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BMI Calculator

Calculate your Body Mass Index (BMI) using metric (kg/cm) or imperial (lbs/ft-in) measurements. Results update instantly as you type and include the calculation steps and your healthy weight range.

Units:
BMI: 24.2 — Normal weight
Healthy weight range for your height: 53.5 – 72.0 kg
BMI Formula: BMI = weight(kg) / height(m)²
Weight = 70.0 kg
Height = 1.70 m
BMI = 70.0 / (1.70)²
BMI = 70.0 / 2.8900
BMI = 24.2
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BMI is a screening tool, not a diagnostic measure. Consult a healthcare professional.

What is BMI?

BMI, or Body Mass Index, is a numerical value derived from a person's weight and height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has been used by the World Health Organization (WHO), the Centers for Disease Control and Prevention (CDC), and healthcare systems worldwide as a simple, low-cost screening tool for weight categories that may lead to health problems.

BMI does not directly measure body fat. Instead, it uses the ratio of weight to height squared to estimate whether a person's weight falls within a healthy range. Because it requires only a scale and a measuring tape — no blood draw, no imaging, no specialist equipment — it became the standard first-line screening tool in clinical settings and public health research.

Despite its widespread use, BMI has well-documented limitations (discussed below). It is best understood as one data point among several, not a definitive verdict on health.

How to Calculate BMI

The BMI formula is straightforward, but the exact numbers depend on which unit system you are using.

Metric Formula

BMI = weight (kg) ÷ height (m)²

Convert height from centimetres to metres first by dividing by 100. Then square the height in metres and divide the weight in kilograms by that squared value.

Worked example (metric):

Weight = 70 kg, Height = 170 cm = 1.70 m
Step 1 — Square the height: 1.70 × 1.70 = 2.89 m²
Step 2 — Divide weight by height squared: 70 ÷ 2.89 = 24.2

A BMI of 24.2 falls in the Normal weight range (18.5 – 24.9).

Imperial Formula

BMI = (weight (lbs) ÷ height (in)²) × 703

Convert height to total inches (feet × 12, plus remaining inches). Square the total inches. Divide the weight in pounds by that squared value. Multiply by the conversion factor 703.

Worked example (imperial):

Weight = 154 lbs, Height = 5 ft 7 in = 67 inches
Step 1 — Square the height: 67 × 67 = 4,489 in²
Step 2 — Divide: 154 ÷ 4,489 = 0.034305
Step 3 — Multiply by 703: 0.034305 × 703 = 24.1

The factor 703 is derived from the unit conversion between the metric and imperial systems. It makes the imperial result equal to the metric result for the same person.

BMI Categories (WHO Classification)

The WHO defines the following standard BMI categories for adults aged 18 and older:

BMI Range Category Health Risk
Below 18.5 Underweight Malnutrition, osteoporosis, immune suppression
18.5 – 24.9 Normal weight Lowest risk; optimal range for most adults
25.0 – 29.9 Overweight Slightly elevated risk of metabolic disease
30.0 – 34.9 Obese — Class I Moderate risk of type 2 diabetes, heart disease
35.0 – 39.9 Obese — Class II High risk; surgical weight-loss often considered
40.0 and above Obese — Class III (severe) Very high risk; significant mortality association

Some health organisations use slightly different cutoffs for specific ethnic groups. For example, the WHO recommends lower BMI thresholds for Asian populations (23.0 for overweight, 27.5 for obese) because research shows metabolic risk appears at lower BMI values in these groups.

BMI Limitations

BMI is a useful population-level screening tool, but it has significant limitations when applied to individuals. Understanding these limitations helps you interpret your result more accurately.

It Does Not Distinguish Fat from Muscle

BMI measures total body mass relative to height. It cannot tell whether that mass is fat, muscle, or bone. A professional athlete with 8% body fat and a sedentary person with 35% body fat may have identical BMIs. For muscular individuals, BMI routinely overestimates health risk.

It Ignores Fat Distribution

Where fat is stored matters as much as how much fat there is. Visceral fat — fat stored around the organs in the abdominal cavity — is metabolically active and strongly linked to insulin resistance, type 2 diabetes, and cardiovascular disease. Subcutaneous fat (under the skin) carries much lower risk. BMI cannot detect the difference. Waist circumference or waist-to-hip ratio are better predictors of abdominal fat.

Age and Sex Affect Interpretation

Older adults tend to have more body fat at the same BMI compared to younger adults, because muscle mass naturally decreases with age (sarcopenia). Women generally have more body fat than men at the same BMI. A BMI in the high-normal range in an older adult may indicate more body fat than the same BMI in a young adult.

Ethnicity Affects Risk Thresholds

Multiple large-scale studies have shown that South Asian, East Asian, and other Asian populations develop metabolic disease at lower BMI values than European populations. Conversely, some African populations have lower cardiovascular risk at the same BMI. This means the standard WHO cutoffs are not universally optimal.

Better Complementary Metrics

For a fuller picture of body composition and metabolic health, consider these measurements alongside BMI:

Frequently Asked Questions

What is a normal BMI for adults?

According to the World Health Organization (WHO), a BMI between 18.5 and 24.9 is considered normal weight for adults. A BMI below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese.

Is BMI accurate for athletes and muscular people?

BMI is less accurate for highly muscular individuals because muscle is denser than fat. An athlete may have a BMI in the overweight range while having very low body fat. In these cases, body fat percentage or waist circumference measurements give a more accurate health picture.

How do I calculate BMI in imperial units (pounds and inches)?

The imperial BMI formula is: BMI = (weight in pounds ÷ height in inches²) × 703. For example, a person who weighs 154 lbs and is 5 ft 7 in (67 inches) tall: BMI = (154 ÷ 67²) × 703 = (154 ÷ 4489) × 703 = 0.03431 × 703 = 24.1.

Does BMI differ for men and women?

The BMI formula is the same for men and women. However, at the same BMI, women typically have a higher body fat percentage than men. Some health professionals use sex-specific interpretations, but the standard WHO categories apply to both sexes.

What is the BMI formula for children and teenagers?

Children and teens use the same BMI formula as adults, but the results are interpreted differently. Instead of fixed cutoffs, pediatric BMI is compared against age- and sex-specific growth charts using BMI-for-age percentiles. A child is considered overweight if their BMI is at or above the 85th percentile, and obese at or above the 95th percentile.

What are the limitations of using BMI as a health measure?

BMI does not measure body fat directly. It cannot distinguish between fat, muscle, and bone mass. It also does not account for fat distribution — abdominal (visceral) fat is more dangerous than fat stored elsewhere. Age, ethnicity, and sex affect what BMI values are associated with health risks. Waist circumference, body fat percentage, blood pressure, cholesterol, and blood sugar together give a much fuller picture of metabolic health.

Medical Disclaimer: This BMI calculator is for informational purposes only. BMI is a screening tool and not a diagnostic measure. It cannot assess your overall health, fitness level, or disease risk on its own. Consult a qualified healthcare professional before making any health or dietary decisions based on your BMI result.

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