BMI: what the number means and what it cannot tell you

Why you’d do this

Body mass index turns up everywhere: a clinic form, a gym sign-up, a visa medical, a fitness app. It is one number made from two measurements, and it gets treated as a verdict far more often than it deserves. The useful thing to know before you calculate it is what it is: a rough screening measure that compares your weight with your height. It says nothing about where the weight sits on your body, how much of it is muscle, or how you feel and function day to day.

Calculating it yourself, and reading the result with its limits in view, is a sensible way to understand what that clinic or app number refers to. This guide covers the BMI Calculator and the Body Fat Calculator, which looks at a different question from tape measurements.

How, with the named tool

BMI Calculator works on your device. Nothing you type is uploaded.

1. Choose your units. Press cm / kg or ft-in / lb. The height and weight fields switch to match.
2. Enter height and weight. Fill in Height and Weight. In imperial mode, height is split into ft and in.
3. Add your age if you want the check. The Age (optional) field lets the tool decide whether adult categories apply to you at all (see below).
4. Read the result. Under Your BMI you get the number, its category, a marker on a scale, and Healthy weight for your height as a range in kg or lb.

The arithmetic is simple: weight in kilograms divided by height in metres, squared. A person of 175 cm and 70 kg comes out at 22.9. The tool uses the WHO adult bands:

• below 18.5: Underweight
• 18.5 to 24.9: Normal range
• 25 to 29.9: Overweight
• 30 and above: Obese range

The healthy-weight range it shows is just those same band edges turned back into weights for your height. At 175 cm, BMI 18.5 to 24.9 corresponds to roughly 56.7 to 76.3 kg. That is a statement about the formula, not a target for you.

If you want a second, different measurement, Body Fat Calculator estimates body-fat percentage with the US Navy tape-measure method. Press Metric (cm) or Imperial (in), choose Male or Female, then enter Height, Waist (at navel level, tape horizontal), Neck (just below the larynx) and, for women, Hip (at the widest point). It shows Estimated body fat together with the typical error against a DXA scan, about ±3 to 4 percentage points. It also lists four older ideal-weight formulas (Devine, Robinson, Miller, Hamwi) side by side without choosing one.

What to watch for

Muscle and bone weigh the same as anything else. BMI counts all weight equally, so a muscular person can land in the overweight or obese range with little body fat, and someone who has lost muscle can sit in the normal range. The Body Fat Calculator’s own note says the formula tends to underestimate body fat in muscular or high-BMI people and overestimate it in very lean women. Neither tool measures body composition directly.

Age changes what the number means. The bands were written for adults. If you type an age under 18, the BMI Calculator does not show a category. It replaces the result with a note that adult categories do not apply to children and teens, and that growth is assessed with clinician-guided charts. The Body Fat Calculator does the same, because its formula comes from adult data. For anyone under 18, a clinician is the right person to interpret growth, not this page. Among adults, body composition also shifts with age even when weight stays the same, which a single formula cannot reflect.

One set of cut-offs for everyone is a simplification. The tool applies the same WHO adult bands to every body, and it does not adjust for ethnicity, build or frame size. Health organisations have noted that the relationship between BMI and body fat differs between populations, so the same number can mean different things for different people. The calculator does not adjust for any of that, and neither should you assume it does.

The healthy range is not a goal. It shows where the formula’s normal band falls for your height. It does not know your history, your medical conditions, your training, or what is right for you, and this guide gives no weight targets or diet advice. If a number worries you, or you are thinking of changing how you eat or train, that conversation belongs with a clinician.

The ideal-weight formulas are not goals either. The Body Fat Calculator labels them as mid-20th-century population regressions, and notes that Devine is used clinically for drug dosing, not as a goal weight.

FAQ

Why does BMI call a muscular person overweight?

Because it only sees weight and height. A kilogram of muscle counts the same as a kilogram of fat, so the formula cannot tell them apart. This is why BMI is called a screening measure. A tape-measure estimate such as the Body Fat Calculator’s looks at a different signal, though it carries its own error of about ±3 to 4 percentage points.

Does BMI work for children and teenagers?

No. The adult bands do not apply under 18. Enter an age below 18 in the BMI Calculator and it shows a note instead of a category, because growth in children and teens is assessed with clinician-guided charts.

Is my data sent anywhere?

No. Both tools run on your device, and the height, weight, age and measurements you enter are not uploaded.

BMI is a rough screening measure, not a diagnosis — talk to a clinician for personal advice.