BMI Calculator (Body Mass Index)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/5/2026
BMI is calculated by dividing weight (in kg) by the square of height (in meters). Example: for 70 kg and 1.70 m, BMI is 70 ÷ 1.70² ≈ 24.2, which falls in the normal weight category according to the WHO.
Explanation
Body mass index is a screening indicator, not a diagnosis: it compares weight to height without accounting for body composition (muscle mass, fat mass, frame size). A very muscular person can therefore have a high BMI without excess body fat, and the indicator is less reliable for older adults, pregnant women, or elite athletes. The World Health Organization defines four categories for an adult: underweight (BMI < 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), and obesity (30 and above). For an estimate of your daily energy needs, see also our calorie needs calculator.
Example: 70 kg for 1.70 m
Inputs
Weight: 70 kg. Height: 170 cm.
Calculation
Convert height to meters: 170 cm = 1.70 m. Square the height: 1.70² = 2.89. Finally, divide the weight by that square: 70 ÷ 2.89 ≈ 24.2.
Result
BMI ≈ 24.2, which falls in the normal weight category according to WHO thresholds.
Frequently asked questions
Is BMI reliable for everyone?
No. BMI doesn't distinguish muscle mass from fat mass: a very muscular athlete can show a "high" BMI without excess fat. It's also less relevant for children, pregnant women, and older adults, whose body composition differs from the general adult population the thresholds were established on.
Does a BMI in the "normal" category guarantee good health?
No, BMI is just one indicator among many. Other factors (waist circumference, physical activity, diet, medical history) matter just as much for assessing overall health. A healthcare professional is best placed to interpret your BMI in your personal context.
What should I do if my BMI is outside the normal range?
A BMI outside the 18.5-24.9 range doesn't automatically mean a health problem, but it's worth discussing with a doctor or healthcare professional, who can assess your situation as a whole rather than from a single number.