BMI Calculator

Medically reviewed by Dr. Muhammad Sharif (FCPS, MCPS). Last updated 20-Oct-2026.

Quick answer: BMI (Body Mass Index) is your weight in kilograms divided by your height in meters squared. For adults age 20 and older, a healthy BMI is 18.5 to 24.9. Below 18.5 is underweight, 25.0 to 29.9 is overweight, and 30.0 or higher is obesity. BMI is a screening number. It does not diagnose a condition or measure body fat.

What Is BMI?

BMI stands for Body Mass Index. It is one number that compares a person's weight with their height. Doctors use it as a fast first check to see whether weight may be low, healthy, or high for a given height.

Belgian mathematician Adolphe Quetelet developed the idea in the 1830s. Physiologist Ancel Keys named it Body Mass Index in 1972. The CDC and the World Health Organization still use it because it is quick, free, and needs only two measurements.

How to Use the BMI Calculator

This calculator is intended for adults aged 20 and older, including women who are not pregnant.

1. Pick US units (feet, inches, pounds) or metric (centimeters, kilograms).
2. Enter your height and your current weight.
3. Press Calculate BMI.
4. Read your number, your category, and the ideal body weight for your height.

Your numbers stay in your browser, and we do not save them. Three common input mistakes change results:

• Mixing units, such as pounds with centimeters.
• Typing 5.10 for 5 feet 10 inches. Use the feet and inches boxes separately.
• Rounding height. A two inch error can shift BMI by more than a full point.

What Is a Healthy BMI?

A healthy BMI for adults is 18.5 to 24.9, according to the CDC. The ranges are the same for men and women at every adult age.

Obesity has three classes. Class 1 is 30.0 to 34.9, class 2 is 35.0 to 39.9, and class 3 is 40.0 or higher. A higher BMI is linked with a greater chance of high blood pressure, type 2 diabetes, and heart disease. A very low BMI can point to poor nutrition or a weaker immune system.

How Is BMI Calculated?

BMI equals weight in kilograms divided by height in meters squared. With pounds and inches, divide weight by height in inches squared, then multiply by 703.

For a person who is 5 feet 4 inches (64 inches) and weighs 130 pounds: 130 ÷ 4,096 × 703 = 22.3. See our BMI formula guide for more examples in both unit systems.

Is BMI Accurate?

BMI is exact arithmetic, but it is only a rough guide to body fat and health. These are the main limits.

• Muscle: BMI cannot tell muscle from fat, so a strong, lean person can land in the overweight range.
• Age: older adults often lose muscle and a little height, which BMI can miss.
• Background: NICE in the UK uses lower action points of 23 and 27.5 for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African, and African Caribbean backgrounds.
• Pregnancy: do not use BMI during pregnancy.
• Fat location: BMI does not show where fat is stored.

Waist size fills part of that gap. The CDC lists a waist over 40 inches (102 cm) for men, or over 35 inches (88 cm) for women who are not pregnant, as a sign of higher risk. Measure just above your hip bones after you breathe out. Our BMI chart explains each limit in more detail.

BMI for Children and Teens

Adult cutoffs do not apply from ages 2 to 19. The CDC compares a child's BMI with other children of the same age and sex and reports a percentile.

What to Do With Your Result

• Below 18.5: talk with a doctor. Low weight can come from low food intake, stress, or a medical cause.
• 18.5 to 24.9: keep up your habits. Blood pressure, blood sugar, and cholesterol still matter.
• 25 or higher: this is a signal to look closer, not a diagnosis. Measure your waist and ask your doctor about those same checks. Health agencies such as the CDC and NIH note that losing 5 to 10 percent of body weight can improve them. A pace of 1 to 2 pounds a week is easier to keep.

Frequently Asked Questions

What is a good BMI for my age?

For any adult age 20 or older, a healthy BMI is 18.5 to 24.9. The range does not change as you get older. Under age 20, BMI is read as a percentile.

Is BMI different for men and women?

The BMI categories are the same for men and women, but body composition differs. Women usually carry more body fat than men at the same BMI. Learn more about BMI for women and how to interpret BMI based on sex.

Why do two BMI calculators show different results?

The usual causes are mixed units or rounding. The standard formula uses only height and weight, so a tool that asks for age, sex, or activity may adjust the result.

Why is my BMI high when I am fit?

BMI counts all weight the same. Muscle, bone, and water add to the total, so a lean, strong person can show an overweight BMI. Check your waist size and ask a doctor or trainer about body fat testing.

Is BMI or body fat percentage better?

They answer different questions. BMI is a quick screen from height and weight. Body fat percentage shows how much of your weight is fat, but it needs tools such as calipers, a smart scale, or a DEXA scan, and home methods vary in accuracy. Many doctors start with BMI and add other tests if needed.

Is the BMI formula wrong for tall or short people?

Some mathematicians say so. Oxford's Nick Trefethen argued that dividing by height squared makes short people look lighter and tall people look heavier than they are, and he proposed using height to the power of 2.5. The CDC and WHO still use the standard formula, so the categories on this page follow it.

What BMI is considered obese?

A BMI of 30.0 or higher. Class 1 is 30.0 to 34.9, class 2 is 35.0 to 39.9, and class 3 is 40.0 or higher.

Is BMI outdated?

It has limits, which is why doctors pair it with waist size, blood tests, and a physical exam. It remains a common first check because it is quick, free, and easy to repeat.

Does this calculator save my height and weight?

No. The math runs in your browser, and your numbers are not saved on our servers.

How often should I check my BMI?

Every few months is enough for most adults. Body weight changes from day to day, so frequent checks add little.

This page is for information only and is not medical advice. Read our Medical Disclaimer and our Medical Review Policy.

Sources: CDC, Adult BMI Categories. NHLBI