How BMI is calculated
Body Mass Index is your weight in kilograms divided by your height in metres squared: BMI = kg ÷ m². For example, 70 kg at 1.75 m gives 70 ÷ (1.75 × 1.75) = 22.9, which is in the normal range.
If you work in pounds and inches, convert first: pounds divided by 2.205 gives kilograms, and inches multiplied by 2.54 gives centimetres. A 154 lb, 69 in person is 70 kg and 175 cm, the same figures as the example above. Entering imperial numbers without converting is the most common mistake with this calculator and inflates the result badly. Because BMI ignores composition, many people pair it with the body fat calculator.
BMI categories
| Category | BMI range |
|---|---|
| Underweight | below 18.5 |
| Normal | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obese | 30 and above |
What BMI does and doesn't tell you
BMI is a quick screening figure, not a diagnosis. It doesn't distinguish muscle from fat, so very muscular people can score high without excess fat, and it isn't ideal for children, pregnant people or the elderly. Use it as a rough guide alongside other health measures. A healthy-range target weight for your height comes from the ideal weight calculator.
Two other measures fill the gaps BMI leaves. Waist circumference shows where fat sits and a body fat percentage separates muscle from fat. If your BMI sits just outside the normal band, one of those usually explains why. It is a general screening estimate, not medical advice. For a measure that accounts for waist size, try the body roundness index calculator.
Frequently asked questions
- What is a healthy BMI?
- For most adults a BMI between 18.5 and 24.9 is considered healthy.
- Does BMI work for athletes?
- Not well — muscle is dense, so very muscular people can have a high BMI without excess body fat.
- Is BMI accurate for everyone?
- It's a general screening tool. It's less reliable for children, pregnant people, athletes and older adults.
- What if my weight is in pounds and my height in inches?
- Convert before entering: divide pounds by 2.205 for kilograms and multiply inches by 2.54 for centimetres. A 154 lb, 69 in person becomes 70 kg and 175 cm. Typing imperial figures straight in gives a badly inflated result.
- Can my BMI change without my weight changing?
- Yes, if the height you enter changes. Height is measured slightly shorter later in the day and with age, and because BMI squares height, a 2 cm difference moves the result by a few tenths. BMI is a screening estimate, not medical advice.