BMI — Body Mass Index — is the most widely used screening tool for weight classification. While it has limitations, it remains the standard starting point used by doctors and health organizations worldwide. Here’s everything you need to know about how to calculate it, what the numbers mean, and when to look beyond BMI.
What Is BMI?
Body Mass Index is a numerical value calculated from your height and weight. It was developed in the 1830s and is still used today as a quick, cost-free way to classify weight status in adults. It does not directly measure body fat, but it correlates well with more precise measures in most populations.
How to Calculate BMI
Metric Formula (kg / m²)
BMI = Weight (kg) ÷ Height (m)²
Example: Weight = 70 kg | Height = 1.75 m
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
Imperial Formula (lbs / inches²)
BMI = (Weight in lbs ÷ Height in inches²) × 703
Example: Weight = 154 lbs | Height = 5’9″ (69 inches)
BMI = (154 ÷ 69²) × 703 = (154 ÷ 4,761) × 703 = 22.7
BMI Categories (Adults)
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased |
| 18.5 – 24.9 | Normal weight | Lowest |
| 25.0 – 29.9 | Overweight | Increased |
| 30.0 – 34.9 | Obese Class I | High |
| 35.0 – 39.9 | Obese Class II | Very High |
| 40.0 and above | Obese Class III | Extremely High |
Limitations of BMI
BMI is a useful screening tool, but it has real limitations you should be aware of:
- It doesn’t measure body fat directly — a muscular athlete may show as “overweight”
- It doesn’t account for fat distribution — belly fat is more dangerous than thigh fat
- It varies by ethnicity — Asian populations have higher health risks at lower BMI values
- It’s less accurate for the elderly — muscle loss with age can mask higher fat percentages
BMI for Children and Teens
For anyone under 18, BMI is interpreted differently. Instead of fixed thresholds, a child’s BMI is compared to age- and sex-specific growth charts to determine a percentile. A BMI at or above the 95th percentile is considered obese.
What to Do If Your BMI Is High
A high BMI is a signal to investigate further — not a diagnosis. Talk to your doctor. Consider complementary metrics like waist circumference, body fat percentage, and blood work. Lifestyle changes (diet and exercise) remain the most evidence-based approach to reducing health risk.
For calorie needs alongside BMI, check our Calorie Calculator.
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