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BMI Calculator

Calculate your Body Mass Index (BMI) instantly. Get your WHO classification, healthy weight range, and personalized recommendations.

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BMI calculator. Body mass index from weight and height.
A BMI calculator computes your body mass index by dividing your weight by the square of your height. It categorizes the result into underweight, normal, overweight, or obese ranges according to WHO standards.

What Is Body Mass Index (BMI)?

Body Mass Index (BMI) is a numerical value calculated from a person's weight and height that serves as a screening tool to categorize weight status. A BMI between 18.5 and 24.9 is considered normal weight, while values below 18.5 indicate underweight and values of 25 or above signal overweight or obesity.
Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI remains the most widely used population-level metric for assessing weight-related health risks. The World Health Organization (WHO), the Centers for Disease Control and Prevention (CDC), and the National Institutes of Health (NIH) all use BMI as a primary screening tool in clinical guidelines.
BMI does not measure body fat directly. It cannot distinguish between muscle, bone, and fat tissue, which means athletes or highly muscular individuals may receive misleadingly high BMI readings. Despite this limitation, research consistently shows that at the population level, higher BMI values correlate with increased risk of type 2 diabetes, cardiovascular disease, certain cancers, and all-cause mortality. For individuals, BMI is most useful as a starting point — best interpreted alongside other measures like waist circumference, body fat percentage, and overall fitness level.

How to Calculate BMI Step by Step

To calculate BMI, you divide your weight by the square of your height. The formula differs slightly depending on whether you use metric or imperial units, but both produce the same result.
Using the imperial formula (pounds and inches):
1. Measure your weight in pounds and your height in inches. For example, 5'10" equals 70 inches.
2. Square your height: 70 × 70 = 4,900.
3. Divide your weight by the squared height: 170 ÷ 4,900 = 0.03469.
4. Multiply by 703 (the imperial conversion factor): 0.03469 × 703 = 24.4.
5. Your BMI is 24.4, which falls in the normal weight range.
Using the metric formula (kilograms and meters):
1. Measure your weight in kilograms and your height in meters. For example, 77 kg and 1.78 m.
2. Square your height: 1.78 × 1.78 = 3.1684.
3. Divide your weight by the squared height: 77 ÷ 3.1684 = 24.3.
4. Your BMI is 24.3.
Our calculator above handles both unit systems automatically and instantly shows your WHO classification, healthy weight range for your height, and how much weight you would need to gain or lose to reach the normal BMI range.

BMI Formula

BMI=weight (kg)height (m)2BMI = \frac{weight \, (kg)}{height \, (m)^2}
  • BMIBMI = Body Mass Index in kg/m²
  • weightweight = Body weight in kilograms (kg)
  • heightheight = Height in meters (m)
For imperial units (pounds and inches), the formula includes a conversion factor of 703:
BMI=weight (lbs)height (in)2×703BMI = \frac{weight \, (lbs)}{height \, (in)^2} \times 703
The conversion factor 703 accounts for the difference between metric and imperial units (1 kg = 2.20462 lbs, 1 m = 39.3701 in). Both formulas produce identical BMI values.
The WHO classifies adult BMI into these categories: Severe Thinness (below 16.0), Moderate Thinness (16.0–16.9), Mild Thinness (17.0–18.4), Normal Weight (18.5–24.9), Overweight (25.0–29.9), Obese Class I (30.0–34.9), Obese Class II (35.0–39.9), and Obese Class III (40.0 and above). These ranges apply equally to men and women aged 20 and older.

BMI Calculation Examples

Example: 5'4" Woman Weighing 140 Pounds

A woman who is 5 feet 4 inches tall (64 inches) and weighs 140 pounds can calculate her BMI as follows: 140 ÷ (64 × 64) × 703 = 140 ÷ 4,096 × 703 = 24.0. Her BMI of 24.0 places her in the normal weight category (18.5–24.9). The healthy weight range for someone 5'4" is approximately 108–145 pounds. She is 5 pounds below the upper limit of normal weight, so even a modest weight gain would move her into the overweight category.

Example: 5'10" Man Weighing 210 Pounds

A man standing 5 feet 10 inches tall (70 inches) at 210 pounds: 210 ÷ (70 × 70) × 703 = 210 ÷ 4,900 × 703 = 30.1. His BMI of 30.1 places him just into Obese Class I (30.0–34.9). The healthy weight range for his height is 129–174 pounds, meaning he would need to lose approximately 36 pounds to reach the upper boundary of normal weight (BMI 24.9). However, if this person is muscular and physically active, his high BMI may reflect lean mass rather than excess fat — a common limitation of BMI that makes waist circumference or body composition analysis a useful complement.

Example: 6'0" Man Weighing 155 Pounds

A 6-foot-tall man (72 inches) weighing 155 pounds: 155 ÷ (72 × 72) × 703 = 155 ÷ 5,184 × 703 = 21.0. His BMI of 21.0 is well within the normal weight range. The healthy weight range for someone 6'0" is approximately 137–183 pounds, giving him a comfortable margin of 28 pounds before reaching overweight status. This BMI range is often associated with the lowest overall health risks in population studies.

Tips for Understanding and Using Your BMI

  • Treat BMI as a starting point, not a diagnosis. A normal BMI does not guarantee good health, and an elevated BMI does not automatically mean you are unhealthy. Combine BMI with waist circumference measurement (below 40 inches for men, below 35 inches for women) for a more complete picture.
  • Measure yourself consistently. Weigh yourself at the same time of day — ideally in the morning before eating — and measure your height without shoes. Even small measurement errors can shift your BMI by a full point.
  • Understand that BMI does not distinguish muscle from fat. If you strength train regularly or are naturally muscular, your BMI may classify you as overweight despite having a healthy body fat percentage. Consider a body composition test (DEXA scan, bioelectrical impedance) for a more accurate assessment.
  • Track your BMI trend over time rather than fixating on a single reading. A gradual increase in BMI over months or years may signal lifestyle changes worth addressing, even if each individual reading is within the normal range.
  • Know the different health risk thresholds. A BMI of 30+ significantly increases risk for type 2 diabetes, heart disease, and sleep apnea. A BMI below 18.5 is associated with nutrient deficiencies, weakened immunity, and bone loss. Both extremes warrant a conversation with a healthcare provider.
  • Use BMI alongside other health metrics. Blood pressure, fasting blood glucose, cholesterol levels, and physical fitness often tell a more complete health story than weight and height alone. The AMA recommends that BMI never be used as the sole diagnostic criterion.

Frequently Asked Questions About BMI

What is a healthy BMI for adults?

A healthy BMI for adults is between 18.5 and 24.9 kg/m², according to the World Health Organization and the CDC. This range is associated with the lowest statistical risk of weight-related health problems. For example, a person who is 5'6" has a healthy weight range of about 115–150 pounds, while someone 5'10" should weigh between 129–174 pounds to stay in the normal BMI range.

Is BMI the same for men and women?

Yes, the BMI formula and classification ranges are identical for men and women. However, women naturally carry a higher body fat percentage than men at the same BMI — typically 20–25% body fat for a healthy woman versus 10–20% for a healthy man. This means BMI may underestimate body fat in women and overestimate it in muscular men. Some researchers have proposed gender-specific BMI thresholds, but the WHO currently uses the same scale for both sexes.

Why is BMI not accurate for athletes and bodybuilders?

BMI cannot differentiate between muscle mass and fat mass. Since muscle is denser than fat, highly muscular individuals often have BMIs in the overweight or obese range despite very low body fat. For example, a 5'10" bodybuilder weighing 210 pounds at 10% body fat would have a BMI of 30.1 (obese), which clearly misrepresents their health status. Athletes should use body fat percentage, DEXA scans, or waist-to-hip ratio instead of relying on BMI alone.

What is the difference between BMI categories: overweight vs. obese?

Overweight is defined as a BMI of 25.0 to 29.9, while obesity begins at a BMI of 30.0. Obesity is further divided into three classes: Class I (30.0–34.9), Class II (35.0–39.9), and Class III or severe obesity (40.0 and above). Each step up significantly increases health risks. A BMI of 30 carries roughly double the diabetes risk of a BMI of 25, and a BMI of 40+ is associated with a 6- to 14-year reduction in life expectancy.

How much should I weigh for my height?

Your healthy weight depends on your height and corresponds to a BMI of 18.5–24.9. Here are healthy weight ranges for common heights: 5'2" (104–136 lbs), 5'4" (108–145 lbs), 5'6" (115–150 lbs), 5'8" (125–163 lbs), 5'10" (129–174 lbs), 6'0" (137–183 lbs), 6'2" (144–194 lbs). These are general guidelines — your ideal weight also depends on your muscle mass, frame size, age, and overall health profile.

Does BMI change with age?

The BMI formula itself does not change with age for adults 20 and older. However, body composition shifts with aging: people tend to lose muscle mass and gain fat tissue, especially after age 50. This means an older adult with a "normal" BMI of 24 may actually carry more body fat than a younger person with the same BMI. Some studies suggest that a slightly higher BMI (25–27) in adults over 65 may be associated with lower mortality risk — a phenomenon known as the "obesity paradox."

Is waist circumference a better health indicator than BMI?

Waist circumference is often a better predictor of health risk than BMI because it specifically measures visceral fat — the deep abdominal fat surrounding internal organs that is most strongly linked to heart disease, diabetes, and metabolic syndrome. A waist circumference above 40 inches (102 cm) for men or 35 inches (88 cm) for women indicates elevated risk, regardless of BMI. The NHLBI recommends using both BMI and waist circumference together for the most accurate risk assessment.

What are the health risks of being underweight (BMI below 18.5)?

Being underweight is associated with serious health risks including weakened immune function, osteoporosis and increased fracture risk, anemia, fertility problems, and nutrient deficiencies. A BMI below 16.0 (severe thinness) significantly increases mortality risk and may indicate an eating disorder or underlying medical condition. If your BMI consistently falls below 18.5, consult a healthcare provider to rule out medical causes and develop a safe plan to reach a healthy weight.

Key Terms

Body Mass Index (BMI)

A numerical value derived from dividing weight in kilograms by the square of height in meters (kg/m²), used as a screening tool to classify weight status.

Normal Weight

A BMI between 18.5 and 24.9, the range associated with the lowest risk of weight-related health complications according to WHO guidelines.

Obesity

A BMI of 30.0 or higher, divided into Class I (30–34.9), Class II (35–39.9), and Class III (40+). Associated with increased risk of diabetes, heart disease, and certain cancers.

Visceral Fat

Fat stored deep in the abdominal cavity around internal organs. More strongly linked to metabolic disease than subcutaneous fat and better measured by waist circumference than BMI.

Waist Circumference

The measurement around the abdomen at the level of the navel. A waist over 40 inches (men) or 35 inches (women) indicates elevated health risk regardless of BMI.

Body Composition

The proportion of fat, muscle, bone, and water in the body. Unlike BMI, body composition analysis distinguishes between fat mass and lean mass.

BMI Prime

The ratio of a person's actual BMI to the upper limit of normal BMI (25.0). A BMI Prime of 1.0 means you are at the threshold of overweight; below 1.0 is normal.