The BMI Formula
BMI (Body Mass Index) is a simple ratio of weight to height:
Metric: BMI = weight (kg) / height (m)²
Imperial: BMI = (weight in lbs x 703) / (height in inches)²
Quick Example
For someone 5’9” (69 inches) weighing 170 lbs:
BMI = (170 x 703) / (69²) = 119,510 / 4,761 = 25.1
BMI Categories
The World Health Organization defines these ranges for adults:
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased (nutritional deficiency) |
| 18.5 - 24.9 | Normal weight | Lowest health risk |
| 25.0 - 29.9 | Overweight | Moderately increased |
| 30.0 - 34.9 | Obesity Class I | High |
| 35.0 - 39.9 | Obesity Class II | Very high |
| 40.0+ | Obesity Class III | Extremely high |
Healthy Weight Ranges by Height
A “normal” BMI (18.5-24.9) corresponds to these weight ranges:
| Height | Healthy Weight Range |
|---|---|
| 5’0” | 97 - 128 lbs |
| 5’2” | 104 - 136 lbs |
| 5’4” | 110 - 145 lbs |
| 5’6” | 118 - 155 lbs |
| 5’8” | 125 - 163 lbs |
| 5’10” | 132 - 174 lbs |
| 6’0” | 140 - 184 lbs |
| 6’2” | 148 - 194 lbs |
| 6’4” | 156 - 205 lbs |
Note: These ranges are broad for a reason. A 5’10” person at 132 lbs and 174 lbs are both considered “healthy” by BMI — their body composition, fitness level, and other factors determine actual health.
When BMI Is Misleading
BMI is a screening tool, not a diagnostic one. It fails for several groups:
Athletes and Muscular People
Muscle is denser than fat. A 6’0” bodybuilder at 210 lbs has a BMI of 28.5 (“overweight”) but might have 10% body fat — exceptionally lean.
Older Adults
After 65, a slightly higher BMI (25-27) is actually associated with lower mortality than the “normal” range. Muscle loss with aging means the same BMI represents more fat and less muscle.
Tall and Short People
BMI slightly overestimates fatness in tall people and underestimates it in short people because it uses height squared (not cubed), which doesn’t perfectly scale with body volume.
Different Ethnicities
Health risks at the same BMI vary by ethnicity. Asian populations face elevated health risks at lower BMIs (23+), while some Pacific Islander and Black populations may have lower risk at the same BMI due to different body composition patterns.
Better Alternatives to BMI
If BMI doesn’t tell the full story, what does?
Waist Circumference
- Men: Above 40 inches = elevated health risk
- Women: Above 35 inches = elevated health risk
Belly fat (visceral fat) is more dangerous than fat stored in hips/thighs, and waist measurement captures this directly.
Waist-to-Height Ratio
Keep your waist circumference below half your height. A 5’10” (70 inch) person should aim for a waist under 35 inches. Simple, effective, and more predictive than BMI alone.
Body Fat Percentage
| Rating | Men | Women |
|---|---|---|
| Essential fat | 2-5% | 10-13% |
| Athletic | 6-13% | 14-20% |
| Fit | 14-17% | 21-24% |
| Average | 18-24% | 25-31% |
| Obese | 25%+ | 32%+ |
Measured via DEXA scan (most accurate), bioelectrical impedance scales (convenient), or skinfold calipers (cheap but requires skill).
Fitness Level
A “overweight” person who exercises regularly and has good cardiovascular fitness often has better health outcomes than a “normal weight” person who is sedentary. Fitness matters more than the scale number.
What To Do With Your BMI
If your BMI is 18.5-24.9: You’re in the statistically lowest-risk range. Focus on maintaining fitness and healthy habits rather than the number itself.
If your BMI is 25-29.9: Don’t panic. Check your waist circumference and activity level. If you exercise regularly and carry muscle, you may be perfectly healthy. If not, a modest calorie reduction (300-500 calories/day) and regular movement can make a meaningful difference.
If your BMI is 30+: This correlates with increased health risks regardless of fitness. Consider speaking with a healthcare provider about a sustainable plan. Losing even 5-10% of body weight significantly reduces risk factors.
If your BMI is below 18.5: Being underweight carries its own risks (weakened immune system, bone loss, nutritional deficiency). If unintentional, consult a doctor.
The Bottom Line
BMI is useful as a quick population-level screening tool, but it’s a blunt instrument for individuals. Use it as one data point alongside:
- How you feel and your energy levels
- Your fitness and physical capabilities
- Waist circumference
- Blood work (cholesterol, blood sugar, etc.)
- Body composition (if available)
No single number defines your health. BMI is a starting point, not a verdict.