Health
How to Calculate BMI (Body Mass Index)
Learn the BMI formula in both metric and imperial units, understand BMI categories, and know the limitations of this common health metric.
- 5 min read
What Is BMI?
Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. It provides a simple screening tool to categorize individuals into weight categories such as underweight, normal weight, overweight, and obese. BMI is used worldwide by healthcare providers as a quick initial assessment, although it has important limitations.
The BMI Formula
Metric Formula
Imperial Formula
Metric Example
Weight: 75 kg, Height: 1.75 m:
- Square the height: 1.75 × 1.75 = 3.0625
- Divide weight by result: 75 ÷ 3.0625 = 24.5 BMI
Imperial Example
Weight: 165 lbs, Height: 5'9" (69 inches):
- Square the height: 69 × 69 = 4,761
- Multiply weight by 703: 165 × 703 = 115,995
- Divide: 115,995 ÷ 4,761 = 24.4 BMI
BMI Categories
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased risk of nutritional deficiency |
| 18.5 - 24.9 | Normal weight | Lowest health risk |
| 25.0 - 29.9 | Overweight | Moderately increased risk |
| 30.0 - 34.9 | Obese (Class I) | High risk |
| 35.0 - 39.9 | Obese (Class II) | Very high risk |
| 40.0 and above | Obese (Class III) | Extremely high risk |
BMI Reference Table by Height
Here is what different BMI values correspond to at common heights:
| Height | Normal (BMI 22) | Overweight (BMI 27) | Obese (BMI 32) |
|---|---|---|---|
| 5'2" (157 cm) | 121 lbs / 55 kg | 148 lbs / 67 kg | 175 lbs / 80 kg |
| 5'6" (168 cm) | 136 lbs / 62 kg | 167 lbs / 76 kg | 198 lbs / 90 kg |
| 5'10" (178 cm) | 153 lbs / 69 kg | 188 lbs / 85 kg | 223 lbs / 101 kg |
| 6'0" (183 cm) | 162 lbs / 74 kg | 199 lbs / 90 kg | 236 lbs / 107 kg |
| 6'2" (188 cm) | 172 lbs / 78 kg | 211 lbs / 96 kg | 250 lbs / 113 kg |
Limitations of BMI
BMI is a screening tool, not a diagnostic tool. It does not directly measure body fat, muscle mass, or overall health.
- Muscle mass: Athletes and bodybuilders may have a high BMI but low body fat. A muscular person at 5'10" weighing 200 lbs has a BMI of 28.7 (overweight), even if they have low body fat.
- Age: Older adults tend to have more body fat than younger adults at the same BMI.
- Sex: Women generally have more body fat than men at the same BMI.
- Ethnicity: Health risks at specific BMI values vary among different ethnic groups.
- Fat distribution: BMI does not account for where fat is stored. Abdominal fat is more dangerous than fat stored in hips and thighs.
Better Alternatives to Consider
- Waist circumference: A waist over 40 inches (men) or 35 inches (women) indicates increased health risk.
- Waist-to-hip ratio: Provides insight into fat distribution.
- Body fat percentage: Measured with calipers, DEXA scan, or bioelectrical impedance.
- Consult a healthcare professional: BMI should be one of many factors considered in a health assessment.
Frequently Asked Questions
How do you calculate BMI?
Divide your weight in kilograms by your height in metres squared: BMI = kg ÷ m². For 75 kg and 1.75 m that is 75 ÷ 3.0625 = 24.5. In pounds and inches, multiply the weight by 703 first: BMI = (lb × 703) ÷ in².
What is a healthy BMI range?
A BMI of 18.5 to 24.9 counts as normal weight. Below 18.5 is underweight, 25.0 to 29.9 is overweight, and 30.0 or more is classed as obese.
Is BMI accurate for everyone?
No. BMI is a screening tool, not a diagnosis: it cannot tell muscle from fat, so a lean athlete can score as overweight, and it says nothing about where fat is stored. Read it alongside a waist measurement and medical advice.