BMI Calculator

Calculate your body mass index

Body mass index (BMI) is a quick way to compare weight with height. Enter your height and weight in metric or imperial units to calculate your BMI and see the standard adult category. The result updates as you type.

BMI is a screening measure, not a diagnosis. It is useful for starting a conversation about weight and health, but it cannot measure body fat directly or account for every person’s build, age, activity level, or medical history.

The BMI formula

In metric units:

BMI = weight (kg) ÷ height (m)²

For example, a person who weighs 70 kg and is 175 cm tall has a BMI of:

70 ÷ (1.75 × 1.75) = 22.9

In imperial units, the equivalent formula is:

BMI = weight (lb) ÷ height (in)² × 703

The calculator applies the correct formula for the units you select, so metric and imperial entries produce the same BMI.

Standard adult BMI categories

BMI	Standard adult category
Below 18.5	Underweight
18.5 to below 25	Normal weight
25 to below 30	Overweight
30 or above	Obesity

These ranges are commonly used for adults. They are not targets or diagnoses, and they are not interpreted the same way for children, teenagers, or during pregnancy.

What BMI can and cannot tell you

BMI is convenient because it only needs height and weight, but that simplicity is also its limit. It does not distinguish muscle from fat, show where body fat is stored, or measure blood pressure, blood sugar, fitness, diet, or overall wellbeing.

• Muscular people may have a high BMI despite a low body-fat level.
• Older adults may have a BMI in the usual range while having less muscle and more body fat than BMI suggests.
• Children and teenagers need BMI-for-age percentiles that account for age and sex.
• Pregnancy and some health conditions require individual clinical advice rather than a standard BMI category.

If you are concerned about your weight or health, a healthcare professional can assess BMI alongside waist measurement, medical history, blood tests, and other relevant information.

Using BMI responsibly

A single BMI is best treated as one data point, not a verdict about health or a reason for a drastic change. Trends over time, how you feel, your activity, and guidance from a qualified clinician all matter. Seek professional advice promptly if weight changes unexpectedly, eating feels difficult or distressing, or you are managing a medical condition.

Frequently asked questions

How is BMI calculated?

In metric units, BMI is weight in kilograms divided by height in metres squared: kg ÷ m². In imperial units, the equivalent is weight in pounds divided by height in inches squared, multiplied by 703. Both formulas produce the same result.

What are the standard adult BMI categories?

For most adults, a BMI below 18.5 is classified as underweight; 18.5 to less than 25 as normal weight; 25 to less than 30 as overweight; and 30 or above as obesity. These are screening categories, not a diagnosis.

Is BMI accurate for everyone?

No. BMI is a quick population-level screening measure, not a direct measure of body fat or health. It can overestimate body fat in very muscular people and underestimate it in some older adults or people with less muscle. Pregnancy, childhood, adolescence, and some ethnic backgrounds also need different interpretation. A clinician can put BMI in context with your health history, waist measurement, and other measures.

Can I use this BMI calculator for a child or teenager?

Not by itself. Children and teenagers are assessed using age- and sex-specific BMI percentiles rather than the adult category cutoffs shown here. Use a child BMI percentile tool or ask a healthcare professional.

Is my height or weight saved?

The calculation runs in your browser and the tool does not upload or store your measurements. The values may appear in the page address when you share a specific calculation.