BMI Calculator
Calculate your Body Mass Index (BMI) instantly using height and weight in metric or imperial units. See your BMI category, healthy weight range, BMI Prime, and a full explanation of what your result means and its limitations.
Your BMI Result
Understanding Your BMI Result
Body Mass Index (BMI) is a number calculated from your height and weight that is widely used as a screening tool for weight categories. The World Health Organization (WHO) and most health authorities use the following standard categories:
| BMI Range | Category | Health Implication |
|---|---|---|
| Below 18.5 | Underweight | May indicate nutritional deficiency or underlying health issue |
| 18.5 – 24.9 | Normal Weight | Associated with lowest health risk in most populations |
| 25.0 – 29.9 | Overweight | Increased risk of cardiovascular disease, type 2 diabetes |
| 30.0 – 34.9 | Obese (Class I) | High risk; weight loss is recommended |
| 35.0 – 39.9 | Obese (Class II) | Very high risk |
| 40.0 and above | Obese (Class III) | Extremely high risk; medical intervention often advised |
The BMI Formula
BMI is calculated using this formula (metric units):
In imperial units, the formula is adjusted: BMI = (Weight in lbs ÷ Height in inches2) × 703
Example: A person who weighs 70 kg and is 1.75 m tall has a BMI of: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 — Normal Weight.
What Is BMI Prime?
BMI Prime is your BMI divided by 25 (the upper boundary of the Normal range). A value of exactly 1.0 means you're at the top of Normal weight. Below 1.0 is normal or underweight; above 1.0 is overweight or obese. It gives a quick sense of how far you are from the threshold — for example, a BMI Prime of 1.2 means you are 20% above the upper limit of normal weight.
Important Limitations of BMI
BMI is a useful population-level screening tool, but it has well-documented limitations you should understand before interpreting your result:
- It cannot distinguish muscle from fat. A highly muscular athlete may have a BMI in the "Overweight" range while having very low body fat. BMI would incorrectly flag them as unhealthy.
- It ignores fat distribution. Where fat is stored matters as much as how much you have. Visceral fat (around the abdomen) carries higher health risk than fat stored on the hips and thighs, but BMI measures neither.
- It varies by age, sex, and ethnicity. Older adults naturally lose muscle mass, meaning BMI can underestimate body fat. Research also suggests that Asian populations face elevated health risks at lower BMI thresholds than the standard WHO categories.
- It is not diagnostic. BMI is a screening tool, not a diagnosis. A high BMI does not mean you are unhealthy, and a normal BMI does not guarantee good health.
Better Measures to Use Alongside BMI
For a more complete picture of your health, consider these complementary measurements:
- Waist circumference: A waist measurement above 35 inches (women) or 40 inches (men) is associated with higher metabolic risk, regardless of BMI.
- Waist-to-height ratio: A ratio below 0.5 (waist circumference ÷ height) is a strong predictor of healthy weight. Some researchers consider this more accurate than BMI.
- Body fat percentage: Measured via DEXA scan, hydrostatic weighing, or a bioelectrical impedance scale. Healthy ranges are roughly 14–24% for men and 21–31% for women, depending on age.
Use our Body Fat Calculator to estimate your body fat percentage using the U.S. Navy method.
BMI for Children and Teens
This calculator is designed for adults aged 18 and older. BMI for children and teenagers (ages 2–19) is interpreted differently because body composition changes significantly during growth. For children, BMI is plotted on age- and sex-specific growth charts and expressed as a percentile. Consult a pediatrician for a child's weight assessment.
Frequently Asked Questions
According to WHO standards, a BMI between 18.5 and 24.9 is considered Normal Weight for adults. However, optimal healthy weight varies by age, sex, ethnicity, and body composition. A BMI in the normal range is a useful indicator but not a complete measure of health.
No. BMI can significantly overestimate body fat in muscular or athletic individuals because muscle is denser than fat. A bodybuilder or competitive athlete may register as "Overweight" or "Obese" by BMI while having very low body fat. For athletes, body fat percentage is a more meaningful measure.
BMI Prime is your BMI divided by 25, the upper limit of the Normal range. A BMI Prime of 1.0 means you are exactly at the normal/overweight boundary. Below 1.0 is normal or underweight; above 1.0 is overweight. A BMI Prime of 1.2, for example, means you are 20% above the upper normal limit.
No. Children and teenagers (ages 2–19) use a different BMI scale that accounts for age and sex because body composition changes significantly during growth. For children, BMI is expressed as a percentile compared to peers of the same age and sex. This calculator is designed for adults 18 and older.
BMI does not measure fitness, cardiovascular health, nutrition, or where body fat is distributed. A person can have a normal BMI but carry dangerous levels of visceral (abdominal) fat, have poor metabolic health, or low muscle mass — all of which affect wellbeing. BMI is a starting point, not a complete health assessment.
A high BMI is one signal worth discussing with a healthcare provider, who can do a more complete assessment including blood pressure, blood sugar, cholesterol, and other markers. Do not make significant dietary or medical decisions based solely on your BMI result.
BMI is an indirect estimate calculated only from height and weight. Body fat percentage directly measures how much of your body mass is fat versus lean tissue (muscle, bone, water). Body fat percentage is more informative but requires additional measurement methods such as DEXA scans, hydrostatic weighing, or skinfold calipers.
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