How much of your weight is fat, and how much is muscle?
The bathroom scale shows one number: but two people weighing the same can look completely different. What matters is body composition: how much of that weight is fat versus muscle, bone, organs, water (that's lean mass).
This calculator uses the Navy method: you measure 3-4 circumferences with a tape (neck, waist, hips for women) plus height and weight: and the calculator returns your % body fat.
Accuracy: ±3-4% vs a DEXA scan (the gold standard). Better than BMI, worse than an expensive clinic scan, ideal for tracking everyday progress.
How to use it
- Toggle units (cm/kg or in/lb) and sex: the formula differs for men and women (women also measure hips).
- Enter weight and height: same as on your bathroom scale.
- Measure your neck with a tape, just below the larynx. Men: waist at navel level. Women: waist at the narrowest point + hips at the widest (around the buttocks).
- Look at the % body fat (big number) and the category (Athlete / Fitness / Average / Obese).
- Target slider, type a goal %, the calculator tells you how many kilos of fat you need to drop to get there.
- Tip: take 3 measurements of each circumference, average them. Tape without compression, body upright, breathe normally.
When this is useful
Six situations where body fat % tells you more than weight:
- Tracking training progress: scale stuck, but clothes looser? You're losing fat and building muscle: the scale won't show it, the calculator will. Measure every 2 weeks, watch the trend.
- Planning a cut (fat loss), you know you're at 22% and want to hit 15%. The calculator says: drop 5 kg of pure fat (while keeping muscle). Concrete goal instead of "lose some weight".
- Doctor asks about body composition: before a visit to a dietitian, endocrinologist, cardiologist. Better signal than BMI, which mistakes a muscular person for an obese one.
- Bodybuilder before a contest: checking if you're already at "stage condition" (5-8% for men, 12-15% for women). The calculator won't replace DEXA, but it shows the trend week by week.
- BMI says you're overweight, but you aren't: athletes often have BMI > 25 (technically "overweight") but real BF below 12%. Muscle weighs more than fat. The calculator shows the truth.
- Checking essential fat: women losing their period when % drops too low (below 14%), men with hormone drops below 6%. The number tells you when to stop.
Questions and answers
It's what percent of your weight is fat, and what is everything else (muscle, bone, organs, water). Example: you weigh 80 kg with 20% BF. That means: 16 kg of fat + 64 kg of everything else (muscle, bone, water, your "lean mass"). Lower % = more muscle relative to fat. Not "lower is always better", everyone needs a minimum (essential fat, the minimum needed for the body to function) to stay healthy.