Bmi Calculator

What is BMI?

BMI (Body Mass Index) measures a person’s leanness or fullness based on their height and weight. It helps estimate whether a person has a healthy body weight.

The BMI value places a person into categories like:

• Underweight
• Normal weight
• Overweight
• Obese

These categories may vary slightly by region and age. BMI is not perfect, but it is a useful starting point to know if you need further health checks.

BMI Table for Adults (Age 20+)

Recommended by the World Health Organization (WHO). Applies to both men and women.

Classification	BMI Range (kg/m²)
Severe Thinness	Less than 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	More than 40

BMI Chart for Adults (Visual Summary)

A graph shows BMI categories in different colors:

• Underweight (various shades)
• Normal (green zone)
• Overweight (yellow/orange)
• Obese (red shades)

Gray contour lines show exact BMI values for different height and weight combinations.

BMI Table for Children and Teens (Age 2–20)

Recommended by the CDC. Based on percentiles compared to same age and sex.

Category	Percentile Range
Underweight	Less than 5%
Healthy weight	5% – 85%
At risk of overweight	85% – 95%
Overweight	More than 95%

BMI Charts for Children and Teens

The CDC provides separate growth charts:

• Chart for boys (BMI-for-age percentiles)
• Chart for girls (BMI-for-age percentiles)

These help track growth patterns over time.

Health Risks of Being Overweight

According to the CDC, overweight increases risk of:

• High blood pressure
• High LDL cholesterol (“bad” cholesterol), low HDL cholesterol, high triglycerides
• Type 2 diabetes
• Coronary heart disease and stroke
• Gallbladder disease
• Osteoarthritis (joint cartilage breakdown)
• Sleep apnea and breathing problems
• Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
• Lower quality of life
• Mental illness (depression, anxiety)
• Body pain and physical limitations
• Higher risk of early death

Ideally, keep BMI below 25. Always talk to your doctor before making major lifestyle changes.

Health Risks of Being Underweight

Being underweight also carries risks:

• Malnutrition and vitamin deficiencies
• Anemia (lower ability to carry oxygen in blood)
• Osteoporosis (weak bones, higher fracture risk)
• Weakened immune system
• Growth and development problems (especially in children)
• Hormonal issues in women (irregular periods, higher miscarriage risk)
• Surgery complications
• Higher risk of early death

If you or someone you know is underweight without a clear reason, consult a doctor. It could be a sign of an eating disorder like anorexia.

Limitations of BMI

BMI is a useful estimate, but it has limits. It does not measure body fat directly or account for:

• Muscle vs. fat mass
• Bone density
• Age
• Sex
• Ethnicity
• Activity level

In Adults:

• Older adults often have more body fat than younger adults with the same BMI.
• Women usually have more body fat than men at the same BMI.
• Athletes and bodybuilders may have high BMI but low body fat (muscle weighs more than fat).

In Children and Teens:

• Height and sexual maturation affect BMI.
• BMI works better for identifying obesity in children than for overweight.
• Thin children may have low BMI due to low fat or low muscle mass.

Despite these limits, BMI is accurate for 90–95% of people when used along with other health measures.

How to Calculate BMI

US Units (pounds and inches)

Formula:
BMI = 703 × (weight in lbs) ÷ (height in inches)²

Example for a 5’10” (70 inches), 160 lb person:
BMI = 703 × (160 ÷ 70²) = 703 × (160 ÷ 4900) = 23.0

Metric Units (kilograms and meters)

Formula:
BMI = weight in kg ÷ (height in meters)²

Example for 72.57 kg, 1.778 m tall:
BMI = 72.57 ÷ (1.778²) = 23.0

What is BMI Prime?

BMI Prime = Your BMI ÷ 25 (the upper limit of normal BMI)

• Less than 0.74 → Underweight
• 0.74 to 1 → Normal weight
• 1 to 1.2 → Overweight
• More than 1.2 → Obese

BMI Prime Table

Classification	BMI Range	BMI Prime Range
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

BMI Prime helps compare different populations or limits.

What is Ponderal Index (PI)?

PI is similar to BMI but cubes the height instead of squaring it. It works better for very tall or very short people.

US Units Formula

PI = height (inches) ÷ ∛weight (lbs)

Example (70 inches, 160 lbs):
PI = 70 ÷ ∛160 = 70 ÷ 5.43 = 12.9

Metric Units Formula

PI = weight (kg) ÷ height³ (meters)

Example (72.57 kg, 1.778 m):
PI = 72.57 ÷ (1.778³) = 12.9

PI avoids extreme values that BMI sometimes gives for very tall or short individuals.