BMI Calculator

What is BMI?

Body Mass Index (BMI) is a numerical value derived from a person's height and weight, designed as a quick, population-level indicator of whether someone falls into a healthy weight range. The World Health Organization (WHO) has used BMI since the 1990s as a standardized screening tool to classify underweight, healthy weight, overweight, and obesity in adults. It is inexpensive, non-invasive, and easy to calculate, which is why primary-care clinicians, public-health agencies, and nutritionists around the world still rely on it as a first-pass indicator.

BMI does not measure body fat directly. It assumes a broadly typical ratio of fat, muscle, bone, and water for a given height and weight, which works reasonably well for the majority of sedentary and moderately active adults but breaks down at the extremes. Very muscular athletes often land in the "overweight" or even "obese" category despite single-digit body fat percentages; older adults who have lost muscle mass can show a "normal" BMI while carrying a dangerous amount of visceral fat. BMI also says nothing about where fat is distributed, yet abdominal (visceral) fat carries substantially higher cardiometabolic risk than fat stored around the hips and thighs.

For these reasons, BMI is best understood as a starting point rather than a diagnosis. Used alongside waist circumference, body-composition measurement, blood pressure, and blood markers, it provides valuable context. Used in isolation, it can mislead — which is why modern clinical guidelines increasingly emphasize combining multiple indicators.

How is BMI Calculated?

The BMI formula is simple: BMI = weight (kg) ÷ height (m)². For imperial units: BMI = 703 × weight (lb) / height (in)².

Worked example:A person 1.75 m tall weighing 75 kg. BMI = 75 / (1.75 × 1.75) = 75 / 3.0625 ≈ 24.5 — on the upper end of the WHO-defined healthy range (18.5–24.9). Remember to convert height from centimetres to metres (170 cm = 1.70 m).

WHO BMI Classification

Limitations of BMI

BMI cannot distinguish lean mass from fat mass, ignores fat distribution, treats adult men and women with a single scale, and can misclassify athletes, older adults, pregnant women, and some ethnic groups (for example, lower obesity thresholds are used for many Asian populations because cardiometabolic risk rises at lower BMI values). Used alongside waist circumference — above 102 cm for men or 88 cm for women signals elevated risk — BMI becomes considerably more informative.

Tips for Maintaining a Healthy BMI

• Combine 150+ minutes of moderate aerobic activity per week with two strength sessions.
• Emphasize whole foods: vegetables, fruit, legumes, lean protein, whole grains, healthy fats.
• Avoid extreme or very-low-calorie diets — sustainable deficits of 300–500 kcal work best.
• Prioritize sleep (7–9 hours) and stress management — both directly affect weight.
• Track trends, not daily fluctuations; weight naturally moves 1–2 kg with hydration and meals.

Frequently Asked Questions