Body Fat Estimator

What each input means

The calculator asks for 10 pieces of information. Every field is validated before the maths runs, so an impossible value is refused rather than quietly producing a wrong answer.

A worked example using the default inputs

With units of Metric (kg, cm), sex at birth of Male, weight (optional) of 75 kg, height of 175 cm, neck circumference of 38 cm, the calculator returns 17.8%.

The same arithmetic runs whatever you type into the form above, so you can reproduce any figure by matching the inputs listed here.

How this is calculated

Men: %fat = 86.010·log₁₀(waist − neck) − 70.041·log₁₀(height) + 36.76. Women: 163.205·log₁₀(waist + hip − neck) − 97.684·log₁₀(height) − 78.387. All circumferences in inches.

Working values are kept at full precision and rounded only for display. Impossible dates, zero denominators, negative amounts where they make no sense, and out-of-range values are rejected before any result is shown.

The equations these health tools use

Health calculators are published regression equations, not proprietary formulas. Each was fitted to a population sample, which is why they are reported as estimates and why two tools measuring the "same" thing can differ slightly.

The energy equations assume a healthy adult at rest and do not know whether weight is muscle or fat. Two people with identical height and weight can have maintenance needs twenty per cent apart, which is why the calorie tools present a band rather than a single number and why you should compare your own trend over two to three weeks against the estimate.

BMI is a population screening measure, not a description of an individual. It reads high for muscular athletes and low for older adults who have lost muscle, and it was never designed for children, pregnancy or some ethnic groups.

Three worked examples from this section

BMI for 70 kg at 175 cm

Convert height to metres, square it, then divide weight by that figure: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9. That sits inside the 18.5 to 24.9 band the World Health Organization labels as the healthy range.

Maintenance calories for a 30-year-old, 70 kg, 175 cm, lightly active

Mifflin–St Jeor gives 10 × 70 + 6.25 × 175 − 5 × 30 + 5 ≈ 1,649 kcal at complete rest. Multiplying by 1.375 for light activity gives roughly 2,267 kcal a day to hold weight.

Resting energy (BMR)

≈ 1,649 kcal

Lightly active maintenance

≈ 2,267 kcal

A 15% deficit

≈ 1,927 kcal

A 15% surplus

≈ 2,607 kcal

Pace for 5 km in 25:00

Twenty-five minutes over five kilometres is exactly 5:00 per kilometre. In miles that is 5:00 × 1.609 ≈ 8:03 per mile, and the same effort projects a 10 km of about 52 minutes.

Projections assume you hold the same pace over the longer distance. In practice most runners slow by three to eight per cent per doubling of distance, which is why race-day predictions from a short run tend to be optimistic.

What health estimates can and cannot tell you

These tools apply published equations to the numbers you enter. The equations were derived from population samples, so they describe an average body with your measurements, not your body. Individual variation of around 10 per cent is normal and is not a sign that you entered something wrong.

• They do not know your medical history, medication, body composition, pregnancy, disability or family risk.
• A single number is not a diagnosis. Trends over weeks, measured under the same conditions, carry far more information than any one reading.
• If a result surprises you, or you are planning a significant change to training or diet, use it as a prompt for a conversation with a clinician or accredited practitioner rather than as an instruction.

Sources and further reading

• World Health Organization: obesity and overweight
• Centers for Disease Control and Prevention: adult BMI