BMI Calculator
Calculate your Body Mass Index in metric or imperial — get your BMI score, WHO weight category, healthy weight range, and BMI Prime instantly.
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Enter your weight and height to instantly calculate your BMI and healthy weight range.
What Is BMI?
Body Mass Index (BMI) is a number calculated from your weight and height that provides a simple screening tool for weight categories — underweight, normal weight, overweight, and obese. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is now the most widely used measure of body weight status globally.
BMI does not directly measure body fat, but research shows it is moderately correlated with more direct measures of body fat obtained from skinfold thickness, bioelectrical impedance, and dual-energy X-ray absorptiometry (DEXA). The WHO and CDC use BMI cutpoints to screen populations, track obesity trends, and guide public health interventions.
How to Calculate BMI in Metric Units (kg and cm)
The metric BMI formula divides your weight in kilograms by the square of your height in metres. The result is a dimensionless number.
Convert height to metres
height (m) = height (cm) ÷ 100
Example: 175 cm → 1.75 m
Apply the formula
BMI = weight (kg) ÷ height (m)²
Example: 70 kg ÷ (1.75)² = 70 ÷ 3.0625 = 22.9
Metric Worked Example
Weight: 70 kg Height: 175 cm → 1.75 m
BMI = 70 ÷ (1.75 × 1.75)
BMI = 70 ÷ 3.0625
BMI = 22.9 → Normal weight
How to Calculate BMI in Imperial Units (lbs and inches)
The imperial BMI formula multiplies a conversion factor of 703 by your weight in pounds, then divides by the square of your total height in inches.
Convert height to total inches
height (in) = (ft × 12) + in
Example: 5 ft 9 in = (5 × 12) + 9 = 69 in
Apply the imperial formula
BMI = 703 × weight (lbs) ÷ height (in)²
Example: 703 × 154 ÷ 69² = 108262 ÷ 4761 = 22.7
Imperial Worked Example
Weight: 154 lbs Height: 5 ft 9 in = 69 in
BMI = 703 × 154 ÷ (69 × 69)
BMI = 108,262 ÷ 4,761
BMI = 22.7 → Normal weight
BMI Formula Reference
Metric Formula
BMI = weight (kg) ÷ height (m)²
Imperial Formula
BMI = 703 × weight (lbs) ÷ height (in)²
BMI Prime (ratio to upper normal)
BMI Prime = BMI ÷ 25
Values above 1.0 exceed the upper normal boundary. Values below 0.74 indicate underweight.
BMI Categories and What They Mean
The World Health Organization (WHO) defines six adult BMI categories. These cutpoints apply to men and women of all ages, though some health organisations use adjusted cutpoints for Asian populations.
| BMI Range | Category | Health Risk |
|---|---|---|
| < 18.5 | Underweight | Increased risk of malnutrition, osteoporosis |
| 18.5 – 24.9 | Normal weight | Lowest risk for weight-related conditions |
| 25.0 – 29.9 | Overweight | Elevated risk of cardiovascular disease, type 2 diabetes |
| 30.0 – 34.9 | Obese (Class I) | High risk — consult a healthcare provider |
| 35.0 – 39.9 | Obese (Class II) | Very high risk — medical evaluation recommended |
| ≥ 40.0 | Obese (Class III) | Extremely high risk — specialist care advised |
What Is a Healthy BMI Range?
A healthy BMI is between 18.5 and 24.9. This corresponds to a different weight range depending on your height. The table below shows approximate healthy weight ranges for common heights.
| Height (Metric) | Height (Imperial) | Healthy Weight (kg) | Healthy Weight (lbs) |
|---|---|---|---|
| 155 cm | 5 ft 1 in | 44.4 – 59.8 kg | 97.9 – 131.8 lbs |
| 160 cm | 5 ft 3 in | 47.4 – 63.7 kg | 104.5 – 140.4 lbs |
| 165 cm | 5 ft 5 in | 50.4 – 67.7 kg | 111.1 – 149.3 lbs |
| 170 cm | 5 ft 7 in | 53.5 – 71.9 kg | 117.9 – 158.5 lbs |
| 175 cm | 5 ft 9 in | 56.7 – 76.3 kg | 125.0 – 168.2 lbs |
| 180 cm | 5 ft 11 in | 59.9 – 80.7 kg | 132.1 – 177.9 lbs |
| 185 cm | 6 ft 1 in | 63.3 – 85.2 kg | 139.5 – 187.8 lbs |
Limitations of BMI
BMI is a useful population-level screening tool, but it has important limitations when applied to individuals. Understanding these helps you interpret your result in proper context.
Athletes and muscular people
Muscle is denser than fat, so highly muscular individuals often have a BMI in the overweight or obese range despite having very low body fat. An NFL lineman or powerlifter may have BMI 30+ but minimal health risk for their body composition.
Older adults
Older adults tend to lose muscle mass and gain fat with age — a process called sarcopenia. This means an older person can have a "normal" BMI while carrying a disproportionately high fat percentage and a significant cardiovascular risk.
Ethnic differences
Research shows that people of Asian descent have higher cardiometabolic risk at lower BMI values. Several Asian countries use adjusted cutpoints: overweight is defined as BMI ≥ 23 and obese as BMI ≥ 27.5 in some guidelines.
Pregnancy and children
This calculator is for adults. BMI during pregnancy does not apply in the same way. for children and teenagers, age- and sex-specific BMI percentile charts (CDC growth charts) must be used instead of adult cutpoints.
BMI Calculator FAQ
What is a healthy BMI for adults?
According to the World Health Organization, a healthy BMI for adults is between 18.5 and 24.9. A BMI below 18.5 is underweight, 25.0–29.9 is overweight, and 30 or above is classified as obese. These cutpoints are the same for men and women.
How do I calculate my BMI without a calculator?
In metric: divide your weight in kilograms by your height in metres squared. for example, if you weigh 70 kg and are 1.75 m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9. In imperial: multiply your weight in pounds by 703, then divide by your height in inches squared.
What is the difference between metric and imperial BMI?
The underlying BMI value is identical — only the input units differ. The metric formula uses kg and metres. The imperial formula uses lbs and inches, and multiplies by 703 to convert the result to the same dimensionless BMI scale. Our calculator handles both automatically.
Is BMI accurate for athletes or muscular people?
Not reliably. Muscle weighs more than fat per unit of volume, so athletes with high muscle mass often score in the overweight or obese BMI range despite having very low body fat. In these cases, waist circumference, body fat percentage (measured via DEXA or skinfold testing), or waist-to-hip ratio are better indicators of health risk.
What does a BMI over 30 mean?
A BMI of 30 or above is classified as obese. Obesity is associated with increased risk of type 2 diabetes, cardiovascular disease, certain cancers, sleep apnoea, and other conditions. BMI 30–34.9 is Class I (moderate risk), 35–39.9 is Class II (severe risk), and 40 or above is Class III (very severe). A healthcare provider can help determine the most appropriate course of action.
What is BMI Prime and how is it different from BMI?
BMI Prime is the ratio of your BMI to the upper limit of the normal weight category (25.0). A BMI Prime of 1.0 exactly equals a BMI of 25. Values below 0.74 indicate underweight, 0.74–1.0 is normal, and values above 1.0 indicate overweight or obesity. It makes it easier to see at a glance how far from the upper normal boundary a person is.
Can BMI tell me my body fat percentage?
BMI cannot directly measure body fat percentage. It is a proxy — a screening tool, not a diagnostic one. Research shows a moderate correlation between BMI and body fat in population studies, but for an individual the same BMI can correspond to very different body fat levels depending on age, sex, ethnicity, and muscle mass. DEXA scans and air displacement plethysmography are gold-standard methods for measuring body fat.
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