What BMI is and what it’s for
Body mass index relates your weight to your height with a very simple formula: BMI = weight (kg) ÷ height² (m), or BMI = 703 × weight (lb) ÷ height² (in) in US units. It was proposed by the Belgian statistician Adolphe Quetelet in the 19th century, and the key is what he designed it for: describing populations, not diagnosing individuals.
It’s still used because it’s cheap, fast and only needs a scale and a tape measure. As a first screening it works well. As a personal verdict, it doesn’t.
A worked example
Someone who is 5′9″ (69 inches) and weighs 160 lb: 703 × 160 ÷ 69² = 23.6. That’s a healthy weight, and the healthy range for that height runs from about 125 to 169 lb.
BMI categories for adults
| BMI | Category |
|---|---|
| Below 16 | Severely underweight |
| 16 – 18.4 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 – 34.9 | Obesity class 1 |
| 35 – 39.9 | Obesity class 2 |
| 40 or more | Obesity class 3 |
These ranges are for adults. For children and teens, and for older adults, BMI has to be read differently.
When BMI is misleading
- Lots of muscle. Muscle is denser than fat. A strength athlete can score 27 with 10% body fat.
- Older adults. With age you lose muscle and gain fat without the scale moving. A “healthy” BMI at 75 can hide muscle loss.
- Very tall or very short people. Because height is squared, BMI tends to penalize very tall people and flatter very short ones.
- Where the fat is. Two people with the same BMI face very different risks if one carries fat around the belly and the other around the hips and thighs.
Measures that complement BMI
Waist circumference
Measure around your waist just above the hip bones, standing and breathing out normally. It’s a direct sign of visceral fat, the kind around your organs that’s linked to type 2 diabetes and heart disease. Risk rises above 35 inches for women and 40 inches for men.
Waist-to-height ratio
Even simpler: waist ÷ height, in the same units. Below 0.5 is good. Put another way, your waist should measure less than half your height. It works the same for men and women.
Healthy weight by height
The weight range that corresponds to a BMI between 18.5 and 24.9:
| Height | Healthy weight |
|---|---|
| 5′0″ | 95 – 127 lb |
| 5′2″ | 101 – 136 lb |
| 5′4″ | 108 – 145 lb |
| 5′6″ | 115 – 154 lb |
| 5′8″ | 122 – 164 lb |
| 5′10″ | 129 – 174 lb |
| 6′0″ | 136 – 184 lb |
| 6′2″ | 144 – 194 lb |
| 6′4″ | 152 – 205 lb |
An important note
This calculator is for information only and doesn’t replace a visit to your doctor. A number outside the range isn’t a diagnosis, and a number inside it doesn’t guarantee you’re healthy. If you’re worried about your weight, talk to your doctor, who can look at your bloodwork, body composition and history, things a two-variable formula can’t see. To estimate how much to eat for your goal, try the calorie calculator.