BMI Calculator
Calculate your Body Mass Index (BMI) using our free BMI calculator. Enter your height and weight in metric or imperial units to instantly see your BMI score and weight category. The BMI calculator works for adults aged 20 and older.
Formula reviewed for accuracy. Our methodology & sources
BMI Calculator
health calculator
How It Works
BMI, or Body Mass Index, is a simple ratio of your weight to your height that estimates whether you are at a healthy weight for your size. In metric units it is your weight in kilograms divided by the square of your height in metres; in imperial units the equivalent formula is 703 times your weight in pounds divided by the square of your height in inches. Squaring the height is what lets a single number apply across a wide range of body sizes, so a tall person and a short person can be compared on the same scale. Once it has your BMI, the calculator places you into one of the four categories defined by the World Health Organization: underweight below 18.5, normal from 18.5 to 24.9, overweight from 25 to 29.9, and obese at 30 or above. It also reports the weight range that would put you in the healthy band for your height, by working the formula in reverse at the 18.5 and 24.9 thresholds. For example, a person 1.75 m tall weighing 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9, which sits comfortably in the normal range; to stay in that band they would aim for roughly 56.7 to 76.3 kg. Use BMI as a quick, free screening check — it is the tool doctors reach for first to flag possible weight-related health risk across a population. But respect its limits, because this is where most people misread it. BMI cannot distinguish muscle from fat, so a lean, muscular athlete can register as "overweight" despite low body fat, while someone with little muscle may sit in the normal range yet carry unhealthy fat. It also does not capture where fat is stored (abdominal fat carries more risk), and it is less reliable for older adults, pregnant women, and different ethnic groups, some of whom face elevated risk at lower BMI values. For children and teens aged 2–19 the same formula is used but interpreted against age- and sex-specific percentiles rather than these adult cut-offs. Treat BMI as a starting point for a conversation with a healthcare professional, not a diagnosis.
Formula
BMI = weight (kg) / height² (m) BMI (Imperial) = 703 × weight (lbs) / height² (inches)
Examples
70kg, 175cm adult
Average adult male in the normal BMI range.
Frequently Asked Questions
What is a healthy BMI?
According to the World Health Organization, a healthy BMI for adults ranges from 18.5 to 24.9. Below 18.5 is classed as underweight, 25 to 29.9 as overweight, and 30 or above as obese. These thresholds apply to adults aged roughly 20 and older, not to children.
Is BMI an accurate measure of health?
BMI is a useful, quick screening tool but not a complete measure of health. It cannot tell muscle from fat, ignores where fat is stored, and does not account for age, sex, or ethnic differences in body composition. A muscular athlete may register as overweight despite low body fat, so BMI is best used alongside other measures like waist circumference.
How is BMI calculated for children?
For children and teenagers aged 2 to 19, BMI is calculated with the same weight-over-height-squared formula, but it is interpreted using age- and sex-specific percentile charts rather than the fixed adult categories. A child's result is expressed as a percentile compared with others of the same age and sex, because healthy body composition changes throughout growth.
How can I lower my BMI safely?
BMI falls when you reduce body weight through a sustainable combination of a balanced, calorie-controlled diet and regular physical activity. Even a modest 5–10% reduction in body weight can meaningfully improve blood pressure, blood sugar, and cholesterol. Aim for gradual loss of about 0.5 kg (1 lb) a week rather than crash dieting, and consult a healthcare professional before starting.
Why does BMI overestimate risk for muscular people?
BMI only looks at total weight relative to height and has no way to separate lean muscle from body fat. Because muscle is denser and heavier than fat, athletes and heavily trained individuals can have a high BMI while carrying very little fat. For these individuals, body-fat percentage or waist measurements give a far better picture of health than BMI alone.
Does BMI work the same for men and women?
The BMI formula and the adult category thresholds are identical for men and women. However, at the same BMI women naturally tend to carry a higher percentage of body fat than men, so the number alone does not reflect body composition differences between the sexes. This is one reason BMI is treated as a population screening tool rather than an individual diagnosis.
What is the difference between BMI and body fat percentage?
BMI estimates weight status from height and weight only, while body fat percentage directly measures how much of your body is fat versus lean tissue. Body fat percentage is more accurate for assessing individual health but requires special methods like calipers, bioelectrical impedance, or a DEXA scan. BMI is faster and free, which is why it remains the standard first-line screen.
Sources
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