What is the difference between BMI and BMI prime?

BMI prime is your BMI divided by 25, the upper limit of the normal range. A BMI prime below 0.74 is underweight, 0.74 to 1 is normal, above 1 is overweight, and above 1.2 is obese. It is a dimensionless number useful for comparing populations with different BMI cutoffs.

BMI Calculator

Calculate your body mass index from height and weight. Switch between metric and imperial units for instant results.

Metric

kg and cm

Imperial

lb, ft and in

Height (cm)

Weight (kg)

Your BMI

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Select units and enter height and weight

Quick Examples

What Is BMI?

Body mass index (BMI) is a number calculated from your weight and height that estimates whether you are underweight, a healthy weight, overweight, or obese. Belgian statistician Adolphe Quetelet developed it in the 1830s after observing that weight scales with the square of height in adults, which lets one scale compare people of very different heights.

For adults, BMI provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems. It is not a diagnostic tool on its own: your doctor combines it with medical history, physical exam findings, and lab results.

The BMI Formula

There are two versions of the formula depending on your unit system. Both give the same result.

Metric formula

BMI = weight (kg) / height (m)²

Convert centimeters to meters first by dividing by 100. A person who is 170 cm tall is 1.70 m tall.

Imperial formula

BMI = 703 x weight (lb) / height (in)²

The number 703 is a conversion factor that makes the imperial calculation match the metric one. Square the total height in inches first, then divide.

Worked Examples

Metric: 65 kg at 170 cm. Height = 1.70 m, so BMI = 65 / 1.70² = 65 / 2.89 = 22.5 (normal).
Metric: 85 kg at 180 cm. Height = 1.80 m, so BMI = 85 / 1.80² = 85 / 3.24 = 26.2 (overweight).
Imperial: 145 lb at 5 ft 7 in. Height = 67 in, so BMI = 703 x 145 / 67² = 101,935 / 4,489 = 22.7 (normal).
Imperial: 160 lb at 5 ft 10 in. Height = 70 in, so BMI = 703 x 160 / 70² = 112,480 / 4,900 = 23.0 (normal).
Imperial: 210 lb at 5 ft 10 in. BMI = 703 x 210 / 4,900 = 147,630 / 4,900 = 30.1 (obesity class I).

WHO BMI Categories for Adults

The World Health Organization defines the following BMI ranges for adults aged 20 and older. They apply to both men and women.

Classification	BMI (kg/m²)
Severe thinness	Below 16
Moderate thinness	16 - 17
Mild thinness	17 - 18.5
Normal (healthy)	18.5 - 24.9
Overweight	25 - 29.9
Obesity class I	30 - 34.9
Obesity class II	35 - 39.9
Obesity class III	40 and above

Some Asian health authorities use lower cutoffs (23 for overweight, 27.5 for obesity), because risks rise at lower BMI values in those populations.

Your Healthy Weight Range

A single BMI number is useful, but a healthy weight range is more practical. Compute it by multiplying your height squared by 18.5 and by 24.9:

Healthy weight = 18.5 x height² to 24.9 x height²

At 1.70 m: 18.5 x 2.89 = 53.5 kg to 24.9 x 2.89 = 72.0 kg.
At 1.778 m (5 ft 10 in): 18.5 x 3.161 = 58.5 kg to 24.9 x 3.161 = 78.7 kg, or about 129 to 174 lb.

The calculator above shows your personal range after each calculation.

What BMI Can and Cannot Tell You

BMI is a good first screen, but it has clear limits. It estimates excess weight, not excess body fat, and it cannot see body composition.

BMI is a screening measure and is not intended to diagnose disease or illness. Always interpret it alongside other health information, ideally with a healthcare provider.

Muscle vs. fat: muscle is denser than fat. Athletes, bodybuilders, and people with physically demanding jobs can have a high BMI while being lean and healthy.
Age: older adults tend to carry more body fat than younger adults at the same BMI, because muscle mass declines with age.
Sex: women tend to have more body fat than men at an equivalent BMI.
Fat distribution: BMI says nothing about where fat is stored. Abdominal fat carries higher risk than fat stored on the hips and thighs, which is why waist circumference is often measured alongside BMI.
It misses "skinny fat": an inactive person with a normal BMI can still have unhealthy levels of body fat and low muscle mass.

Despite these limits, BMI remains useful: it tracks body fat fairly well for 90 to 95 percent of the population when combined with other measures.

BMI for Children and Teenagers

For ages 2 to 19, the same formula is used but the result is interpreted with BMI-for-age percentiles, which account for sex and normal growth patterns:

Category	Percentile range
Underweight	Below 5th percentile
Healthy weight	5th to 85th percentile
Overweight	85th to 95th percentile
Obese	Above 95th percentile

Percentiles compare a child to others of the same age and sex. Adults aged 20 and older use the standard adult categories above.

Health Risks Linked to BMI

Weight extremes carry documented health risks. According to the CDC, excess weight raises the risk of high blood pressure, unhealthy cholesterol, type 2 diabetes, heart disease, stroke, osteoarthritis, sleep apnea, and certain cancers. Being underweight brings its own risks: malnutrition, anemia, osteoporosis, weakened immunity, and hormonal disruption. Unexplained low weight deserves a doctor's visit, as it can signal an underlying condition.

Common BMI Mistakes

Using centimeters without converting: the metric formula needs meters — 170 cm = 1.70 m. Dividing kilograms by centimeters squared gives a wildly wrong answer.
Forgetting to square the height: BMI = weight / height², not weight / height.
Mixing unit systems: never combine kilograms with inches or pounds with meters.
Treating BMI as a diagnosis: a BMI of 26 in a muscular athlete means something very different from 26 in a sedentary adult. Context decides.

Frequently Asked Questions

How is BMI calculated?

BMI equals weight in kilograms divided by height in meters squared (kg/m^2). In imperial units: 703 x weight in pounds divided by height in inches squared. Example: 65 kg at 1.70 m gives 65 / 2.89 = 22.5.

What BMI is considered healthy?

For most adults, 18.5 to 24.9 is the healthy (normal) range per the World Health Organization. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese.

What is a good BMI for my height and weight?

Multiply your height squared by 18.5 and 24.9 to get your healthy weight range. At 5 ft 10 in (1.778 m) that is 58.5 to 78.7 kg, roughly 129 to 174 lb. The calculator above computes your personal range automatically.

Is BMI accurate for everyone?

No. BMI is a screening tool that does not distinguish muscle from fat and does not account for age, sex, or ethnicity differences. Athletes with high muscle mass can have a high BMI while being healthy. It is a screening measure, not a diagnosis.

How do I calculate BMI in pounds and inches?

Use 703 x weight (lb) / height (in)^2. For 5 ft 7 in (67 in) and 145 lb: 703 x 145 / 4489 = 22.7, in the normal range.

Why is my BMI high if I am muscular?

BMI measures excess weight, not excess body fat, and muscle is denser than fat. Bodybuilders and strength athletes often land in overweight or obese BMI categories with low body fat. Body fat percentage or waist measurements are better indicators for muscular people.

Does BMI work for children and teenagers?

The formula is the same, but ages 2 to 19 use BMI-for-age percentiles by sex: below the 5th percentile is underweight, 5th-85th is healthy, 85th-95th is overweight, and above 95th is obese.

Can BMI tell me my body fat percentage?

No. BMI correlates with body fat for about 90-95% of people, but it cannot measure body composition directly. Two people with the same BMI can have very different body fat levels depending on muscle mass and age.

What should I do if my BMI is above 25?

A BMI above 25 is overweight and above 30 is obese, both linked to higher health risks. Talk to a healthcare provider, who can combine BMI with waist circumference, blood pressure, and lab results before recommending changes.

Related Calculators

These tools pair well with the BMI calculator:

Weight Converter: convert kilograms, pounds, stones, and ounces.
Length Converter: convert centimeters, feet, inches, and meters.
Percentage Calculator: track percentage weight change toward a goal.
Mean Median Mode Calculator: analyze BMI readings over time.

Conclusion

Body mass index distills height and weight into a single number: weight in kilograms divided by height in meters squared, or 703 times pounds divided by inches squared. The WHO bands are simple — under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, 30 and above obese — and your personal healthy range is 18.5 to 24.9 times your height squared.

Use BMI as a starting screen, not a final verdict: pair it with waist measurement, activity level, and professional advice. Use the calculator at the top of this page for instant metric or imperial results with your category and healthy weight range.