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Sources & Methodology
Adult BMI: weight (kg) ÷ height (m)². Imperial: (weight (lbs) ÷ height (in)²) × 703. Reverse BMI: target weight (kg) = goal BMI × height (m)². BMI Prime: actual BMI ÷ 25. Ponderal Index: weight (kg) ÷ height (m)³. Child BMI-for-age percentile: approximated using CDC LMS method with Box-Cox power transformation, interpolating from published CDC median and standard deviation tables by age and sex.
Last reviewed: April 2026
Complete Guide to BMI — Formula, Categories, and Interpretation for Every Person
Body Mass Index (BMI) is the world’s most widely used screening tool for weight-related health assessment. Calculated from height and weight alone, it provides a quick estimate of whether an individual’s weight may pose health risks. Understanding how BMI is calculated, what the categories mean, and crucially, where BMI has significant limitations, is essential for interpreting your result in context.
The BMI Formula — Metric and Imperial
BMI was first developed by Belgian mathematician Adolphe Quetelet in the 1830s and was adopted by the WHO as a global screening standard in the 1990s. The formula is identical for adults of all ages, sexes, and ethnicities — though interpretation of the result varies significantly across these groups.
Imperial: BMI = [weight (lbs) ÷ height (in)²] × 703
Example (metric): 77 kg ÷ (1.75 m)² = 77 ÷ 3.0625 = 25.1
Example (imperial): [170 lbs ÷ (69 in)²] × 703 = [170 ÷ 4761] × 703 = 25.1
Healthy weight range from height:
Min weight (kg) = 18.5 × height (m)²
Max weight (kg) = 24.9 × height (m)²
WHO Adult BMI Categories — Complete Classification
The WHO uses a detailed 8-tier classification for adults aged 20 and older. Most people are familiar with the four main categories, but the full classification includes severity tiers for underweight and obesity.
| BMI Range | Category (WHO) | Health Risk | Asian Threshold |
|---|---|---|---|
| < 16.0 | Severely underweight | Very high | <16.0 |
| 16.0–16.99 | Moderately underweight | High | 16.0–16.99 |
| 17.0–18.49 | Mildly underweight | Moderate | 17.0–18.49 |
| 18.5–22.99 | Normal weight | Low | 18.5–22.99 |
| 23.0–24.99 | Normal weight | Low | Overweight risk |
| 25.0–29.99 | Overweight (pre-obese) | Increased | Obese |
| 30.0–34.99 | Obese Class I | High | Obese |
| 35.0–39.99 | Obese Class II | Very high | Very high |
| ≥ 40.0 | Obese Class III (morbid) | Extremely high | Extremely high |
BMI for Women — What the Numbers Mean Differently
The BMI formula produces the same number for men and women at identical heights and weights — but the health interpretation is not identical. At the same BMI, women have approximately 10% higher body fat than men, because women carry more essential fat for reproductive functions (hormonal regulation, pregnancy, lactation). This means a woman with BMI 22 and a man with BMI 22 have very different body compositions.
Healthy body fat percentages by sex according to the American Council on Exercise (ACE): For women aged 20–39, healthy body fat is 21–33%. For men aged 20–39, healthy body fat is 8–19%. A woman at the top of the normal BMI range (BMI 24.9) typically has about 32% body fat — which is within normal range for women but would indicate obesity for a man.
| BMI | Approx. Body Fat % (Women) | Approx. Body Fat % (Men) | Category (Both) |
|---|---|---|---|
| 17 | ~15% | ~8% | Underweight |
| 20 | ~22% | ~12% | Normal |
| 22 | ~28% | ~16% | Normal |
| 25 | ~34% | ~22% | Overweight boundary |
| 28 | ~38% | ~26% | Overweight |
| 32 | ~42% | ~32% | Obese Class I |
Child and Teen BMI — Why Percentiles Instead of Fixed Cutoffs
Children and teenagers ages 2 to 19 cannot be evaluated with the same fixed BMI cutoffs used for adults. A BMI of 18 means something very different for a 7-year-old versus a 17-year-old, and boys and girls have different growth trajectories. The CDC developed sex-specific BMI-for-age growth charts that compare a child’s BMI to thousands of children of the same age and sex in a reference population.
The CDC BMI percentile categories for children are: below the 5th percentile = underweight; 5th to below 85th percentile = healthy weight; 85th to below 95th percentile = overweight; 95th percentile and above = obese; 120% of the 95th percentile or BMI ≥ 35 = severe obesity. A child at the 60th percentile has a BMI higher than 60% of children the same age and sex — this is normal and healthy.
Reverse BMI — Your Target Weight for Any Goal BMI
Reverse BMI answers the question: “What weight do I need to reach to achieve a BMI of X?” This is the most practical application of the BMI formula for people working toward a health goal. The formula is simply the BMI formula solved for weight: target weight (kg) = goal BMI × height (m)².
For a person who is 5 ft 6 in (167.6 cm) and wants to achieve a BMI of 22: target weight = 22 × (1.676)² = 22 × 2.809 = 61.8 kg (136.3 lbs). If they currently weigh 80 kg, they need to lose 18.2 kg (40.1 lbs) to reach their goal. The reverse BMI calculator above computes this automatically with any height, goal BMI, and current weight.
BMI Prime, Ponderal Index, and the Limitations of BMI
BMI Prime — How Far from Normal Are You?
BMI Prime is the ratio of your actual BMI to 25 (the upper boundary of the normal range). It is calculated as: BMI Prime = actual BMI ÷ 25. A BMI Prime of exactly 1.0 means your BMI is exactly 25. Below 0.74 is underweight, 0.74 to 1.0 is normal, 1.0 to 1.2 is overweight, and above 1.2 is obese. The advantage of BMI Prime is that it gives an intuitive sense of scale: a BMI Prime of 1.3 means your BMI is 30% above the normal upper limit.
Ponderal Index — Better for Tall and Short People
The Ponderal Index (PI) divides weight by the cube of height rather than the square: PI = weight (kg) ÷ height (m)³. Normal range is approximately 11 to 14 kg/m³. BMI tends to classify very tall people as overweight even when they are not, because weight scales roughly with the cube of height while BMI only squares height. The PI corrects for this by using the cube. It is more reliable for individuals below 5 ft (152 cm) or above 6 ft 4 in (193 cm).
Where BMI Fails — Muscle, Ethnicity, Age
BMI has well-documented limitations that every user should understand. Muscle vs. fat: BMI cannot distinguish between muscle and fat. A professional bodybuilder with 8% body fat may have a BMI of 30 (classified as obese) while an elderly person with 40% body fat may have a BMI of 22 (classified as normal). BMI systematically misclassifies athletes and bodybuilders. Ethnicity: Asian populations develop metabolic complications at lower BMI values than European populations, which is why the WHO recommends an overweight threshold of BMI 23 (not 25) for South Asian, East Asian, and Southeast Asian individuals. Older adults: Adults over 65 lose muscle mass with age, so their BMI understates body fat. A BMI of 22–27 is often considered a better healthy range for older adults than the standard 18.5–24.9.
Healthy Weight Range by Height
| Height | Min Healthy (BMI 18.5) | Max Healthy (BMI 24.9) | Asian Min (BMI 18.5) | Asian Max (BMI 23.0) |
|---|---|---|---|---|
| 5’0” / 152 cm | 43 kg (94 lbs) | 58 kg (128 lbs) | 43 kg | 53 kg (117 lbs) |
| 5’3” / 160 cm | 47 kg (104 lbs) | 64 kg (141 lbs) | 47 kg | 59 kg (130 lbs) |
| 5’6” / 168 cm | 52 kg (115 lbs) | 70 kg (155 lbs) | 52 kg | 65 kg (143 lbs) |
| 5’9” / 175 cm | 57 kg (125 lbs) | 76 kg (168 lbs) | 57 kg | 70 kg (155 lbs) |
| 6’0” / 183 cm | 62 kg (136 lbs) | 83 kg (184 lbs) | 62 kg | 77 kg (170 lbs) |
| 6’3” / 191 cm | 67 kg (148 lbs) | 91 kg (200 lbs) | 67 kg | 84 kg (185 lbs) |