BMI Calculator
Find your Body Mass Index and see which weight category it falls into.
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What is BMI and how is it calculated?
Body Mass Index (BMI) is a simple screening number that compares your weight to your height. The formula is weight in kilograms divided by height in meters squared (kg/m²), or in imperial units, 703 × weight in pounds divided by height in inches squared. It was developed in the 19th century by Belgian statistician Adolphe Quetelet as a population-level measure, and public health bodies still use it today because it's quick to calculate and needs no special equipment.
A BMI under 18.5 is generally classed as underweight, 18.5–24.9 as normal weight, 25–29.9 as overweight, and 30 or above as obese. These bands are useful starting points, but BMI has real limitations: it cannot tell the difference between muscle and fat, so a muscular athlete may show a high BMI without carrying excess body fat. It also doesn't account for where fat is stored, age-related muscle loss, pregnancy, or differences between ethnic groups, some of which have adjusted thresholds in clinical guidance.
Use this calculator as a first checkpoint rather than a verdict. If your result sits outside the "normal" range, pair it with other measures — waist circumference, body fat percentage, or a conversation with a healthcare provider — before drawing conclusions. Small, sustainable changes to diet and activity tend to move BMI more reliably than short bursts of extreme dieting, and tracking the trend over months matters more than any single reading.
Frequently asked questions
Is a BMI of 25 automatically unhealthy? Not necessarily. It's the line where "overweight" starts in the standard chart, but someone at 25 with good fitness and no other risk markers is in a very different position than someone at 25 with high blood pressure and low activity. Treat 25 as a prompt to look closer, not a diagnosis.
Why does BMI use height squared? Weight tends to scale with the square of height across a population, so dividing by height² removes most of the effect of being simply taller or shorter, leaving a number that reflects body size more than stature.
How often should I recheck my BMI? Monthly is plenty for most people. Body weight fluctuates day to day with water, food, and hormones, so a single reading matters less than the direction it moves over several weeks.
Worked example
Take a 34-year-old who is 170 cm tall and weighs 75 kg. BMI = weight(kg) ÷ height(m)². Height in meters is 1.70, squared is 2.89. 75 ÷ 2.89 = 25.9, which falls in the "overweight" band (25–29.9). Moving to 68 kg at the same height would bring BMI to 23.5, inside the "normal" range (18.5–24.9).
Does BMI mean the same thing for everyone? No. It doesn't distinguish muscle from fat, so a muscular athlete can show a “high” BMI while carrying very little body fat. It's a population-level screening tool, not a diagnosis.
Is BMI accurate for older adults? Less so. Muscle mass naturally declines with age, so an older adult with a “normal” BMI can still have a higher fat percentage than a younger person with the same number. Pairing BMI with the Body Fat Calculator gives a fuller picture.