BMI Calculator
Calculate your Body Mass Index (BMI) instantly. Get your WHO classification, healthy weight range, and personalized recommendations.
Aquesta calculadora encara no s'ha traduït al Català.
De moment mostrem la versió en Anglès mentre acabem la traducció.
Your BMI
24.1
Classification
Normal weight
Healthy weight range
118.1 – 159 lbs
BMI Scale
1018.5253040+
WHO BMI Classification
| Classification | BMI (kg/m²) |
|---|---|
Severe thinness | 0 – 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 |
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 Use the BMI Calculator
The calculator turns two everyday measurements into a full weight-status readout. You do not need to convert anything yourself — pick your unit system and type in the numbers.
1. Choose your unit system: metric (kilograms and centimeters) or imperial (pounds, feet, and inches).
2. Enter your weight.
3. Enter your height.
4. Read your BMI value and the WHO category it falls into — underweight, normal, overweight, or one of the three obesity classes.
5. Check the healthy weight range shown for your height, and how many pounds (or kilograms) separate you from the normal range.
Because BMI is a screening estimate rather than a diagnosis, treat the category as a starting point: it tells you whether your weight is worth a closer look, not whether you are healthy. If you would rather reproduce the number by hand, the formula is in the next section.
BMI Formula
- = Body Mass Index in kg/m²
- = Body weight in kilograms (kg)
- = Height in meters (m)
The metric formula divides weight in kilograms by height in meters squared. For imperial units (pounds and inches), you multiply by a conversion factor of 703:
The factor 703 reconciles the two unit systems (1 kg = 2.20462 lbs, 1 m = 39.3701 in), so both formulas return an identical BMI.
To work it out by hand in imperial units, take someone 5'4" (64 inches) weighing 140 pounds: square the height (64 × 64 = 4,096), divide the weight by that result (140 ÷ 4,096 = 0.0342), then multiply by 703 to get a BMI of 24.0. In metric, someone 1.78 m tall weighing 77 kg squares their height (1.78 × 1.78 = 3.17) and divides (77 ÷ 3.17 = 24.3).
The calculator above does this automatically in either unit system and maps the result onto the WHO categories shown in the reference table below — from underweight through the three classes of obesity. These ranges apply equally to men and women aged 20 and older.
BMI Categories and What They Mean
| Category | BMI (kg/m²) | What it means |
|---|---|---|
| Underweight | Below 18.5 | May signal undernutrition, low bone density, or an underlying condition; below 16.0 is classed as severe thinness. |
| Normal weight | 18.5 – 24.9 | The range linked with the lowest weight-related health risk in population studies. |
| Overweight | 25.0 – 29.9 | Raised risk of type 2 diabetes and heart disease; a good point to check your waist size. |
| Obese class I | 30.0 – 34.9 | Roughly double the diabetes risk of a normal BMI; even modest weight loss helps early. |
| Obese class II | 35.0 – 39.9 | High risk of metabolic and cardiovascular disease; medical guidance is recommended. |
| Obese class III | 40.0 and above | Severe obesity, associated with a 6–14 year reduction in life expectancy. |
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.
Example: 5'3" (160 cm) Adult Weighing 176 Pounds (80 kg)
An adult who is 5 feet 3 inches (160 cm) tall and weighs 176 pounds (80 kg) has a BMI of 31.2: in metric, 80 ÷ (1.60 × 1.60) = 80 ÷ 2.56 = 31.2. That falls in Obese Class I (30.0–34.9). The healthy weight range for this height is about 104–140 pounds (47.4–63.7 kg), so reaching the normal range means losing roughly 36 pounds (16.3 kg). Because BMI is a screening estimate, that target is not all-or-nothing: losing even 5–10% of body weight — around 9–18 pounds here — already lowers blood pressure and blood sugar risk long before the number reaches 24.9.
What BMI Does Not Measure
BMI is a screening estimate, not a diagnosis, and it does not measure body fat directly. Because the formula uses only weight and height, it cannot tell muscle from fat, cannot see where fat sits on your body, and cannot detect the visceral fat around your organs that drives most metabolic risk.
Specifically, BMI does not capture body-fat percentage, visceral (abdominal) fat, muscle mass, bone density, or fat distribution between your waist and hips. Two people with an identical BMI of 27 can have very different health profiles — one carrying excess belly fat, the other lean and muscular.
BMI was designed to describe populations, not to diagnose individuals. At the population level it correlates strongly with the risk of type 2 diabetes, heart disease, and several cancers, which is why the WHO, CDC, and NIH still use it as a first-line screen. For a single person, though, it is only a starting point. Body composition tests (DEXA scans, skinfold calipers, bioelectrical impedance, or our Body Fat Calculator using the US Navy method) measure fat directly, and waist circumference — above 40 inches (102 cm) for men or 35 inches (88 cm) for women — flags the visceral fat that BMI misses.
BMI also reads differently across groups. Older adults lose muscle and can carry more fat at the same BMI, and some studies find a slightly higher BMI (25–27) is linked with lower mortality after age 65 — the so-called "obesity paradox." Athletes and people of different ethnic backgrounds may need adjusted thresholds. The American Medical Association recommends that BMI never be the sole diagnostic criterion. Interpret your result alongside waist circumference, blood pressure, blood glucose, cholesterol, and fitness, and treat it as one input to a conversation with a healthcare provider — not a verdict.
Common Mistakes When Reading BMI
- Treating BMI as a diagnosis. BMI is a screening estimate, not a ruling on your health. A number in the overweight range is a prompt to look closer, not proof of disease.
- Assuming a normal BMI means you are healthy. Someone with a BMI of 23 can still carry excess abdominal fat and low muscle — sometimes called "normal-weight obesity." Pair BMI with a waist measurement rather than trusting the category alone.
- Forgetting that muscle inflates BMI. Muscle is denser than fat, so lifters, athletes, and naturally muscular people often land in the overweight or obese range despite low body fat. Use body-fat percentage instead of BMI to judge their status.
- Reading the 25.0 boundary as a cliff. A BMI of 24.9 and 25.1 describe almost identical bodies. The categories are broad bands, not switches, so track your trend over months instead of fixating on a single decimal.
- Applying adult BMI to the wrong group. The 18.5–24.9 range is for adults aged 20 and over. Children and teens are assessed with age-and-sex percentiles, and BMI is unreliable during pregnancy and in the frail elderly.
- Ignoring where the fat sits. BMI cannot see fat distribution, yet visceral fat around the organs drives most metabolic risk — so a waist over 40 inches (102 cm) for men or 35 inches (88 cm) for women matters even when BMI looks fine.
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 115–154 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 sharply raises the risk of type 2 diabetes and heart disease compared with the normal range, 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" (101–136 lbs), 5'4" (108–145 lbs), 5'6" (115–154 lbs), 5'8" (122–164 lbs), 5'10" (129–174 lbs), 6'0" (136–184 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.
Is this BMI calculator free to use?
Yes. The BMI calculator works in your browser and supports both metric (kg/cm) and imperial (lb/ft-in) units. Enter your height and weight and it shows your BMI, WHO category, the healthy range for your height, and the weight change needed to reach it.
How accurate is this BMI calculator?
The math is exact — the calculator uses the WHO formula (weight in kg divided by height in meters squared) and rounds to one decimal place. As a health measure, though, BMI is only a screen: it estimates weight status and does not measure body fat, muscle, or diagnose disease.
What should I do after calculating my BMI?
If your BMI is in the normal range (18.5–24.9), keep tracking it over time. If it is high, use the healthy-range figure as a target and remember that losing even 5–10% of your body weight measurably lowers blood pressure and blood-sugar risk. Because BMI is only a screen, pair it with a waist measurement, and for any result outside the normal range, discuss it with your doctor. For a fat-specific reading, try our Body Fat Calculator.
What is BMI Prime?
BMI Prime is your BMI divided by 25 — the upper limit of the normal range. A BMI Prime of 1.0 means you are exactly at the overweight threshold; below 1.0 is within normal weight, and 1.36 means you are 36% above the upper limit. It is a quick way to see how far your BMI sits from the healthy ceiling and to compare people whose countries use different BMI cut-offs.