BMI Calculator
Calculate your Body Mass Index from height and weight, see your category, and understand what the number does — and doesn't — tell you.
+ Add waist & hip measurements (optional — unlocks waist-to-hip and waist-to-height ratio)
Ideal weight range (4 clinical formulas)
| 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.
Waist-based ratios
Quick answer
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.
- BMI cannot distinguish muscle mass from fat mass.
- Four separate clinical formulas (Devine, Robinson, Miller, Hamwi) exist for "ideal weight," and they disagree with each other.
- Waist-based ratios capture fat distribution, which BMI alone cannot.
| 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.
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.
What BMI actually measures
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.
How this calculator works
Enter your height and weight in either metric or imperial units:
The formula behind the calculation
The calculator uses the standard formula:
- Metric: BMI = weight in kilograms ÷ (height in metres)²
- Imperial: BMI = 703 × weight in pounds ÷ (height in inches)²
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.
Worked examples
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.
BMI categories at a glance
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 |
Beyond your BMI number
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:
- Healthy weight range — a direct weight-range estimate for your height, independent of the BMI formula.
- Body fat percentage — the composition measurement BMI can't distinguish from muscle.
- Daily calorie needs — how many calories to maintain, lose, or gain weight from here.
- TDEE — total daily energy expenditure, factoring in activity level.
- Protein target — a daily protein range based on your weight and goal.
- Macro breakdown — protein, carbs, and fat split for your calorie target.
Benefits and limitations
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.
Expert tips
- Treat BMI as one data point, not a full picture — waist circumference and body composition give more individual context.
- If you're very muscular or an athlete, don't rely on BMI alone; it commonly overstates weight-related risk for this group.
- Track trends over time rather than a single reading — a gradual shift is more informative than one number.
- Bring your result to a healthcare provider for interpretation alongside your personal and family health history.
Common mistakes
- Applying adult BMI bands directly to children or teens, who need age- and sex-specific percentile charts instead.
- Treating BMI as a diagnosis rather than a screening signal.
- Ignoring muscle mass — a muscular person can have a "high" BMI with low body fat.
- Comparing BMI across very different ages without accounting for how healthy ranges shift with age.
Glossary
- BMI (Body Mass Index)
- A ratio of weight to height squared, used as a quick population-level weight-category screen.
- Body composition
- The proportion of fat, muscle, bone, and water that make up body weight — not captured by BMI.
- Visceral fat
- Fat stored around internal organs, more strongly linked to health risk than overall body fat percentage.
- Healthy weight range
- The span of weights, at a given height, that fall within the "normal" BMI band.
Beyond BMI
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.
Body Mass Index (BMI): everything to know
-
Definition
BMI is a screening ratio of weight to height, used to sort adults into underweight, normal, overweight, and obese categories at a population level.
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Attributes
- Height
- Weight
- Sex
- Age
- Waist circumference
- Hip circumference
-
Dimensions
- BMI value
- BMI category
- Ponderal Index
- BMI Prime
- Waist-to-height ratio
- Waist-to-hip ratio
-
Measurements
- Height (cm or ft/in)
- Weight (kg or lb)
- Waist circumference
- Hip circumference
-
Classifications
- Underweight
- Normal weight
- Overweight
- Obese Class I–III
-
Context
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.
-
Core questions
- What is my BMI?
- What is a healthy weight for my height?
- Is BMI accurate for muscular people?
- What is my ideal weight?
-
Comparisons
- BMI vs. body fat percentage
- BMI vs. Ponderal Index
- Waist-to-hip ratio vs. BMI as a risk indicator
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Decision guidance
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.
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Examples
- 170 cm / 65 kg → BMI 22.5 (normal)
- 5'7" / 160 lb → BMI 25.1 (overweight)
-
Common mistakes
- Applying adult BMI bands to children
- Ignoring muscle mass in athletes
- Treating BMI as a diagnosis
Go beyond this number
A single metric rarely tells the whole story. These feed into the same decision:
- Ideal / Healthy Weight A weight range instead of a single BMI number.
- Calories Daily intake needed to reach or hold that weight.
- Macros How those calories split across protein, carbs, and fat.
- Protein A target gram range to support lean mass.
- Exercise (TDEE) How activity level changes the calorie target.
- Body Fat A composition check BMI alone can't capture.
Frequently asked questions
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.
Facts, regions & standards
- Aliases
- Body Mass Index, Quetelet Index
- Synonyms
- Weight-to-height ratio
- Countries / regions
- Global — WHO adult bands used near-universally; pediatric charts vary by country (CDC in the US, WHO growth standards elsewhere)
- Units
- kg/m², lb/in² (via the 703 constant)
- History
- Originally the "Quetelet Index," devised by Belgian statistician Adolphe Quetelet in the 1830s; renamed BMI and popularized for clinical screening in the 1970s–80s.
- Standards bodies
- https://www.who.int, https://www.cdc.gov
- Relationships
- Ponderal Index, BMI Prime, Waist-to-hip ratio, Waist-to-height ratio, Body fat percentage
Sources & references
- About Adult BMI — Centers for Disease Control and Prevention (CDC) Government health body
- Body Mass Index (BMI) — World Health Organization (WHO) International health authority
- Assessment of Obesity and Its Clinical Implications — National Institutes of Health (NIH) Government research institute
- Waist Circumference and Waist-Hip Ratio: Report of a WHO Expert Consultation — World Health Organization (WHO) International health authority
- CDC Child and Teen BMI Calculator — Centers for Disease Control and Prevention (CDC) Government health body — use for ages 2–19
Author & reviewer
Why trust this tool
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.
Research process
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.
Update history
- — Added Ideal Weight (Devine/Robinson/Miller/Hamwi), Ponderal Index, BMI Prime, waist-to-height and waist-to-hip ratios; added entity layer, knowledge graph, and AI Overview block.
- — Added information-gain links to related calculators and a static category chart.
Turn your BMI into a full health picture
Pair it with calorie, macro, and body-fat targets built from the same inputs.