Calculate your Body Mass Index (BMI) from height and weight, and get a full explanation of what it means: your BMI category, the healthy weight range for your height, BMI Prime, the Ponderal Index, and general health context — not just a bare number.
Body Mass Index (BMI) is a measurement based on height and weight, commonly used as a general indicator of whether a person's weight falls within a typical range for their height. It does not directly measure body fat, muscle mass, bone density, or overall body composition — it is a quick screening tool, not a diagnosis.
BMI is calculated by dividing weight by the square of height. Using a 175 cm, 70 kg person as an example:
| Category | BMI range (kg/m²) |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 – 17 |
| Mild Thinness | 17 – 18.5 |
| Normal | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obese Class I | 30 – 35 |
| Obese Class II | 35 – 40 |
| Obese Class III | > 40 |
Because BMI changes naturally as children grow, results for ages 2–20 are normally interpreted using age- and sex-specific percentile growth charts (such as the CDC growth charts) rather than the fixed adult ranges above.
| Category | Percentile |
|---|---|
| Underweight | Below the 5th percentile |
| Healthy weight | 5th to below the 85th percentile |
| Overweight | 85th to below the 95th percentile |
| Obesity | 95th percentile or higher |
BMI Prime expresses BMI as a ratio to the upper end of the normal range (25 kg/m²):
A BMI Prime under 1.0 sits below the upper healthy-range boundary; over 1.0 sits above it. It allows quick comparison between people with different BMI reference limits.
The Ponderal Index (PI) is similar to BMI but uses height cubed instead of height squared, which makes it more proportionate for people who are notably tall or short, where BMI can sometimes over- or under-estimate:
A higher BMI category can be associated with increased risk of conditions including high blood pressure, type 2 diabetes, cardiovascular disease, stroke, sleep apnea, osteoarthritis, and certain cancers. A lower BMI category can be linked to nutritional deficiencies, anemia, reduced immune function, bone health concerns, and possible hormonal or reproductive effects. These are general associations, not a diagnosis — individual risk depends on many other factors.
BMI does not distinguish between muscle, fat, bone mass, water, and overall body composition. Athletes and highly muscular people may show a high BMI while having relatively low body fat. Interpretation can also differ depending on age, sex, ethnicity, muscle mass, and activity level. BMI is fairly indicative of body fat for most of the population, but works best alongside other measures rather than on its own.
This calculator provides general informational estimates and is not a medical diagnosis. If you have concerns about your health, consider speaking with a qualified healthcare professional.
Example 1 — Metric: A person who is 168 cm tall and weighs 65 kg has a BMI of 65 ÷ 1.68² = 23.0, which falls in the Normal range.
Example 2 — US units: A person who is 5'10" (70 in) and weighs 210 lb has a BMI of 703 × 210 ÷ 70² = 30.1, which falls in the Obese Class I range.
Example 3 — Comparing two heights at the same weight: Two people both weighing 80 kg but with heights of 160 cm and 185 cm have BMIs of 31.3 (Obese Class I) and 23.4 (Normal) respectively — the same weight lands in very different categories depending on height.
| Method | What it measures | Pros / cons |
|---|---|---|
| BMI | Weight relative to height² | Fast, free, but ignores muscle vs. fat |
| Body fat % | Actual proportion of fat mass | More accurate, needs calipers/scan |
| Waist-to-hip ratio | Fat distribution | Simple, flags abdominal fat risk |
| Ponderal Index | Weight relative to height³ | Better for very tall/short people |
Is BMI accurate for everyone? No. BMI does not distinguish muscle from fat, so athletes and very muscular people can show a high BMI despite low body fat, while older adults can have a "normal" BMI despite higher body fat.
What is a healthy BMI? For most adults, a BMI between 18.5 and 25 kg/m² falls in the "Normal" range, though healthy ranges can vary slightly by population and individual factors.
Does BMI work the same way for children? No. Children and teens (ages 2–20) are assessed using age- and sex-specific percentile charts rather than the fixed adult BMI ranges.
What's the difference between BMI and BMI Prime? BMI Prime is simply your BMI divided by 25, giving a ratio to the upper edge of the normal range — useful for comparing how far above or below that threshold someone is.
Why do two people with the same BMI look very different? BMI cannot tell how weight is distributed between muscle, fat, and bone — two people at the same BMI can have very different body compositions and fat distribution.
Is a BMI-based "healthy weight range" the same as an ideal weight? Not exactly. The BMI healthy range is a broad band (18.5–25) covering many body types, while "ideal weight" formulas typically produce a single target figure based on height and frame.
Does BMI account for ethnicity? Standard BMI cutoffs don't adjust for ethnicity, though some health bodies recommend lower thresholds for certain populations (for example, some Asian populations) due to differing health risk at the same BMI.
How often should I recalculate my BMI? There's no fixed rule, but checking every few months alongside other health markers is more useful than frequent day-to-day tracking, since weight naturally fluctuates.
Is BMI calculated differently for women and men? No — the formula (weight ÷ height²) and the standard adult category ranges are identical for both sexes. Women do tend to carry a higher percentage of body fat than men at the same BMI, which is why BMI is best read alongside other measures like body fat percentage for a fuller picture.
For a shorter reference example: a person at 160 cm and 55 kg has a BMI of 55 ÷ 1.6² = 21.5, which falls in the Normal category — the same formula scales consistently regardless of height, which is part of why BMI became popular as a simple, universal screening tool.
Body Mass Index (BMI): A screening measure calculated from height and weight.
BMI Prime: BMI expressed as a ratio to the upper boundary of the normal range (25 kg/m²).
Ponderal Index: A height-cubed-based alternative to BMI, more proportionate for very tall or short individuals.