men is the raised-risk zone.
Very low body fat is not a target. Below about 15% in women is associated with menstrual disruption and bone density loss; below about 6% in men with hormonal decline and poor recovery, which is why the figures you see in photographs are almost always temporary states squeezed into the week of a contest. Nobody lives there.
Trust the change measured by the same equation, not the absolute value. The equation may carry a consistent bias, but that bias stays put over time — so an estimate falling from 30.5% to 27.8% over three months shows the DROP correctly even if it's two points off the true figure. One reading is uncertain; the series is reliable.
Watch the lean mass figure too. If body fat is falling while lean mass holds steady, you're doing it right. If lean mass is falling as well, the deficit is too harsh or protein is too low, and both causes can be corrected with the calorie and macro tools on this site without making the plate any bigger. Only the split changes.
Frequently asked questions
Why don't you ask for waist and neck measurements?
Because those measurements don't repeat well at home. The tape's height, its tension and where you are in your breath all shift a little each time, and a 2 cm drift at the waist moves the result by around 1.5 points. The Navy equation is more accurate on paper, but that accuracy never materialises if you can't take the same measurement twice. Height and weight, by contrast, are measured almost without error. The same logic runs the other way: if you know how to handle a tape and can do it consistently, the Navy equation is the better option and this page isn't for you. Pointing you to the right tool is part of being accurate.
My smart scale shows body fat — which one is right?
Both are estimates, and each misses in its own direction. Smart scales use bioelectrical impedance: a weak current passes through your feet and resistance is measured. Fat holds little water, so its resistance is high. The logic is sound, but the result is so sensitive to hydration that a fasted morning reading and one taken after dinner can sit 3–4 points apart, and all of that gap is fluid. Foot-to-foot models also only measure the lower body and estimate the rest. The practical answer: pick one, stay with it, always measure under the same conditions (morning, fasted, after the bathroom) and read the curve rather than the absolute value.
What should I do to lower my body fat?
Body fat percentage is a ratio, so it falls two ways: reduce fat or add lean mass. Doing both at once works best. A moderate deficit (300–400 calories a day), resistance training two or three times a week and 1.6–2.2 grams of protein per kilo is the one combination that lowers the numerator while holding the denominator. Making the deficit harsher speeds fat loss but drags lean mass down with it; the ratio then doesn't fall as much as you expected, because total weight in the denominator is dropping too. That's why "just eat less" is on its own the slowest route.
Related tools
- BMI calculatorEnter your height and weight to see your body mass index, which category it falls in and the healthy weight range for your height. A body fat estimate comes with it — it says what BMI alone cannot.
- Waist-to-hip ratio calculatorEnter your waist, hip and height to see three separate risk measures at once and your target waist. In the sample body the ratio is 0.94, the target waist 89 cm and the gap 9 cm.
- Ideal weight calculatorWork out the healthy weight range for your height. We don't hand you a single "ideal weight" number — because there isn't one. You get the range, where you sit in it, and which end fits you.
- Macro calculatorTurn your daily calorie target into grams of protein, carbohydrate and fat. We publish the ratio we use and why we chose it, instead of offering preset templates with no explanation.
This tool is for information only and is not a substitute for medical diagnosis or treatment. Talk to a healthcare professional before making a lasting change to how you eat.