⚖️ BMI Calculator
Calculate your Body Mass Index in metric or imperial units.
<18.5 Normal
18.5–24.9 Overweight
25–29.9 Obese
≥30
| Category | BMI Range |
|---|---|
| Underweight | < 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obese | ≥ 30 |
What Is a BMI Calculator?
A BMI (Body Mass Index) calculator determines your body mass index — a number derived from your height and weight that serves as a screening tool for weight categories associated with health risks. The formula is: BMI = weight (kg) ÷ height (m)². In imperial units, it's BMI = 703 × weight (lbs) ÷ height (inches)². The result places you in one of four categories: underweight (below 18.5), normal weight (18.5–24.9), overweight (25–29.9), or obese (30 and above).
BMI was developed by Belgian statistician Adolphe Quetelet in the 1830s as a population-level statistical measure, not an individual health diagnostic tool. The CDC and WHO use it as a quick, inexpensive screening tool that correlates with body fat at population scale — not as a definitive measure of individual health. A person with high muscle mass (athletes, bodybuilders) may have a high BMI despite low body fat. Older adults may have a "normal" BMI despite high body fat percentage due to muscle loss with age.
Despite its limitations, BMI remains widely used in clinical settings because it requires no equipment beyond a scale and measuring tape. Research published in the New England Journal of Medicine found that excess BMI is associated with increased risk for type 2 diabetes, cardiovascular disease, and certain cancers at the population level. For adults, every 5-unit increase in BMI above 25 is associated with approximately 29% higher all-cause mortality risk in long-term studies.
BMI thresholds also differ by ethnicity. The WHO has proposed lower cutoff points for Asian populations, where health risks associated with excess body fat appear at lower BMI values — with overweight potentially starting at 23 and obesity at 27.5 for some Asian ethnic groups.
How to Use This BMI Calculator
- Select your unit system — choose metric (kg/cm) or imperial (lbs/inches) based on what's most natural for you.
- Enter your weight — use your current weight. For the most accurate reading, weigh yourself in the morning before eating, without shoes or heavy clothing.
- Enter your height — use your actual measured height, not what you think it is. Many people overestimate height by 1–2 inches, which meaningfully affects BMI.
- View your BMI and category — the calculator displays your BMI number and which category (underweight, normal, overweight, obese) it falls in.
- Track changes over time — recalculate monthly as a simple way to monitor weight trends, keeping in mind that BMI is a screening tool, not a comprehensive health assessment.
Why BMI Is a Starting Point, Not a Verdict
BMI is most useful as one data point in a broader health picture that includes waist circumference, blood pressure, blood glucose, cholesterol levels, and physical fitness. The American Heart Association recommends waist circumference as an additional metric — health risks increase significantly when waist circumference exceeds 35 inches for women or 40 inches for men, independent of BMI. Using BMI alongside waist-to-height ratio gives a more complete picture of metabolic risk than either measure alone.
Related Tools
- Calorie Calculator — calculate daily calorie needs based on your BMI and activity level
- Age Calculator — BMI health ranges vary slightly by age group
- Sleep Calculator — sleep deprivation is associated with weight gain and elevated BMI
- Net Worth Calculator — track financial health alongside physical health metrics
- Tip Calculator — quick calculator for everyday math needs
Frequently Asked Questions
Is a BMI of 25 always unhealthy?
Not necessarily. BMI 25 sits at the border of "normal" and "overweight," but health impact depends heavily on where body fat is distributed (visceral vs subcutaneous fat), muscle mass, fitness level, and individual metabolic health markers. Many people with a BMI of 25–27 have entirely healthy cholesterol, blood pressure, and blood sugar levels. Conversely, some people with "normal" BMI have metabolic dysfunction. BMI is a population-screening tool, not an individual diagnosis.
Why doesn't BMI apply to children the same way?
For children and teens (ages 2–19), BMI is interpreted using age- and sex-specific percentile charts rather than fixed cutoffs. A BMI of 22 might be in the 85th percentile for a 10-year-old boy (overweight) but perfectly average for an adult. The CDC provides BMI-for-age percentile charts, where below 5th percentile is underweight, 5th–84th is healthy weight, 85th–94th is overweight, and 95th percentile or above is obese — all referenced against a population of same-age, same-sex peers.
What is the ideal BMI range?
For adults, 18.5–24.9 is classified as normal weight. Research on longevity suggests the lowest mortality risk is associated with BMI roughly between 22 and 24 for most populations. Being slightly above "normal" (BMI 25–27) appears to carry lower mortality risk than being underweight (below 18.5) in some large-scale studies — a finding sometimes called the "obesity paradox." For Asian populations, the WHO suggests a normal range may be 18.5–22.9, with increased risk beginning at 23.
How much weight do I need to lose to lower my BMI by 1 point?
BMI decreases by 1 point for every approximately 6.7 pounds (3 kg) lost for a person 5'7" (170 cm) tall. For a person 5'4" (163 cm), losing 1 BMI point requires about 5.8 pounds. For someone 6'0" (183 cm), it takes about 7.9 pounds per BMI point. The formula is: weight change for 1 BMI point = height(m)² × 1. Since BMI = weight/height², changing BMI by 1 means changing weight by height(m)² kg.
Should I use BMI or body fat percentage?
Body fat percentage is a more direct and accurate measure of body composition, but it requires tools like DEXA scans, hydrostatic weighing, or skinfold calipers — all less accessible than a scale and measuring tape. BMI is a reasonable first-pass screening tool for most people. If your BMI is borderline (24–27) and you're athletic or muscular, measuring body fat percentage gives a more meaningful picture. For clinical screening and population research, BMI remains the standard due to its simplicity and accessibility.