BMI Calculator

Our BMI Calculator determines your body‑mass index and the associated weight category, factoring in your age. Switch to the “Metric Units” pane for SI measurements or choose the “Other Units” pane to convert between US and metric systems. The tool also provides the Ponderal Index alongside BMI, which we’ll explain further below.

BMI introduction

BMI gauges a person’s thinness or bulk by relating height to weight, essentially estimating body tissue mass. It’s a common quick check to see if someone’s weight aligns with health standards for their stature. The resulting figure places an individual into categories—underweight, normal, overweight or obese—based on predefined ranges that can differ by region, age, and sometimes include finer splits such as severe underweight or extreme obesity. Because both excess and insufficient weight can affect health, BMI, while imperfect, serves as a useful trigger for further evaluation. See the table below for the specific BMI brackets employed by this calculator.

BMI table for adults

Displayed here are the WHO’s suggested adult body‑weight ranges derived from BMI values, applicable to men and women aged 20 and up.

Classification	BMI range - kg/m2
Severe Thinness	< 16
Moderate Thinness	16 - 17
Mild Thinness	17 - 18.5
Normal	18.5 - 25
Overweight	25 - 30
Obese Class I	30 - 35
Obese Class II	35 - 40
Obese Class III	> 40

BMI chart for adults

The chart visualizes BMI categories according to WHO data. Colored zones correspond to the primary BMI groups, while the grey contour lines indicate whole‑number BMI values across various height‑weight pairings.

BMI table for children and teens, age 2-20

The Centers for Disease Control and Prevention (CDC) recommends BMI categorization for children and teens between age 2 and 20.

Category	Percentile Range
Underweight	<5%
Healthy weight	5% - 85%
At risk of overweight	85% - 95%
Overweight	>95%

BMI chart for children and teens, age 2-20

The Centers for Disease Control and Prevention (CDC) BMI-for-age percentiles growth charts.

Chart for boys
Chart for girls

Risks associated with being overweight

Carrying excess weight raises the likelihood of several serious illnesses. The CDC outlines the following health risks associated with overweight.

• High blood pressure
• Higher levels of LDL cholesterol, which is widely considered "bad cholesterol," lower levels of HDL cholesterol, considered to be good cholesterol in moderation, and high levels of triglycerides
• Type II diabetes
• Coronary heart disease
• Stroke
• Gallbladder disease
• Osteoarthritis, a type of joint disease caused by breakdown of joint cartilage
• Sleep apnea and breathing problems
• Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
• Low quality of life
• Mental illnesses such as clinical depression, anxiety, and others
• Body pains and difficulty with certain physical functions
• Generally, an increased risk of mortality compared to those with a healthy BMI

The preceding list demonstrates the many adverse—sometimes life‑threatening—consequences of excess weight. Ideally, individuals aim for a BMI under 25 kg/m², though a medical professional should be consulted to assess any necessary lifestyle adjustments.

Risks associated with being underweight

Being underweight has its own associated risks, listed below:

• Malnutrition, vitamin deficiencies, anemia (lowered ability to carry blood vessels)
• Osteoporosis, a disease that causes bone weakness, increasing the risk of breaking a bone
• A decrease in immune function
• Growth and development issues, particularly in children and teenagers
• Women may experience fertility problems stemming from hormonal disruptions that alter menstrual cycles, and underweight females face an elevated risk of first‑trimester miscarriage.
• Potential complications as a result of surgery
• Generally, an increased risk of mortality compared to those with a healthy BMI

Occasionally, low body weight signals an underlying health issue, such as anorexia nervosa, which carries its own dangers. If you or someone you know appears underweight without an apparent cause, seek medical advice.

Limitations of BMI

While BMI remains a popular gauge of healthy weight, it has notable drawbacks. The metric merely estimates mass relative to height and ignores body composition. Because muscle, bone, and fat distribute differently across individuals, BMI should be interpreted alongside additional measurements rather than serving as the sole health metric.

In adults:

BMI falls short of perfection because it reflects excess body mass, not specifically excess fat. Various factors—age, gender, ethnicity, muscle mass, fat proportion, activity level—also shape the reading. For instance, an elderly individual with a “normal” BMI but a sedentary lifestyle may harbor considerable fat, posing health risks, whereas a younger person with the same BMI but higher muscle content could be fit. Athletes, especially bodybuilders, often register as overweight due to dense muscle, yet their body composition may be perfectly healthy. The CDC highlights these nuances.

• Older adults tend to have more body fat than younger adults with the same BMI.
• Women tend to have more body fat than men for an equivalent BMI.
• Muscular individuals and highly trained athletes may have higher BMIs due to large muscle mass.

In children and adolescents:

The same reasons that make BMI less reliable for grown‑ups also affect youngsters and teens. A child’s height and stage of puberty also play a role in how BMI relates to actual body fat. For kids who are obese, BMI gives a clearer picture of excess fat than it does for those who are merely overweight, where the index can reflect either extra fat or extra lean mass (muscle, bone, water, organs…). In slim children, variations in BMI often stem from differences in lean tissue rather than fat.

That being said, BMI is fairly indicative of body fat for 90-95% of the population, and can effectively be used along with other measures to help determine an individual's healthy body weight.

BMI formula

Below are the equations used for calculating BMI in the International System of Units (SI) and the US customary system (USC) using a 5'10", 160-pound individual as an example:

USC Units:

BMI = 703× mass (lbs) height2 (in)  = 703× 160 702  = 23.0

SI, Metric Units:

BMI =  mass (kg) height2 (m)  =  72.57 1.7782  = 23.0

BMI Prime

BMI prime is the ratio of a person's measured BMI to the upper limit of BMI that is considered "normal," by institutions such as the WHO and the CDC. Though it may differ in some countries, such as those in Asia, this upper limit, which will be referred to as BMI_upper is 25 kg/m².

The BMI prime formula is:

BMI prime =

BMI  25

BMI‑prime is simply the ratio of a person's BMI to the upper bound of the normal BMI range, making it a unit‑free figure. Values below 0.74 denote underweight, 0.74‑1.0 correspond to a normal weight, 1.0‑1.2 indicate overweight, and anything above 1.2 classifies as obese. The following table matches these BMI‑prime ranges with the corresponding weight categories.

Classification	BMI	BMI Prime
Severe Thinness	< 16	< 0.64
Moderate Thinness	16 - 17	0.64 - 0.68
Mild Thinness	17 - 18.5	0.68 - 0.74
Normal	18.5 - 25	0.74 - 1
Overweight	25 - 30	1 - 1.2
Obese Class I	30 - 35	1.2- 1.4
Obese Class II	35 - 40	1.4 - 1.6
Obese Class III	> 40	> 1.6

Using BMI‑prime lets you gauge how far an individual's BMI deviates from the highest normal value, and it also enables you to compare groups that have different normal‑range cut‑offs.

Ponderal Index

The Ponderal Index (PI) works like BMI by relating a person's weight to their height, but it raises height to the third power instead of squaring it. While BMI is handy for large‑scale studies, it is not the best tool for judging a single person's leanness. PI shares some of BMI’s drawbacks, yet it performs better for people who are extremely tall or short, where BMI can misjudge body‑fat levels. Below is the formula for calculating PI, illustrated with a 5‑foot‑10‑inch, 160‑pound person.

USC Units:

PI =  height (in) ∛mass (lbs)  =  70 ∛160  = 12.9

SI, Metric Units:

PI =  mass (kg) height3 (m)  =  72.57 1.7783  = 12.9

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