Body fat percentage, with a tape measure.
No calipers, no scan, no smart scale. Three measurements and your height give a body fat estimate accurate enough to track — plus how your weight splits between fat and everything else.
Where the formula comes from
This is the U.S. Navy circumference method, developed by Hodgdon and Beckett in 1984 to screen personnel without a laboratory. It works because girth at the waist tracks fat storage, while the neck — and for women the hips — give the equation a frame of reference for body size and shape.
Women: 495 ÷ (1.29579 − 0.35004·log₁₀(waist + hip − neck) + 0.22100·log₁₀(height)) − 450
It is a regression fitted to a normal range of bodies. Feed it extreme measurements and it returns numbers that are not biologically possible, which is why this page refuses to show a result rather than printing something absurd with confidence.
What to do with the number
The single most useful thing body fat percentage tells you is whether a stalled scale is actually a stalled body. Someone eating well and lifting in their first few months can lose three kilos of fat and gain three of muscle, and the bathroom scale will report that nothing happened. Body fat percentage catches it; weight alone does not.
Beyond that, do not check it weekly. Fat loss at a sustainable rate moves this number by about half a percentage point a month — well inside the method's own error bar. Measure every four to six weeks, in the same conditions, and look at the line rather than the dot.
Questions people ask
How accurate is the tape measure method?
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Around three to four percentage points against a DEXA scan for most people. That sounds imprecise, and it is — but it is consistent, which is what matters. If you measure the same way every month, the direction of travel is reliable even when the absolute number is a little off. Chasing the exact figure is the wrong use of this tool.
Where exactly do I measure?
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Neck: just below the larynx, with the tape sloping slightly downward at the front. Waist: at the navel for men, at the narrowest point of the torso for women. Hips (women only): around the widest part with your feet together. Keep the tape flat against the skin, snug but not compressing it, and stay relaxed — do not hold your stomach in.
What is a healthy body fat percentage?
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For men, roughly 14–18% is the fitness range and 18–25% is average. For women it is 21–25% and 25–32%. Women carry more essential fat than men — around 11% versus 3% — because of sex-specific fat stores, so the two scales are genuinely different rather than one being an adjustment of the other.
Why is my body fat different from my smart scale?
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Bathroom scales estimate body fat by passing a small current through your body and measuring resistance. That reading is extremely sensitive to hydration, so it swings with how much you have drunk, how salty dinner was, and whether you have trained. Tape measurements are less sensitive to any of that. Neither is a scan; pick one method and stay with it.
Do I need to know my body fat at all?
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Not really. It is useful in one specific situation: when the scale is not moving and you want to know whether you are losing fat and gaining muscle at the same time. Outside that, waist measurement and how your clothes fit tell you most of what body fat percentage would.
Related calculators
A number is a starting point, not a plan.
This calculator can tell you where you are. It cannot watch your form, adjust when your knee flares up, or keep you consistent in week six. If you want that part, we run a private studio in Kharadi and the first session is free.
This tool provides general fitness and nutrition information for healthy adults aged 18 and over. It is not medical advice, diagnosis or treatment. Speak to a doctor before starting any new exercise or nutrition programme, particularly if you have a health condition, are pregnant or post-partum, or have a history of disordered eating.