BMI Calculator

What Is BMI and How Is It Calculated?

Introduction

Body Mass Index (BMI) is a numerical value derived from a person's weight and height. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI serves as a quick screening tool to categorise individuals into weight status groups — underweight, normal weight, overweight, and obese. Both the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) use BMI as a population-level indicator of weight-related health risk.

Instructions

To use this calculator, simply select either Metric (cm/kg) or Imperial (ft/in/lbs) units. Enter your height and weight, and the calculator will instantly display your BMI score along with the WHO weight category you fall into. It will also calculate your healthy weight range based on your exact height.

The Formula

The metric formula divides weight in kilograms by the square of height in metres:

BMI = weight (kg) — height (m)—

For imperial units, the calculation uses a conversion factor of 703:

BMI = 703 — weight (lbs) — height (in)—

Example Calculation

A 70 kg person standing 1.75 m tall would calculate: 70 — (1.75 — 1.75) = 70 — 3.0625 = 22.9, which falls squarely within the normal range. The same person in imperial units (154 lbs, 68.9 in): 703 — 154 — (68.9)— = 108,262 — 4,747.21 — 22.8 — confirming the formula works identically regardless of unit system.

Use Cases

The international classification system divides BMI results into six categories, each corresponding to a different level of health risk:

Category	BMI Range	Risk Level
Underweight	< 18.5	Moderate
Normal weight	18.5 — 24.9	Low
Overweight	25.0 — 29.9	Increased
Obese Class I	30.0 — 34.9	High
Obese Class II	35.0 — 39.9	Very High
Obese Class III	= 40.0	Extremely High

These thresholds apply to adults aged 18 and older. Risk levels indicate statistical correlation with conditions such as type 2 diabetes, cardiovascular disease, and hypertension — not a certainty of disease. Individual risk depends on many additional factors, including family history, physical activity, diet, and fat distribution.

Limitations of BMI — What It Doesn't Measure

BMI was designed for population studies, not individual diagnosis. It has several well-documented blind spots:

• Muscle mass: Athletes, weightlifters, and manual labourers often register as "overweight" or even "obese" despite carrying minimal body fat. Muscle is roughly 18% denser than fat, so a muscular person weighs more at the same volume.
• Fat distribution: BMI doesn't distinguish where fat is stored. Visceral fat (around internal organs) poses significantly greater health risk than subcutaneous fat (under the skin). Two people with identical BMIs can have very different risk profiles depending on waist circumference.
• Age: Older adults naturally lose muscle and gain fat. A 70-year-old and a 30-year-old with the same BMI likely have different body compositions. Some geriatric researchers suggest the "normal" range for older adults should be shifted upward to 22—27.
• Ethnicity: WHO acknowledges that certain populations — particularly South Asian and East Asian groups — face elevated cardiometabolic risk at lower BMI thresholds. Some countries use adjusted cut-offs: a BMI of 23 or higher is considered overweight in several Asian health guidelines.
• Sex differences: Women typically carry higher body fat than men at the same BMI. Despite this, the standard categories remain identical for both sexes.

For a more complete health assessment, healthcare providers often combine BMI with waist circumference, waist-to-hip ratio, body fat percentage (via DEXA or bioimpedance), blood pressure, and blood lipid profiles.

BMI for Adults vs Children (Key Differences)

For adults 18 and older, BMI categories use fixed cut-off points (18.5, 25, 30). For children and adolescents aged 2 to 17, the raw BMI number is less meaningful because body composition changes rapidly during growth. Instead, paediatricians plot a child's BMI against age- and sex-specific growth charts to produce a BMI-for-age percentile.

The CDC's growth charts classify children as follows: below the 5th percentile is underweight, 5th to 84th percentile is healthy weight, 85th to 94th percentile is overweight, and at or above the 95th percentile is obese. A BMI of 22 might be perfectly healthy for a 16-year-old boy (50th percentile) but indicate underweight in a 40-year-old woman. This calculator is designed for adults; paediatric assessments require age-specific percentile tools.

BMI and Health Risk — What the Research Shows

Large-scale epidemiological studies consistently link BMI to several chronic conditions. A 2016 meta-analysis published in The Lancet, covering 10.6 million participants across four continents, found that all-cause mortality was lowest at a BMI between 20 and 25. Both underweight and obese categories showed elevated mortality, with the risk curve forming a J-shape — moderate underweight carried comparable risk to Class I obesity.

The National Institutes of Health (NIH) notes that overweight and obesity increase risk for type 2 diabetes, coronary heart disease, stroke, certain cancers (including breast, colon, and kidney), sleep apnoea, and osteoarthritis. However, the relationship is probabilistic, not deterministic. Physical fitness, diet quality, metabolic markers (blood glucose, cholesterol, triglycerides), and genetic factors all modulate individual risk. A physically active person with a BMI of 27 may have a better health profile than a sedentary person with a BMI of 23.

BMI is a screening tool, not a diagnostic measure. It can flag potential weight-related health concerns but cannot, on its own, determine whether a specific individual is healthy or unhealthy. Medical professionals use BMI alongside clinical examination, lab work, and patient history to make diagnostic and treatment decisions.

Healthy Weight Ranges by Height Chart

The table below shows the healthy weight range (BMI 18.5—24.9) for common heights. These values apply to adults and are rounded to the nearest 0.5 kg or 1 lb.

Height	Healthy Weight (Metric)	Healthy Weight (Imperial)
152 cm / 5'0?	42.7 — 57.6 kg	94 — 127 lbs
155 cm / 5'1?	44.4 — 59.9 kg	98 — 132 lbs
157 cm / 5'2?	45.6 — 61.5 kg	101 — 136 lbs
160 cm / 5'3?	47.4 — 63.8 kg	104 — 141 lbs
163 cm / 5'4?	49.2 — 66.2 kg	108 — 146 lbs
165 cm / 5'5?	50.4 — 67.8 kg	111 — 150 lbs
168 cm / 5'6?	52.2 — 70.3 kg	115 — 155 lbs
170 cm / 5'7?	53.5 — 71.9 kg	118 — 159 lbs
173 cm / 5'8?	55.4 — 74.5 kg	122 — 164 lbs
175 cm / 5'9?	56.7 — 76.3 kg	125 — 168 lbs
178 cm / 5'10?	58.6 — 78.9 kg	129 — 174 lbs
180 cm / 5'11?	59.9 — 80.7 kg	132 — 178 lbs
183 cm / 6'0?	62.0 — 83.4 kg	137 — 184 lbs
185 cm / 6'1?	63.3 — 85.3 kg	140 — 188 lbs
188 cm / 6'2?	65.4 — 88.0 kg	144 — 194 lbs
191 cm / 6'3?	67.5 — 90.8 kg	149 — 200 lbs

These ranges are based on the standard WHO formula and serve as general guidelines. Individual factors — including frame size, muscle mass, age, and ethnicity — can shift optimal weight outside these numbers. Sources: WHO Global Database on Body Mass Index; CDC Adult BMI Guidelines; NIH National Heart, Lung, and Blood Institute.