BMI Calculator

BMI: the complete guide to Body Mass Index

The BMI (Body Mass Index) is the ratio of your weight to your height squared. Invented in the 1830s by Belgian statistician Adolphe Quetelet and adopted by WHO as the world reference, it sums up your build in one number between 14 and 40+. Formula: BMI = weight (kg) ÷ height² (m). Example: 70 kg at 1.75 m → 70 ÷ 3.0625 = 22.9, right in the normal zone.

Defined in the 1830s by Belgian Quetelet then adopted by WHO, the index has 6 official categories — and according to WHO, 1 adult in 8 was obese worldwide in 2022.

WHO category chart

Ideal weight: WHO range and Lorentz formula

Two complementary methods: the WHO range (any weight giving a BMI between 18.5 and 24.9 for your height) and the Lorentz formula, which refines by build — men: height − 100 − (height − 150)/4; women: height − 100 − (height − 150)/2.5 (height in cm).

BMI for women, men, children: nuances

The index and its thresholds are unisex in adults, but ideal weight differs (~3 kg gap at equal height) and waist circumference refines risk: beyond 94 cm (men) and 80 cm (women), cardio-metabolic risk rises even with normal BMI. For children and teens, never use these thresholds: interpretation uses the health-booklet growth curves, in percentiles by age and sex.

Why does waist circumference matter so much? Belly (visceral) fat predicts diabetes and cardiovascular disease better than BMI alone. Two people at BMI 24 can have very different risks: measure your waist halfway between ribs and hips, belly relaxed, after exhaling.

The honest limits of BMI

BMI ignores body composition: a rugby player at BMI 27 can be in perfect health, while a sedentary person at 23 may lack muscle. It is skewed by pregnancy, growth, edema and muscle loss after 65 (where 24–29 is often preferable). It is an epidemiological warning signal, not an individual diagnosis: cross-check it with waist circumference, activity and, if needed, medical advice.

What to do with your result?

Normal BMI: stay the course (150 minutes of moderate activity per week per WHO). Overweight: aim for −5 to −10% in 6 months, log each weigh-in in the tracking above to see the trend rather than fluctuations. Obesity or associated condition: talk to a doctor or dietitian. In all cases, weigh yourself in the morning, fasting, under the same conditions: measurement consistency is what makes tracking reliable.

Important: this page informs, it does not diagnose. High BMI with abnormal blood pressure, blood sugar or cholesterol, unusual breathlessness or rapid weight gain justify a consultation. Start with your GP, usually with a basic blood test.

Length: from the human body to the stars

The inch comes from the width of a thumb: today it is exactly 2.54 cm since the 1959 international agreement between English-speaking countries. The foot (30.48 cm) matched a man's foot, and the yard (0.9144 m, 3 feet) the distance from nose to thumb of the outstretched arm. The mile (1,609.344 m) descends from Roman thousand paces — a thousand double-steps of 1,480 m, revalued by the British at 8 furlongs.

Mass: pounds, ounces and stones

The pound (453.59237 g exactly) comes from the Roman libra, hence its 'lb' symbol. It divides into 16 ounces (28.35 g), the unit of Anglo-Saxon recipes and jewellery. The stone (6.35 kg, 14 pounds) is still used to weigh people in the UK and Ireland. For freight, beware false twins: the American short ton (907 kg) and the British long ton (1,016 kg) bracket the metric ton (1,000 kg). The metric quintal is exactly 100 kg.

Pressure: from weather to tires

The pascal, the SI unit, is one newton per square meter — too small daily, hence the hectopascal of weather reports (1,013 hPa on average). The bar (100,000 Pa, about atmospheric pressure) serves tires and compressors. Psi (pound per square inch, 6,894.76 Pa) dominates across the Atlantic: 1 bar = 14.5 psi.

Energy: calories, BTU and horsepower

The calorie (4.184 J in thermochemical version) is the energy to heat 1 g of water by 1 °C; the food kilocalorie follows the same definition per kilogram. For power, the French cheval-vapeur (735.5 W) comes from a draught horse's effort, while James Watt's horsepower (745.7 W) slightly overstates the same horse — hence 100 ch = 73.55 kW but 100 hp = 74.57 kW.