Body Fat Calculator

Three measurements with a tailor’s tape

You need weight, height, and a flexible tape of the kind every tailor and most kitchen drawers have. Centimetres, unless you switch the units box, in which case everything including the tape readings is in inches.

• Neck: just below the Adam’s apple, tape level all the way round, snug but not digging in.
• Waist: at the navel, standing straight, after a normal breath out. Not sucked in, and not where the trouser sits, which is lower and reads smaller. This is the measurement the whole formula turns on, so take it twice.
• Hip (women only): the widest point over the buttocks. Men leave it at 0.

Do it first thing in the morning before eating. A waist measured after a wedding dinner is a different waist.

The Navy formula on the default 70 kg man

This is the US Navy circumference method, worked out in the 1980s so a sailor could be assessed with a tape rather than a dunk tank. It is still used because it needs nothing else.

male:   %BF = 495 / (1.0324 − 0.19077 × log₁₀(waist − neck)
                     + 0.15456 × log₁₀(height)) − 450

female: %BF = 495 / (1.29579 − 0.35004 × log₁₀(waist + hip − neck)
                     + 0.22100 × log₁₀(height)) − 450

The default is a man of 70 kg and 165 cm, neck 37 cm, waist 86 cm.

girth       = 86 − 37 = 49 cm
denominator = 1.0324 − 0.19077 × log₁₀(49) + 0.15456 × log₁₀(165)
            = 1.0324 − 0.32244 + 0.34273
            = 1.05269

%BF = 495 / 1.05269 − 450 = 20.22

Body fat	20.2%
Category	Average
Fat mass	14.2 kg
Lean mass	55.8 kg

Put that beside his BMI: 70 kg at 165 cm is 25.7, overweight on the WHO chart and obese on the Indian one. The tape says he is average for a man, with about 14 kg of fat on a 56 kg frame. Both are true. The BMI is reacting to total weight; the tape is reacting to his waist.

Now push the waist to 90 cm and leave everything else alone. Body fat goes to 23.1 percent and fat mass to 16.2 kg. Four centimetres of belly is worth about three percentage points, which is the point of the method: it is really a waist measurement with a correction for frame size.

The thin-fat Indian

In 2004 two diabetes researchers, one Indian and one British, published a photo of themselves side by side. Same BMI, about 22. The Englishman had around 9 percent body fat; the Indian, Chittaranjan Yajnik from Pune, had around 21. The paper called it the Y-Y paradox and it has been in every South Asian obesity lecture since.

That is the pattern. At the same height and weight an Indian body tends to carry less muscle, more fat, and more of that fat around the abdomen and packed around the liver and gut rather than under the skin of the arms and thighs. Yajnik’s Pune group traced it back to birth: Indian babies are lighter than European babies but not proportionately leaner. Thin limbs, a comparatively fat middle.

The consequence is a man who passes every BMI check, wears a size 34, has a small paunch he calls “just a bit of a belly”, and turns up with an HbA1c of 7 at 42. His BMI never flagged anything. His waist would have, ten years earlier, and this calculator reads the waist.

What counts as high for a South Asian

The categories the calculator shows are the general ones used with the Navy method: for men, under 6 percent is essential fat, 6 to 14 athletic, 14 to 18 fitness, 18 to 25 average, and above that high. For women the bands run about seven points higher because female bodies need more fat to function.

Indian researchers tend to draw the obesity line lower than the top of “average”: about 25 percent for men and 30 for women is the pair of numbers most often quoted for Indian adults, and they go with the waist thresholds of 90 cm for men and 80 for women that ICMR uses. Those body fat cut-offs are less settled than the BMI ones, so treat them as a reasonable warning line rather than a rule. The practical reading is that the upper half of “average” is not a place for an Indian man to relax, and the default man at 20 percent with an 86 cm waist is four centimetres from the line.

If your BMI is fine and this page says 24 percent, believe this page and go and get a fasting sugar done. The BMI calculator has the Indian cut-offs, and the two together tell you more than either alone.

Where the tape lies

Trouser waist instead of navel waist. The single biggest error. Trouser size 32 is 81 cm at the waistband, and the navel measurement on the same man is often 88 to 90. Enter the waistband figure and the result is flattering and wrong.

Sucking in. Everyone does it. Breathe out normally and let the stomach sit where it sits.

Tape pulled tight. It should touch the skin all the way round without making a dent. Tight on the neck and loose on the waist, which is the natural tendency, inflates the result.

Mixing units. Height in centimetres and waist in inches, or the other way round, produces either a refusal or a number that is obviously nonsense. Everything follows the one units box.

Expecting DEXA accuracy. The formula is good to about 3 or 4 percentage points against underwater weighing for most people. It is much better than BMI and much worse than a DEXA scan, which costs ₹2,000 to ₹4,000 at a city diagnostic centre. The bioimpedance scale at the gym is not more accurate than the tape; it swings several points with hydration. Where the tape wins is consistency: same time, same place, same tension, and the change from month to month is real even when the absolute figure is a little off.

Once you know the lean mass, the BMR calculator and the calorie calculator make more sense, because 55.8 kg of lean tissue is what is actually burning the calories, and the ideal weight calculator shows that the formula weights for 165 cm sit right at this man’s lean mass plus a healthy amount of fat.