BMI Calculator (Body Mass Index)

Understanding Your BMI: What the Numbers Mean

What Is BMI and How Is It Calculated?

BMI is a measurement of a person's leanness or corpulence based on their height and weight, intended to quantify tissue mass. It's widely used as a general indicator of whether a person carries a healthy body weight for their height. The standard formula, expressed in SI, Metric Units: is:

$$\text{BMI} = \frac{\text{mass (kg)}}{\text{height}^{2}\text{ (m}^{2}\text{)}}$$

In USC Units: (pounds and inches), a correction factor is applied:

$$\text{BMI} = 703 \times \frac{\text{mass (lbs)}}{\text{height}^{2}\text{ (in}^{2}\text{)}}$$

Here's a worked example using a person who is 5'10" (70 inches / 1.778 m) and weighs 160 lbs (72.57 kg):

1. Identify known values: height = 1.778 m, mass = 72.57 kg
2. Apply the SI formula: $$\text{BMI} = \frac{\text{mass}}{\text{height}^{2}}$$
3. Substitute values: $$\text{BMI} = \frac{72.57}{1.778^{2}}$$
4. Calculate: $$\text{BMI} = \frac{72.57}{3.161} = 23.0 \text{ kg/m}^{2}$$
5. Result: BMI = 23.0 kg/m² — this falls in the Normal weight category.

BMI Categories and Ranges for Adults

The World Health Organization (WHO) defines BMI categories for adults aged 20 and older. The ranges apply to both men and women. The table below shows the full WHO classification:

Classification	BMI Range (kg/m2)
Severe Thinness	< 16
Moderate Thinness	16 – 17
Mild Thinness	17 – 18.5
Normal weight:	18.5 – 25
Overweight:	25 – 30
Obese Class I	30 – 35
Obese Class II	35 – 40
Obese Class III	> 40

Maintaining a BMI in the healthy weight range — 18.5 to 24.9 kg/m² — is one of the ways you can support your overall health as you age. That said, your healthcare provider can give you personalized results and guidance based on your full health picture.

BMI Ranges for Children and Teens (Ages 2–20)

For children and teens, BMI is interpreted differently than it is in adults. Rather than fixed numeric thresholds, the CDC uses BMI-for-age percentiles, which account for normal changes in body fat as children grow and differ between boys and girls.

Category	Percentile Range
Underweight:	Below the 5th percentile
Healthy Weight	5th to below the 85th percentile
At Risk of Overweight	85th to below the 95th percentile
Overweight:	95th percentile or above

If you need to assess BMI for a child or teen, use a dedicated BMI Calculator for Children and Teens rather than this adult BMI calculator, since applying adult thresholds to younger individuals can produce misleading results.

Risks Linked to Being Underweight or Overweight

Being overweight increases your risk of a number of serious health conditions. According to the CDC, these include:

• High blood pressure and high cholesterol
• Type 2 diabetes
• Coronary heart disease and stroke
• Sleep apnea and breathing problems
• Certain cancers (endometrial, breast, colon, kidney, liver)
• Low quality of life and mental health conditions such as depression and anxiety
• Generally increased risk of mortality compared to those with a healthy BMI

Being underweight: carries its own serious risks:

• Malnutrition, vitamin deficiencies, and anemia
• Osteoporosis and weakened bones
• Decreased immune function
• Growth and development issues, particularly in children and teens
• Reproductive complications in women due to hormonal imbalances
• Increased risk of complications from surgery

Limitations of BMI as a Health Measure

BMI is a useful and widely used indicator, but it has real limitations worth understanding. Because it only accounts for total body mass relative to height, it cannot distinguish between muscle, fat, and bone. In adults:

• Older adults tend to have more body fat than younger adults at the same BMI.
• Women typically carry more body fat than men at an equivalent BMI.
• Highly trained athletes and bodybuilders may register as overweight due to high muscle mass, even though their body fat percentage is low.
• BMI is influenced by factors such as age, sex, ethnicity, muscle mass, and activity level — none of which are captured in the formula.

In children and adolescents, height and sexual maturation can further influence BMI readings. BMI is a better indicator of excess body fat for obese children than for children who are simply overweight, whose elevated BMI could reflect fat-free mass rather than excess fat. That said, BMI is reasonably accurate for roughly 90–95% of the population and works well as a starting point — especially when used alongside other health information rather than in isolation.

How Age and Race Can Affect BMI Accuracy

Standard BMI thresholds were developed using data that skews toward certain populations, which means the same BMI value can carry different health implications depending on your background. Some examples: people of Asian descent face elevated health risks at lower BMI values than the standard cutoffs suggest, and some countries use adjusted thresholds accordingly. Similarly, older adults naturally accumulate more body fat over time, so a "normal" BMI in an elderly person doesn't necessarily mean a healthy body composition. These are important reasons to treat your BMI result as one piece of information — not the whole story — when assessing your health.