A BMI chart maps your height against your weight to produce a single number, and that number falls into one of four main categories: underweight (below 18.5), healthy weight (18.5 to 24.9), overweight (25.0 to 29.9), or obese (30.0 or higher). Reading the chart itself is straightforward once you know where to look, but understanding what your number actually means takes a bit more context.
How a BMI Chart Is Set Up
Most printed BMI charts use a grid format. Heights run along one axis (usually the left side), and weights run along the other (usually the top or bottom). You find your height, trace across to your weight, and the number at the intersection is your BMI. The grid is often color-coded: green for the healthy range, yellow or orange for overweight, and red for obese.
If you’d rather calculate the number directly, the formula in standard U.S. units is: weight in pounds divided by height in inches squared, then multiplied by 703. In metric units, it’s weight in kilograms divided by height in meters squared. A person who is 5’6″ and 155 pounds, for example, has a BMI of about 25.0, which lands right at the boundary between healthy weight and overweight.
What Each BMI Category Means
Once you have your number, here’s where it falls:
- Below 18.5 (Underweight): May signal nutritional deficiency or an underlying health issue.
- 18.5 to 24.9 (Healthy Weight): Generally associated with the lowest risk of weight-related health problems.
- 25.0 to 29.9 (Overweight): Carries a moderately elevated risk for conditions like high blood pressure and Type 2 diabetes.
- 30.0 to 34.9 (Class 1 Obesity, low risk): Risk for chronic disease increases notably.
- 35.0 to 39.9 (Class 2 Obesity, moderate risk): Higher likelihood of cardiovascular disease, sleep apnea, and joint problems.
- 40.0 or higher (Class 3 Obesity, high risk): Sometimes called severe obesity, associated with the greatest health risks.
These ranges were developed using large population studies that tracked disease rates at different body weights. They’re useful starting points, but they don’t capture the full picture for every individual.
Reading a BMI Chart for Children and Teens
Kids and teenagers use a completely different system. Because children’s body composition changes rapidly as they grow, a fixed number like 25.0 doesn’t mean the same thing for a 10-year-old as it does for a 40-year-old. Instead, a child’s BMI is plotted on a growth chart that compares them to other children of the same age and sex, producing a percentile rather than a flat category.
The CDC’s percentile ranges for anyone under 20 are:
- Below the 5th percentile: Underweight
- 5th to less than the 85th percentile: Healthy weight
- 85th to less than the 95th percentile: Overweight
- 95th percentile or above: Obesity
So if your child’s BMI lands at the 70th percentile, that means they have a higher BMI than 70% of children their age and sex, and they fall squarely in the healthy range. The adult calculator applies starting at age 20.
When the Standard Ranges Don’t Fit
BMI measures total body weight relative to height. It cannot distinguish between muscle, fat, bone, and water. This creates predictable blind spots.
Athletes and people with significant muscle mass often register as overweight or obese on a BMI chart despite carrying very little excess fat. On the other end, older adults tend to lose muscle and gain fat as they age, so their BMI can look “normal” while their actual body fat percentage is elevated. For older individuals, a number in the healthy range may mask meaningful changes in body composition that affect mobility and metabolic health.
Ethnicity matters, too. The World Health Organization recommends lower cut-off points for people of Asian descent: overweight starting at 23 (instead of 25) and obese at 27.5 (instead of 30). Research on Asian American populations found that these adjusted thresholds better predict the actual health risks associated with excess body fat in these groups. If you’re of South Asian, East Asian, or Southeast Asian background, the standard chart may underestimate your risk.
What BMI Doesn’t Tell You
The American Medical Association issued a policy statement noting that BMI has “significant limitations” when used alone in clinical settings, and recommended pairing it with other measurements. The reason is simple: where you carry fat matters as much as how much you weigh.
Waist circumference and waist-to-hip ratio are two of the most practical supplements. A large study on heart attack risk found that waist-to-hip ratio was a stronger predictor of cardiovascular events than BMI. Even among people with a “normal” BMI below 25, a high waist-to-hip ratio still signaled elevated risk. The researchers found that once waist-to-hip ratio was accounted for, the direct relationship between BMI and heart attack risk essentially disappeared. Suggested cut-offs for elevated cardiovascular risk are a waist-to-hip ratio above 0.90 for men and above 0.83 for women.
To measure your own waist-to-hip ratio, wrap a flexible tape measure around the narrowest part of your waist (usually just above your belly button) and then around the widest part of your hips. Divide the waist number by the hip number. This 30-second measurement adds context that a BMI chart alone simply cannot provide.
How to Use Your BMI Practically
Think of BMI as a screening tool, not a diagnosis. It’s a quick, free way to flag whether your weight might be putting you at higher risk for chronic conditions. If your number falls in the healthy range and you don’t have other risk factors, you’re likely in good shape. If it’s elevated, it’s worth looking deeper with waist measurements, blood pressure readings, and basic blood work like cholesterol and blood sugar levels.
Your BMI can also be useful for tracking trends over time. A number that creeps upward by a point or two over several years signals gradual weight gain that might otherwise go unnoticed. Conversely, a steady decline in an older adult could reflect loss of muscle mass rather than healthy weight loss. The direction of change often matters as much as the number itself.