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Free BMI Calculator – Body Mass Index
Calculate your Body Mass Index (BMI) instantly. Understand BMI categories, limitations, and what your number actually means for health.
Quick answer
A person who is 175 cm (5'9") tall and weighs 75 kg (165 lb) has a BMI of 24.5, which falls within the "normal weight" range of 18.5–24.9. BMI is a screening tool, not a diagnostic measure — it does not account for muscle mass, bone density, or fat distribution.
What BMI Measures
Body Mass Index (BMI) is a simple ratio of weight to height, calculated as weight in kilograms divided by height in meters squared: BMI = kg / m². Developed by Adolphe Quetelet in the 1830s, it was designed as a population-level statistical tool, not an individual health diagnostic. Despite its limitations, BMI remains widely used because it is easy to calculate and correlates with health outcomes at the population level.
The World Health Organization classifies BMI as follows: Underweight below 18.5, 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 thresholds were derived from epidemiological data linking BMI ranges to disease risk.
How BMI Is Calculated
The formula is straightforward: BMI = weight (kg) ÷ height (m)². For imperial units: BMI = [weight (lb) ÷ height (in)²] × 703. Example: A person weighing 82 kg and standing 180 cm (1.80 m) tall has a BMI of 82 ÷ (1.80 × 1.80) = 82 ÷ 3.24 = 25.3, placing them just into the "overweight" category.
For a person weighing 60 kg at 165 cm: BMI = 60 ÷ (1.65 × 1.65) = 60 ÷ 2.72 = 22.1, which is well within the normal range. These calculations take seconds but the interpretation requires context about the individual — their muscle mass, age, sex, and ethnicity all influence what a given BMI number actually means for health.
Important Limitations of BMI
BMI does not distinguish between muscle and fat. A muscular athlete with 10% body fat might have a BMI of 28 and be classified as overweight despite being in excellent health. Conversely, a sedentary person with low muscle mass could have a "normal" BMI while carrying excess visceral fat — a condition sometimes called "skinny fat" or metabolically obese normal weight.
BMI thresholds also do not account for ethnic differences. Research shows that health risks associated with a given BMI vary by ethnicity. For example, some Asian populations may experience increased metabolic risk at BMI values considered normal for European-descent populations. The WHO has acknowledged that lower BMI cutoffs may be appropriate for certain ethnic groups.
Age and sex matter too. Women naturally carry more body fat than men at the same BMI, and older adults may have higher body fat percentages at the same BMI as younger adults due to age-related muscle loss. For children and adolescents, BMI is interpreted using age-and-sex-specific percentile charts rather than fixed cutoffs.
When BMI Is Useful — and When It Is Not
BMI works best as a quick screening tool for large populations and as a starting reference for individuals who do not have access to more precise body composition measurements. It is most meaningful for average-build adults who are neither highly muscular nor elderly. For a more complete picture, combine BMI with waist circumference, body fat percentage, and ideally a clinical assessment.
A waist circumference above 94 cm (37 in) for men or 80 cm (31.5 in) for women indicates elevated health risk regardless of BMI. The waist-to-height ratio (keeping waist below half your height) is another simple metric that may be more predictive of cardiovascular risk than BMI alone.
BMI categories and example weights for a 175 cm (5'9") person
| BMI Category | BMI Range | Example Weight (175 cm) |
|---|---|---|
| Underweight | Below 18.5 | Below 56.7 kg (125 lb) |
| Normal Weight | 18.5–24.9 | 56.7–76.3 kg (125–168 lb) |
| Overweight | 25.0–29.9 | 76.6–91.5 kg (169–202 lb) |
| Obese Class I | 30.0–34.9 | 91.9–106.9 kg (203–236 lb) |
| Obese Class II | 35.0–39.9 | 107.2–122.2 kg (236–269 lb) |
| Obese Class III | 40.0+ | Above 122.5 kg (270+ lb) |
Frequently asked questions
Is BMI still a useful health measure?
BMI is useful as a quick, inexpensive screening tool for populations and as a starting reference for individuals. However, it should not be the sole indicator of health. Combining BMI with waist circumference, body fat percentage, and clinical markers provides a far more accurate picture.
Can muscular people have a high BMI and still be healthy?
Yes. BMI does not distinguish between muscle and fat. Athletes, strength trainees, and manual laborers often have elevated BMI due to higher muscle mass while having healthy or low body fat percentages. Body fat measurement is more informative for this population.
What is a healthy BMI for older adults?
Some research suggests that slightly higher BMI values (25–27) may be associated with lower mortality in adults over 65, a phenomenon sometimes called the "obesity paradox." This may be because extra weight provides energy reserves during illness. However, this does not mean gaining weight is beneficial — maintaining muscle mass is the key factor.
How often should I check my BMI?
BMI changes slowly because it depends on weight and height (which is stable in adults). Checking every few months is sufficient for tracking trends. More frequent weigh-ins are better suited to tracking short-term calorie balance than BMI itself.
Is BMI calculated differently for children?
Yes. For children and adolescents (ages 2–19), BMI is plotted on age-and-sex-specific growth charts and expressed as a percentile rather than a fixed category. A child at the 85th percentile is considered overweight, and at the 95th percentile obese, relative to same-age peers.
Related resources
Methodology and trust notes
PrimeMacros uses common nutrition planning equations such as Mifflin-St Jeor for BMR/TDEE estimates, body-weight based protein ranges, and explicit health disclaimers. Results are planning estimates, not diagnosis, treatment or individualized nutrition therapy.
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