BMI Calculator
Calculate your Body Mass Index to understand where you fall on the healthy weight spectrum
Calculate Your BMI
Enter your height and weight
What is BMI?
Body Mass Index (BMI) is a numerical value derived from your weight and height. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI has become the most widely used screening tool for categorizing individuals as underweight, normal weight, overweight, or obese.
While BMI doesn't directly measure body fat, research has shown it correlates reasonably well with more direct measures of body fat for most people. It's used by healthcare professionals worldwide as a quick, inexpensive first assessment of weight-related health risks.
How is BMI Calculated?
BMI uses a simple formula that relates weight to height:
Metric Formula
BMI = weight (kg) / height (m)²
Imperial Formula
BMI = (weight (lbs) × 703) / height (inches)²
For example, a person weighing 154 lbs (70 kg) who is 5'9" (175 cm) tall would have a BMI of 70 / (1.75 × 1.75) = 22.9 kg/m², which falls in the normal range.
BMI Categories (WHO Standards)
The World Health Organization (WHO) defines the following BMI categories for adults:
| Category | BMI Range (kg/m²) |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 - 17 |
| Mild Thinness | 17 - 18.5 |
| Normal | 18.5 - 25 |
| Overweight | 25 - 30 |
| Obese Class I | 30 - 35 |
| Obese Class II | 35 - 40 |
| Obese Class III | > 40 |
Limitations of BMI
While BMI is a useful screening tool, it has important limitations:
- Doesn't measure body composition — BMI can't distinguish between muscle, fat, bone, and water weight. Athletes with high muscle mass may be classified as overweight despite having low body fat.
- Doesn't account for fat distribution — Where you carry fat matters. Abdominal (visceral) fat is more dangerous than subcutaneous fat, but BMI doesn't differentiate between them.
- Varies across demographics — BMI thresholds may not be equally appropriate for all ethnic groups. For example, some Asian populations may have higher health risks at lower BMI values.
- Less reliable at extremes of height — Very tall or very short individuals may get inaccurate readings. The Ponderal Index can be a better alternative for these individuals.
- Age-related changes — Older adults tend to have more body fat and less muscle than younger adults at the same BMI.
For a more complete health picture, consider using BMI alongside waist circumference, body fat percentage, and other health markers.
Beyond BMI: BMI Prime & Ponderal Index
BMI Prime
BMI Prime is the ratio of your BMI to 25 (the upper limit of normal). A value of 1.0 means your BMI is exactly at the overweight threshold. Values below 0.74 suggest underweight, while values above 1.0 indicate overweight. It works across all unit systems, making international comparisons easier.
BMI Prime = BMI / 25
Ponderal Index
The Ponderal Index (also called Corpulence Index) uses the cube of height instead of the square, which makes it more accurate for very tall or very short people. A normal Ponderal Index is approximately 11 to 15 kg/m³.
Ponderal Index = weight (kg) / height (m)³
Frequently Asked Questions
What is BMI?
BMI (Body Mass Index) is a simple screening measure that uses your height and weight to estimate body fat. It's calculated by dividing your weight in kilograms by the square of your height in meters. While not a direct measure of body fat, BMI is a useful starting point for assessing whether you're at a healthy weight.
How is BMI calculated?
BMI is calculated using the formula: weight (kg) / height (m)². For imperial units, the formula is: (weight in pounds × 703) / (height in inches)². For example, a person who weighs 70 kg and is 1.75 m tall would have a BMI of 70 / (1.75 × 1.75) = 22.9.
What is a healthy BMI range?
For adults, a healthy BMI is generally between 18.5 and 25 kg/m². Below 18.5 is considered underweight, 25-30 is overweight, and above 30 is obese. However, these ranges may not apply equally to all populations, ages, or body types.
Is BMI accurate for athletes and muscular people?
BMI has limitations for athletes and very muscular individuals because it doesn't distinguish between muscle mass and fat mass. A muscular person may have a high BMI but a healthy body fat percentage. For these individuals, body fat percentage, waist circumference, or DEXA scans provide more accurate assessments.
Does BMI differ by age and gender?
Standard adult BMI categories are the same for men and women. However, women typically have more body fat than men at the same BMI. For children and teens (ages 2-20), BMI is interpreted using age- and sex-specific percentile charts because body composition changes as children grow.
What is BMI Prime?
BMI Prime is the ratio of your BMI to the upper limit of the normal range (25 kg/m²). A BMI Prime of 1.0 means your BMI is exactly 25. Values below 1.0 indicate normal or underweight, while values above 1.0 indicate overweight or obese. It provides a quick way to see how far you are from the healthy range boundary.
What is the Ponderal Index?
The Ponderal Index (PI) is similar to BMI but uses the cube of height instead of the square: weight (kg) / height (m)³. This makes it more accurate for very tall or very short individuals, where standard BMI can overestimate or underestimate body fat. A normal PI is around 11-15 kg/m³.
What are the health risks of high BMI?
A high BMI (overweight or obese) is associated with increased risk of heart disease, type 2 diabetes, high blood pressure, certain cancers, sleep apnea, and joint problems. However, BMI alone doesn't determine health — factors like fitness level, diet quality, and where you carry fat also matter.
Should I use BMI or body fat percentage?
Both have value. BMI is a quick, free screening tool that works well for most people. Body fat percentage is more accurate but harder to measure (requires calipers, DEXA, or bioimpedance scales). For a comprehensive picture, use BMI as a starting point and supplement with waist circumference or body fat measurements.
How often should I check my BMI?
For most adults, checking BMI every few months is sufficient unless you're actively trying to lose or gain weight. If you're on a weight management program, monthly checks can help track progress. Remember that day-to-day weight fluctuations are normal, so focus on trends over time rather than individual readings.
Go Beyond the Numbers
Track your nutrition, crush fitness challenges, and compete with friends on the JustLame app.
Download JustLame© 2026 JustLame. All rights reserved.