How to Calculate BMI (Body Mass Index)

Body Mass Index (BMI) is a screening number derived from your height and weight. Doctors, insurers, and public health researchers use it as a quick proxy for body fat levels across large populations. While it has real limitations at the individual level, understanding how to calculate BMI and what the result means gives you a useful starting point for health conversations.

This guide walks through the formula, WHO categories, worked examples, and the situations where BMI falls short. For instant results, try our Free BMI Calculator.

Medical Disclaimer: BMI is a screening tool, not a diagnostic measure. It does not account for muscle mass, bone density, body composition, or ethnic differences in health risk. Always consult a qualified healthcare provider before making decisions about your weight or health based on BMI alone.

The BMI Formula

BMI is calculated with a straightforward equation.

Metric Formula:

$BMI = \frac{Weight \ (kg)}{Height \ (m)^2}$

Imperial Formula:

$BMI = \frac{Weight \ (lbs) \times 703}{Height \ (in)^2}$

The constant 703 in the imperial version converts pounds and inches into the same scale as the metric formula.

Steps (Metric):

1. Measure your weight in kilograms.
2. Measure your height in meters.
3. Square your height (multiply it by itself).
4. Divide your weight by the squared height.

Steps (Imperial):

1. Measure your weight in pounds.
2. Measure your height in inches (feet x 12 + remaining inches).
3. Square your height.
4. Divide your weight by the squared height.
5. Multiply by 703.

Worked Examples

Example 1 (Metric): A person weighs 72 kg and stands 1.75 m tall.

• Square the height: 1.75 x 1.75 = 3.0625
• Divide: 72 / 3.0625 = 23.5

Example 2 (Imperial): A person weighs 185 lbs and stands 5’10” (70 inches).

• Square the height: 70 x 70 = 4,900
• Divide weight by squared height: 185 / 4,900 = 0.03776
• Multiply by 703: 0.03776 x 703 = 26.5

Example 3 (Metric): A person weighs 58 kg and stands 1.62 m tall.

• Square the height: 1.62 x 1.62 = 2.6244
• Divide: 58 / 2.6244 = 22.1

WHO BMI Categories

The World Health Organization defines the following adult BMI ranges:

BMI Range	Category
Below 18.5	Underweight
18.5 - 24.9	Normal weight
25.0 - 29.9	Overweight
30.0 - 34.9	Obese (Class I)
35.0 - 39.9	Obese (Class II)
40.0 and above	Obese (Class III)

These cutoffs were developed from population-level data linking BMI ranges to health outcomes such as cardiovascular disease, type 2 diabetes, and overall mortality.

From the examples above: a BMI of 23.5 falls in the normal range, 26.5 falls in the overweight range, and 22.1 falls in the normal range.

When BMI Is Useful

BMI works best as a population screening tool. Epidemiologists studying thousands of people find strong correlations between elevated BMI and chronic disease risk. At this scale, individual variations in muscle mass and body composition average out.

BMI is useful in several specific contexts:

• Primary care screening: Doctors use BMI as a first-pass flag during annual checkups. An elevated number prompts further testing, not an immediate diagnosis.
• Tracking trends over time: If your personal BMI increases from 24 to 29 over three years with no change in exercise habits, that trajectory is meaningful regardless of your body composition.
• Insurance and public health policy: BMI provides a standardized metric that can be applied consistently across millions of individuals.
• Research studies: Large-scale epidemiological research relies on BMI because height and weight data are easy to collect accurately.

When BMI Is Misleading

BMI has well-documented blind spots that matter at the individual level.

Muscle mass: BMI can’t tell the difference between muscle and fat tissue. A powerlifter standing 5’10” at 220 lbs of mostly lean muscle registers a BMI of 31.6 (Obese Class I). Meanwhile, a sedentary person at the same height and weight with high body fat gets the same score. The health profiles of these two people are completely different.

Age: Older adults tend to lose muscle mass and gain fat even when their weight stays stable. A 70-year-old with a “normal” BMI of 23 may carry more visceral fat than a 30-year-old with the same number.

Ethnicity: Research published in The Lancet and other journals has shown that health risks associated with BMI vary across ethnic groups. South Asian and East Asian populations tend to develop metabolic complications at lower BMI thresholds (around 23-24) compared to European populations. Pacific Islander populations may carry less metabolic risk at higher BMI values.

Sex differences: Women naturally carry a higher percentage of body fat than men. Two individuals with a BMI of 25 may have very different body fat percentages depending on sex.

Height extremes: The squared height in the denominator causes BMI to overestimate fatness in very tall people and underestimate it in very short people. A 6’4” person gets penalized more per pound than a 5’4” person.

Children and teens: Standard BMI cutoffs don’t apply to anyone under 20. Pediatric BMI uses age- and sex-specific percentile charts because body composition changes rapidly during growth.

Better Alternatives and Complements

Because BMI has these gaps, health professionals often combine it with other measurements:

• Waist circumference: A waist above 40 inches for men or 35 inches for women indicates elevated visceral fat risk, regardless of BMI.
• Waist-to-hip ratio: Dividing waist measurement by hip measurement provides another indicator of fat distribution.
• Body fat percentage: Methods like DEXA scans, skinfold calipers, or bioelectrical impedance directly measure fat vs. lean tissue. Our Body Fat Calculator uses the Navy method for a quick estimate.
• Waist-to-height ratio: Keeping your waist circumference below half your height is a simple screening guideline supported by recent research.

None of these measures are perfect on their own. The best picture of health comes from combining multiple data points with clinical assessment.

Quick BMI Reference by Height

For quick reference, here are the weights that correspond to a BMI of 25 (the boundary between normal and overweight) at various heights:

Height	Weight at BMI 25
5’2” (157 cm)	136 lbs (62 kg)
5’5” (165 cm)	150 lbs (68 kg)
5’8” (173 cm)	164 lbs (74 kg)
5’10” (178 cm)	174 lbs (79 kg)
6’0” (183 cm)	184 lbs (84 kg)
6’2” (188 cm)	194 lbs (88 kg)

Skip the Math

Our BMI Calculator handles the conversion and categorization instantly. Enter your height and weight in either metric or imperial units and get your BMI with the corresponding WHO category.

Frequently Asked Questions

What is a healthy BMI for adults?

The WHO defines a healthy BMI as 18.5 to 24.9 for adults over 20 years old. This range is associated with the lowest statistical risk for weight-related health conditions in population studies. However, individual health depends on many factors beyond BMI, including fitness level, body composition, family history, and blood markers like cholesterol and blood glucose.

Does BMI apply to children?

Not in the same way. For children and teens aged 2 to 19, BMI is calculated using the same formula but then plotted on CDC growth charts that account for age and sex. A child in the 85th to 94th percentile is considered overweight, and at or above the 95th percentile is considered obese. The raw number thresholds used for adults (25, 30) don’t apply.

Can I have a normal BMI but still be unhealthy?

Yes. This is sometimes called “normal weight obesity” or “thin outside, fat inside” (TOFI). A person with a normal BMI can still carry excess visceral fat around their organs, have high blood pressure, elevated blood sugar, or poor cholesterol ratios. This is why doctors look at multiple indicators, not BMI alone. Waist circumference and blood work provide a more complete picture.

Why is BMI still used if it has so many flaws?

Simplicity and scalability. BMI requires only two measurements that anyone can take at home: height and weight. No special equipment, no trained technician, no lab visit. For public health surveillance across millions of people, this practicality is hard to beat. The key is understanding BMI as a screening tool that flags potential concerns, not as a definitive health verdict.

Should athletes ignore their BMI?

Athletes with significant muscle mass should interpret BMI cautiously. A competitive rugby player, sprinter, or weightlifter will often register as overweight or obese by BMI despite having low body fat. For these individuals, body fat percentage, waist circumference, or a DEXA scan provides much more meaningful data. That said, even athletes benefit from tracking BMI over time as one data point among many.