What is BMI Calculator?
Body mass index is a single number derived from your weight and height, used as a quick screening measure for whether your weight sits in a healthy range for your size. This calculator accepts either metric (kilograms and centimetres) or imperial (pounds, feet and inches) input, converts internally, and reports both the BMI figure and the World Health Organization category it falls into.
Alongside the number, the tool shows the weight range that would place you in the "normal" band at your height. That range is usually the more actionable output: a category label tells you where you are, but a target range tells you what would change it. The scale underneath plots your result visually so you can see how close you are to a boundary rather than treating the categories as hard steps.
BMI is deliberately crude. It uses only two measurements and cannot distinguish muscle from fat, or account for frame size, body composition, age or ethnicity. It was designed to describe populations, not individuals, and it misclassifies muscular people as overweight and people with low muscle mass as healthy. Treat the result as one rough signal among several, not a verdict on your health.
Use Cases
Here are the most common ways people use BMI Calculator every day.
General health check-in
Get a quick sense of whether your weight is in the broadly healthy range for your height, and see the specific weight range that band corresponds to. Useful as a starting point for a conversation with a doctor rather than a conclusion on its own.
Setting a realistic target weight
The healthy-range output converts the abstract 18.5–24.9 BMI band into concrete kilograms or pounds at your height. That gives a target range instead of a single arbitrary number, which is generally a healthier way to set a goal.
Insurance and medical forms
Many health insurance applications, pre-surgical assessments and occupational health forms ask for BMI. This calculates it to the same standard formula those forms expect, in whichever unit system the form uses.
Tracking change over time
Because BMI compresses weight and height into one figure, it is a convenient way to track direction over months. Height is effectively constant in adults, so changes in BMI track changes in weight — the value is in the trend rather than any single reading.
Examples
Metric input
An adult weighing 70 kg at 175 cm.
Weight 70 kg, height 175 cm BMI 22.9 — Normal weight (healthy range 56.7–76.2 kg) Imperial input
An adult weighing 185 lb at 5 ft 11 in.
Weight 185 lb, height 5 ft 11 in BMI 25.8 — Overweight Reading a borderline result
A BMI of 24.8 sits just inside the normal band. The scale marker shows how near the 25 boundary it is, which a category label alone would hide.
Weight 76 kg, height 175 cm BMI 24.8 — Normal weight, close to the overweight boundary Tips for Using BMI Calculator
- BMI is not valid for children or teenagers — they need age- and sex-specific percentile charts, not adult categories.
- It is also unreliable during pregnancy, and for competitive athletes, bodybuilders, and anyone with unusually high muscle mass.
- Waist circumference and waist-to-height ratio predict metabolic risk better than BMI, because they reflect where fat is stored.
- Weigh yourself at the same time of day under the same conditions — normal daily fluctuation of 1–2 kg will move your BMI noticeably.
- A single reading means very little. The trend across several months is the part worth paying attention to.
Frequently Asked Questions
What is a healthy BMI?
The World Health Organization defines 18.5 to 24.9 as the normal range for adults, with under 18.5 classed as underweight, 25 to 29.9 as overweight, and 30 and above as obese, subdivided into classes I, II and III at 35 and 40. These thresholds were derived from population mortality data and are the same worldwide, though some health bodies use lower cut-offs for people of South Asian and East Asian descent, where metabolic risk rises at a lower BMI.
Why does BMI say I am overweight when I am clearly not?
BMI divides weight by height squared and has no way to tell what the weight is made of. Muscle is considerably denser than fat, so a lean, muscular person can produce a BMI in the overweight or even obese range while carrying very little body fat. This is the single best-known failure of the measure. If your BMI conflicts with everything else you know about your body composition, the BMI is probably the thing that is wrong.
Is BMI calculated differently for men and women?
No — the formula and the category thresholds are identical. This is a known limitation rather than a design choice, because women naturally carry a higher proportion of body fat than men at the same BMI. The measure describes body mass relative to height, not body fat, so it cannot reflect that difference.
How do I convert between the metric and imperial formulas?
The metric formula is weight in kilograms divided by height in metres squared. The imperial formula multiplies pounds divided by inches squared by 703, a conversion constant. This calculator converts imperial input to metric before computing, so both paths give the same answer without you needing the constant.