How to Use This BMI Calculator
Enter your weight and height using either metric (kg/cm) or imperial (lbs/ft) units. The calculator instantly displays your BMI score and weight category. The visual indicator shows where you fall on the BMI scale from underweight to obese.
What Is BMI?
Body Mass Index (BMI) is a screening tool that estimates body fat based on height and weight. It was developed in the early 19th century by Belgian mathematician Adolphe Quetelet as a simple way to assess whether someone's weight falls within a healthy range relative to their height.
BMI is the most widely used population-level indicator of healthy weight because it's simple, inexpensive, and correlates reasonably well with body fat percentage in most adults. However, it's important to understand what BMI can and cannot tell you. According to the CDC, BMI is a screening tool but not a diagnostic tool.
BMI Categories
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency risk |
| 18.5 – 24.9 | Normal weight | Lowest risk |
| 25.0 – 29.9 | Overweight | Moderate risk |
| 30.0 – 34.9 | Obese Class I | High risk |
| 35.0+ | Obese Class II+ | Very high risk |
BMI Formula
The BMI formula differs slightly between metric and imperial units:
Example Calculation
For someone weighing 70 kg and measuring 1.75 m (175 cm):
A BMI of 22.9 falls squarely in the "Normal weight" category (18.5–24.9).
Healthy Weight Ranges by Height
Based on a BMI of 18.5–24.9, here are healthy weight ranges for common heights:
| Height | Healthy Weight Range (BMI 18.5–24.9) |
|---|---|
| 5'0" (152 cm) | 114–145 lbs (52–66 kg) |
| 5'4" (163 cm) | 126–162 lbs (57–74 kg) |
| 5'8" (173 cm) | 140–179 lbs (64–81 kg) |
| 6'0" (183 cm) | 154–197 lbs (70–89 kg) |
| 6'4" (193 cm) | 169–217 lbs (77–98 kg) |
Limitations of BMI
BMI is a useful screening tool but has well-documented limitations:
- Muscle mass: Athletes and muscular individuals often have "overweight" or "obese" BMIs despite having low body fat. Muscle is denser than fat, so a muscular person can weigh more without having excess body fat.
- Age: Older adults often have a healthy BMI despite having more body fat, because muscle mass decreases with age.
- Sex differences: Women typically have more body fat than men at the same BMI. The standard BMI ranges apply to both sexes equally, though some researchers suggest slightly higher cutoffs for women.
- Fat distribution: BMI doesn't measure where fat is stored. Abdominal fat (apple shape) carries higher health risks than fat stored in the hips and thighs (pear shape).
- Ethnic differences: Health risks associated with BMI vary by ethnicity. For example, South Asian, Chinese, and Southeast Asian populations have health risks at lower BMIs.
Better Alternatives to BMI
If BMI doesn't give you the full picture, consider these complementary metrics:
- Waist-to-Height Ratio: More accurate predictor of health risk than BMI. Divide your waist circumference by your height. A ratio above 0.5 may indicate increased health risk.
- Waist Circumference: Measure around your natural waist. Risk increases above 40 inches (men) or 35 inches (women).
- Body Fat Percentage: Directly measures the proportion of fat in your body. Use calipers, bioelectrical impedance, or DEXA scans for accuracy.
- Body Roundness Index: A newer metric that estimates body fat and lean mass more accurately than BMI.
How to Use Your BMI Result
BMI is best used as one piece of information among many. Here's how to interpret your result:
- BMI in healthy range (18.5–24.9): Maintain your current habits. Focus on regular physical activity and balanced nutrition.
- BMI below 18.5: Consider consulting a healthcare provider. Being underweight can indicate nutritional deficiencies, eating disorders, or underlying health conditions.
- BMI above 25: BMI alone doesn't mean you need to lose weight, but it's worth discussing with your doctor. They can assess other risk factors like blood pressure, cholesterol, and blood sugar.
Remember: The goal isn't a specific BMI number — it's overall health and wellbeing.