Learn to calculate your Body Mass Index (BMI) using height and weight, and understand what the number means for your health.
The Origin of BMI
The concept behind BMI dates back to the 1830s. Belgian mathematician and statistician Adolphe Quetelet developed what he called the "Quetelet Index" — a ratio of weight to the square of height — while studying the statistical characteristics of human bodies. His goal was not to assess individual health, but to describe the "average man" for social science purposes.
The index remained an academic curiosity for over a century. In 1972, American physiologist Ancel Keys published a landmark paper in the Journal of Chronic Diseases in which he re-evaluated several weight-to-height formulas. He concluded that Quetelet's ratio was the most useful for large-scale population studies and coined the term "Body Mass Index." Keys himself cautioned that BMI was a poor tool for individual clinical assessment.
By 1995, the World Health Organization (WHO) officially adopted BMI as the standard metric for classifying adult weight status — underweight, normal, overweight, and obese — setting the thresholds still used today.
BMI Classification (WHO, Adults)
| Category | BMI Range | Health Risk |
|---|---|---|
| Underweight | < 18.5 | Malnutrition, bone loss, weakened immunity |
| Normal weight | 18.5 – 24.9 | Low risk |
| Overweight | 25 – 29.9 | Moderate risk; metabolic syndrome |
| Obese class I | 30 – 34.9 | High risk; cardiovascular disease, type 2 diabetes |
| Obese class II | 35 – 39.9 | Very high risk; severe complications |
| Obese class III | ≥ 40 | Extremely high risk; life-threatening conditions |
Health Risks of a High BMI
- 1
Cardiovascular disease: Excess body fat, particularly visceral fat around the abdomen, raises LDL cholesterol and triglycerides while lowering HDL. This accelerates arterial plaque build-up, increasing the risk of heart attack and stroke.
- 2
Type 2 diabetes: Obesity promotes insulin resistance, where cells no longer respond efficiently to insulin. The pancreas compensates by producing more insulin until it can no longer keep up, leading to chronically elevated blood sugar.
- 3
Hypertension: Greater body mass forces the heart to pump harder and increases blood volume, raising both systolic and diastolic blood pressure over time.
- 4
Joint stress and osteoarthritis: Each extra kilogram of body weight adds roughly 4 kg of pressure on the knee joints with every step. Long-term excess load accelerates cartilage breakdown.
- 5
Sleep apnea: Fat deposits around the throat narrow the airway, causing repeated breathing interruptions during sleep. This reduces sleep quality and strains the heart.
- 6
Certain cancers: High BMI is associated with an elevated risk of colorectal, breast (post-menopausal), endometrial, kidney, and oesophageal cancers, partly due to chronic inflammation and excess oestrogen production from adipose tissue.
Health Risks of a Low BMI
- 1
Malnutrition: Insufficient caloric and micronutrient intake can cause deficiencies in iron, vitamin B12, folate, and fat-soluble vitamins, impairing nearly every organ system.
- 2
Bone density loss: Low body weight is a major risk factor for osteoporosis. Without adequate calcium and mechanical loading from body mass, bone mineral density decreases, raising fracture risk.
- 3
Weakened immune system: The immune system depends on adequate nutrition. Underweight individuals experience more frequent infections and slower recovery.
- 4
Anaemia: Iron and folate deficiencies common in underweight individuals reduce red blood cell production, causing fatigue, weakness, and shortness of breath.
- 5
Hormonal disruption: In women, very low body fat can suppress oestrogen levels, leading to irregular or absent menstrual cycles (amenorrhoea) and reduced fertility.
Limitations of BMI
Athletes and high muscle mass: Muscle is denser than fat. A highly trained athlete may have a BMI in the "overweight" range while carrying very little body fat. BMI cannot distinguish between fat mass and lean mass.
Elderly and sarcopenia: Older adults often have reduced muscle mass but preserved — or even increased — fat mass. Their BMI may appear normal while they have dangerously low muscle reserves.
Ethnic variations: Research shows that people of Asian descent face higher cardiometabolic risks at lower BMI thresholds. Many Asian countries use a cutoff of 23 for "overweight" rather than 25.
Pregnancy: Weight gain is expected and healthy during pregnancy. Standard BMI classifications do not apply to pregnant individuals.
Children and adolescents: BMI in under-19s must be interpreted using age- and sex-specific percentile charts, not the adult thresholds.
How to Calculate Your BMI
Understand the BMI Formula
BMI = weight (kg) ÷ height (m)². For example, a person who weighs 70 kg and stands 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
Measure Your Height Accurately
Remove shoes and stand flat against a wall. Rest a book horizontally on top of your head and mark the wall. Measure from the floor to the mark in centimetres, then divide by 100 to convert to metres.
Record Your Weight
Weigh yourself first thing in the morning before eating, wearing minimal clothing. Use a digital scale on a hard, flat surface. Note the value in kilograms (divide pounds by 2.205 to convert).
Calculate and Round
Square your height in metres: e.g. 1.75² = 3.0625. Divide your weight by that result: 70 ÷ 3.0625 = 22.9. Round to one decimal place.
Interpret Your Result
Compare your BMI against the WHO table above. Remember BMI is a screening tool — consult a healthcare professional for a complete assessment that accounts for muscle mass, age, and ethnicity.
Try our free tool
BMI Calculator
Frequently Asked Questions
Q: Can I use pounds and inches?
A: Yes. The imperial formula is BMI = (weight in lbs × 703) ÷ (height in inches)². Our BMI Calculator supports both metric and imperial.
Q: Is BMI accurate for athletes?
A: Not always. Muscle weighs more than fat, so muscular people may have a high BMI without excess body fat. Consider body-fat percentage or waist-to-hip ratio for a fuller picture.
Q: Does BMI apply to children?
A: No. Children and teenagers (ages 2–19) use age- and sex-specific percentile charts rather than the standard adult thresholds.