WHO adult category: —
WHO adult screening ranges apply to adults age 20 and older. BMI is a screening ratio, not a diagnosis or direct body-fat measure; adult categories do not apply to children or teens.
BMI (Body Mass Index) is a simple weight-to-height ratio defined as mass in kilograms divided by height in meters squared: BMI = kg/m². It's a screening tool — not a diagnosis — that bins adult weight into Underweight (< 18.5), Normal (18.5–24.9), Overweight (25–29.9), and Obese (≥ 30). Choose US units for pounds plus feet/inches, or metric units for kilograms plus centimeters, and the calculator converts internally before applying the formula.
A 30-year-old adult is 175 cm tall and weighs 70 kg. Calculate the body mass index and identify the WHO classification.
BMI was developed in the 1830s by Adolphe Quetelet as a population-level statistic, not a body-fat measure. It correlates with body fat percentage on average but doesn't distinguish muscle from fat, so it misclassifies athletes and people with dense bone structure as overweight even when their body fat is healthy. WHO ranges apply to adults aged 20+ of average build; the CDC uses age- and sex-specific percentile charts for children and teens. For populations of South Asian descent, lower thresholds (e.g., overweight at ≥ 23) are recommended by several health bodies.
Divide weight in kilograms by height in meters squared: BMI = kg / m². If you have imperial units, the equivalent is BMI = 703 × pounds / inches², but it's easier to let a calculator convert internally.
BMI = weight(kg) ÷ height(m)². Example: a 70 kg person at 1.75 m has BMI = 70 / (1.75)² = 70 / 3.0625 ≈ 22.9.
The WHO classifies 18.5–24.9 as normal (healthy) weight for adults aged 20+. Below 18.5 is underweight; 25–29.9 is overweight; 30 and above is obese.
No. BMI doesn't distinguish muscle from fat, so very muscular people (athletes, bodybuilders) often land in the overweight or obese range despite low body fat. Body-fat percentage or waist-to-height ratio is a better screen for that population.
Adult BMI uses fixed cutoffs (18.5, 25, 30). Children and teens are evaluated against age- and sex-specific percentile curves from the CDC or WHO — a child with BMI 20 might be at the 85th percentile (overweight) at one age and the 50th percentile (normal) at another.
Yes. Health authorities recommend lower thresholds for some populations (e.g., overweight ≥ 23 and obesity ≥ 27.5 for adults of South Asian descent) because cardiometabolic risk rises at lower BMI in those groups. Check your country's clinical guidelines.
Reference:
Body mass index (BMI = mass in kg ÷ height in m²) was devised by Adolphe Quetelet in the 1830s; category thresholds follow standard WHO/CDC adult cutoffs. Centers for Disease Control and Prevention, BMI Categories for Adults. https://www.cdc.gov/bmi/adult-calculator/bmi-categories.html
Sports Performance
A 28-year-old male road cyclist is 178 cm tall and weighs 63 kg after a hard training block. What is his BMI, and how does it compare to the WHO categories for the general population?
BMI ≈ 19.9 kg/m² — Normal (lower end of the healthy range)
Lean endurance athletes often sit near the bottom of the Normal band. BMI under-weights muscle, so for athletic populations a body-fat measurement is a better screen than BMI alone.
Postpartum Clinical
A patient returns for her routine 6-week postpartum visit. She is 165 cm tall and weighs 78 kg. What is her current BMI for the chart, and how does it compare to the WHO classification?
BMI ≈ 28.7 kg/m² — Overweight under WHO adult thresholds
Informational only — not a substitute for clinical judgment. Postpartum weight retention is normal at 6 weeks; clinicians typically reassess at 6–12 months. BMI alone doesn't account for residual fluid, lactation status, or body-composition changes.
Geriatric Primary Care
An active 75-year-old woman is 162 cm tall and weighs 59 kg at her annual wellness visit. Compute her BMI and note where she falls on the WHO adult chart.
BMI ≈ 22.5 kg/m² — Normal weight
Informational only. Several geriatric guidelines recommend a slightly higher BMI floor (around 23–24) in adults over 65 because low BMI is associated with frailty and worse outcomes than mild overweight at that age.
Body Mass Index (BMI) is a single ratio that compares body mass to stature. It uses the Quetelet index, adopted by the WHO and CDC as the standard adult screening metric:
Where:
The WHO adult cut-offs apply to this formula: under 18.5 underweight, 18.5–24.9 normal, 25.0–29.9 overweight, 30.0 and above obese. BMI is a population-level screening tool — it does not distinguish lean mass from fat mass, so muscular athletes and people with high bone density can register as overweight or obese without elevated health risk, and older adults with sarcopenia can register as normal despite excess adiposity. Pair BMI with waist circumference, body fat percentage, or a clinical assessment for individual decisions. This calculator is informational only and is not a substitute for clinical judgment.
The World Health Organization adult BMI cut-offs, with the obese band split into the three WHO obesity classes. These ranges are the same thresholds this calculator uses to label your result above.
| Category | BMI (kg/m²) | Note |
|---|---|---|
| Underweight | Below 18.5 | Possible nutritional deficiency |
| Normal weight | 18.5 – 24.9 | Lowest associated health risk |
| Overweight | 25.0 – 29.9 | Increased health risk |
| Obese Class I | 30.0 – 34.9 | Moderate health risk |
| Obese Class II | 35.0 – 39.9 | Severe health risk |
| Obese Class III (severe) | 40.0 and above | Very severe health risk |
Source: World Health Organization (WHO) adult BMI classification. BMI is a population-level screening tool and does not distinguish lean mass from fat mass; interpret individual results alongside other measures.