Calculate your Body Mass Index from height and weight, see your category, and understand what the number does — and doesn't — tell you.
| Formula | Estimated ideal weight |
|---|---|
| Devine (1974) | — |
| Robinson (1983) | — |
| Miller (1983) | — |
| Hamwi (1964) | — |
| Average of the 4 | — |
Select a sex above to see these — each formula was derived from a different reference population, so treat the spread as a range, not a single target.
BMI is weight in kilograms divided by height in meters squared. A result of 18.5–24.9 is classed as "normal weight" for most adults, but it doesn't distinguish muscle from fat or account for where fat is stored.
| Metric | What it adds beyond plain BMI |
|---|---|
| Ponderal Index | Scales by height cubed instead of squared — better at height extremes |
| BMI Prime | Expresses BMI as a ratio to the normal-range ceiling |
| Waist-to-height ratio | Captures central fat distribution BMI misses |
| Waist-to-hip ratio | WHO-defined cardiometabolic risk indicator, independent of BMI |
A single height/weight input can generate BMI plus four related metrics — Ponderal Index, BMI Prime, and (with waist/hip measurements) two fat-distribution ratios — giving a fuller screening picture than BMI alone.
This calculator uses the exact BMI formula and category thresholds published by the WHO and CDC — no proprietary adjustment. Content is written by a registered dietitian and reviewed by a licensed physician, and every category claim in this article is sourced to the citations below rather than paraphrased from a secondary site.
Category ranges and risk associations are drawn directly from the primary CDC and WHO reference pages linked below. Where guidance differs by age or population (e.g. pediatric percentiles, older-adult thresholds), that's flagged explicitly in the content rather than folded into the general adult ranges.
Quick summary: BMI = weight (kg) ÷ height (m)². A result of 18.5–24.9 is classed as "normal weight," below 18.5 as "underweight," 25–29.9 as "overweight," and 30+ as "obese." BMI is a fast population-level screening tool, not a diagnosis — it doesn't distinguish muscle from fat, and healthy ranges shift for children, older adults, and some ethnic groups.
Body Mass Index (BMI) is a simple ratio of weight to height, used since the 1830s (originally as the "Quetelet Index") as a quick way to screen populations for weight categories associated with health risk. It does not measure body fat directly, muscle mass, bone density, or where fat is distributed — all of which matter more than BMI alone for individual health assessment.
Despite its limitations, BMI remains widely used by clinicians and public health bodies because it's cheap, fast, and correlates reasonably well with health risk at a population level, even though it can mislead for individuals — particularly athletes, older adults, and people with unusual body compositions.
Enter your height and weight in either metric or imperial units:
The calculator uses the standard formula:
The healthy weight range shown is back-calculated from the 18.5–24.9 "normal" BMI band at your entered height. For anyone under 20 or over 65, the calculator adds a note that standard adult BMI bands don't directly apply — pediatric BMI uses age- and sex-specific percentiles rather than the fixed adult category bands used here, and some guidance uses slightly higher thresholds for older adults. We don't yet calculate a pediatric percentile in this tool; for children and teens, use the CDC's BMI-for-age growth charts directly (linked in references below).
Ponderal Index = weight (kg) ÷ height (m)³. Unlike BMI, it scales weight against height cubed rather than squared, which some researchers consider a better fit for very tall or very short people.
BMI Prime = your BMI ÷ 25 (the upper edge of "normal"). A BMI Prime of 1.0 sits exactly at that boundary; below 1.0 is in or under the normal range, above 1.0 is over it — it's a quick way to see "how far" a result is from the normal ceiling.
Ideal weight (4 formulas) — Devine, Robinson, Miller, and Hamwi are all decades-old clinical formulas (originally built for drug-dosing calculations, not general health advice) that estimate a reference body weight from height and sex, each based on a different reference population. They routinely disagree with each other by several kilograms, which is exactly why we show all four side by side instead of picking one.
Waist-to-height ratio = waist circumference ÷ height (same units). Waist-to-hip ratio = waist circumference ÷ hip circumference. Both are measures of fat distribution that BMI cannot capture, and several studies suggest they track cardiometabolic risk at least as well as BMI, independent of it.
Example 1 — Metric: 170 cm and 65 kg → BMI = 65 ÷ (1.70 × 1.70) = 22.5, in the "normal weight" range.
Example 2 — Imperial: 5'7" (67 in) and 160 lb → BMI = 703 × 160 ÷ (67 × 67) ≈ 25.1, just into the "overweight" band.
Example 3 — Healthy range: At 175 cm, the "normal" BMI band (18.5–24.9) corresponds to roughly 56.6–76.3 kg.
Standard adult BMI categories used by most major health bodies:
| Category | BMI range | General association |
|---|---|---|
| Underweight | Below 18.5 | May indicate insufficient nutrition or an underlying condition |
| Normal weight | 18.5 – 24.9 | Generally associated with lowest average health risk |
| Overweight | 25.0 – 29.9 | Associated with somewhat elevated average risk |
| Obese (Class I–III) | 30.0 and above | Associated with higher average risk of related conditions |
BMI alone is one input among several that matter for a fuller picture. This calculator suite has dedicated tools for the numbers BMI leaves out:
Benefits: fast, free, requires no special equipment, and gives a reasonable population-level signal that's useful as a conversation starter with a healthcare provider.
Limitations: BMI can't tell muscle from fat, so athletes and very muscular people are often misclassified as "overweight." It doesn't account for fat distribution (visceral vs. subcutaneous), which matters clinically. Standard bands are also less reliable across different ages, sexes, and some ethnic groups, where risk can shift at different BMI thresholds.
Clinical guidance is increasingly pairing BMI with waist circumference, waist-to-height ratio, or direct body-composition measurement (such as bioelectrical impedance or DEXA scans) to better capture individual risk, rather than relying on BMI in isolation.
BMI is a screening ratio of weight to height, used to sort adults into underweight, normal, overweight, and obese categories at a population level.
BMI sits alongside body composition, fat distribution, and metabolic markers as one of several weight-related health screening signals — none of which is complete on its own.
Once BMI and category are known, the next decisions are usually a calorie target, a macro split, or a body-composition check — all downstream of the same weight/height inputs.
A single metric rarely tells the whole story. These feed into the same decision:
For most adults, a BMI between 18.5 and 24.9 is classified as "normal weight." This range is a population-level guideline, not an individual guarantee of health.
Not reliably. BMI can't distinguish muscle from fat, so athletes and very muscular individuals are often classified as "overweight" or "obese" despite low body fat.
No. Children and teens are assessed using age- and sex-specific BMI percentiles rather than the fixed adult category bands used here.
Standard adult BMI bands are generally intended for ages 20–65. Outside that range, the calculator flags that the category may not apply as directly.
It's the weight span, at your entered height, that falls within the 18.5–24.9 "normal" BMI band — calculated by solving the BMI formula for weight at each end of that range.
No. BMI is a useful screening starting point, but decisions about weight and health are best made with a healthcare provider using additional context like body composition, waist circumference, and personal health history.
Ponderal Index divides weight by height cubed instead of height squared. It's less commonly used than BMI, but some researchers consider it more accurate at the extremes of height, since it scales differently as height increases.
BMI Prime is your BMI divided by 25, the upper bound of the "normal" range. A value of 1.0 means you're exactly at that boundary — it's a quick way to express how far above or below the normal ceiling a result sits.
Devine, Robinson, Miller, and Hamwi were each derived from different reference populations and different original purposes (several were built for drug-dosing calculations). Disagreement of a few kilograms between them is normal — treat the range, not any single formula, as the useful output.
The World Health Organization defines substantially increased risk above 0.90 for men and above 0.85 for women, though this is a population-level guideline, not an individual diagnosis.
A common public-health rule of thumb is keeping waist circumference under half your height (a ratio below 0.5). Ratios of 0.6 or above are generally associated with substantially higher risk.