What is BMI?
BMI, or Body Mass Index, is a simple number that estimates whether your weight is in a healthy range for your height. It is widely used by doctors and health organizations as a quick screening tool because it needs only two measurements: your height and your weight.
BMI does not measure body fat directly, but for most people it correlates well enough to flag whether they may be underweight, at a healthy weight, overweight, or obese.
The BMI formula
Metric: BMI = weight (kg) ÷ height (m)². For example, someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
Imperial: BMI = 703 × weight (lb) ÷ height (in)². For example, 154 lb and 69 inches gives 703 × 154 ÷ (69 × 69) = 22.7.
You do not need to do the math by hand — the free BMI Calculator handles both metric and imperial instantly.
BMI categories for adults
Underweight: below 18.5. Normal weight: 18.5 to 24.9. Overweight: 25 to 29.9. Obese: 30 and above.
These ranges apply to most adults aged 20 and over, regardless of sex. A BMI in the normal range is generally associated with lower health risks, while values at the extremes may warrant a conversation with a doctor.
How to calculate your BMI in seconds
Step 1. Open the free BMI Calculator and choose metric (cm, kg) or imperial (ft/in, lb).
Step 2. Enter your height and weight.
Step 3. Click calculate. Your BMI number and weight category appear instantly, along with a visual scale showing where you fall.
Limitations of BMI
BMI is a useful screening tool but not a complete picture of health. Because it only uses height and weight, it cannot tell muscle from fat. Very muscular athletes may register as overweight despite low body fat, and BMI is less accurate for the elderly, pregnant women, and children, who use age- and sex-specific charts. Treat BMI as a starting point, not a diagnosis, and consult a healthcare professional for personalized advice.
BMI categories at a glance
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
A worked example
Imagine someone who is 1.70 m tall and weighs 68 kg. Using the metric formula: 68 ÷ (1.70 × 1.70) = 68 ÷ 2.89 = 23.5. That falls in the 18.5–24.9 range, so it is classed as a normal, healthy weight. Change the weight to 80 kg and the BMI becomes 27.7 — into the overweight range. Seeing how the number shifts helps you understand what a few kilograms mean for the category.
Why BMI is not the whole story
BMI is a fast screening number, but it has real limits you should know:
- It cannot tell muscle from fat. A muscular athlete may score as overweight while being very healthy.
- It ignores fat distribution. Two people with the same BMI can have very different health risks depending on where fat sits.
- It is calibrated for adults. Children, teens, pregnant women, and the elderly need different references.
- It says nothing about fitness, diet, or blood markers — all of which matter more for overall health.
Think of BMI as one quick indicator on a dashboard, not a diagnosis. Waist measurement, body composition, and a doctor's assessment give a fuller picture.
Common mistakes when calculating BMI
- Mixing units. Using pounds with the metric formula (or kg with the imperial one) gives nonsense. Pick one system.
- Measuring height in centimetres but forgetting to convert to metres. 170 cm is 1.70 m, not 170 in the formula.
- Treating the result as a health verdict. A single number cannot capture your overall health.
Calculate your BMI now
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