How BMI is calculated
Body Mass Index (BMI) is a simple ratio of your weight to your height. In metric units the formula is weight in kilograms divided by height in metres squared:
BMI = weight (kg) ÷ height (m)²
If you think in pounds and inches, the imperial version multiplies by a conversion factor of 703:
BMI = 703 × weight (lb) ÷ height (in)²
Both give the same number — this calculator does the unit conversion for you, so you can enter whichever you know. The result is a single figure, usually between about 15 and 40 for adults, that places you into a broad weight category.
Worked example
Suppose someone is 1.75 m tall and weighs 72 kg. First square the height: 1.75 × 1.75 = 3.0625. Then divide the weight by that figure: 72 ÷ 3.0625 = 23.5. A BMI of 23.5 falls inside the healthy-weight band (18.5–24.9), near the upper-middle of it. In imperial terms the same person is about 5 ft 9 in and 159 lb: 703 × 159 ÷ (69 × 69) = 703 × 159 ÷ 4761 ≈ 23.5 — identical, as expected.
BMI categories for adults
The World Health Organization uses these standard ranges for adults aged 20 and over:
- Under 18.5 — Underweight
- 18.5 to 24.9 — Healthy weight
- 25.0 to 29.9 — Overweight
- 30.0 and above — Obesity (sometimes split into class 1: 30–34.9, class 2: 35–39.9, and class 3: 40+)
What BMI does and doesn't tell you
BMI is a fast, free screening tool — useful for spotting trends across large groups and for flagging when a conversation with a doctor might be worthwhile. But it has real limits. Because it only knows your height and weight, it can't tell muscle from fat. A muscular athlete can register as "overweight" while carrying very little body fat, and an older adult who has lost muscle may sit in the "healthy" range while carrying more fat than is ideal. BMI also doesn't account for where fat is stored — waist measurement is often a better guide to health risk than BMI alone.
For these reasons, treat your BMI as one data point among several, not a verdict. Waist circumference, blood pressure, activity level, and how you actually feel all matter. This tool is for general information only and is not medical advice; talk to a healthcare professional about what your number means for you.
Frequently asked questions
Is BMI accurate for children?
No — children and teens are assessed differently. Because young bodies are still growing, a child's BMI is compared against age- and sex-specific percentile charts rather than the fixed adult categories above. Use a paediatric BMI-for-age chart, or ask your child's doctor.
Does BMI differ for men and women?
The formula and the category thresholds are the same for both. Women tend to carry a higher proportion of body fat than men at the same BMI, which is one more reason BMI is a rough screen rather than a precise body-fat measure.
What's a healthy BMI range?
For most adults, 18.5 to 24.9 is considered healthy. Some health bodies suggest people of South Asian, Chinese, and some other ancestries consider action at slightly lower thresholds, because health risks can appear at a lower BMI.
How often should I check it?
There's no need to track BMI daily — weight fluctuates with hydration, meals, and time of day. Checking every few weeks or months is plenty to see a genuine trend.
Related tools: Unit Converter for switching between kg and lb, and the Percentage Calculator for tracking changes over time.