How to Calculate BMI: Formula, Chart & What Your Number Means
Learn how to calculate BMI using the metric and imperial formulas, what your number means, and its limitations, plus try our free BMI calculator tool today.
Doctors mention it, fitness apps display it, and insurance forms occasionally ask for it, yet most people have never actually worked through how to calculate BMI by hand. The formula itself is short, only requiring height and weight, but knowing what the resulting number actually means takes a bit more context.
This guide covers the BMI formula in both metric and imperial units, a full worked example, what the standard BMI categories mean, whether the calculation differs for women and men, and the well-documented limitations of BMI as a health measure.
What Is BMI?
BMI, or body mass index, is a simple ratio of weight to height, used as a quick screening number to flag whether someone's weight falls within a typical range for their height. It does not measure body fat directly, and it does not account for muscle mass, bone density, or where weight is distributed on the body, which is why it works best as a starting point rather than a complete health assessment.
The BMI Formula (Metric and Imperial)
Metric formula
BMI = Weight (kg) ÷ Height (m)²
Imperial formula
BMI = 703 × Weight (lb) ÷ Height (in)²
Both formulas produce the same BMI for the same person, since the 703 multiplier in the imperial version exists specifically to convert pounds and inches into the equivalent of the metric calculation.
Step-by-Step BMI Calculation Example
Take someone who is 5 feet 7 inches tall (67 inches) and weighs 150 pounds.
| Step | Calculation | Result |
|---|---|---|
| 1. Square the height | 67 × 67 | 4,489 |
| 2. Multiply weight by 703 | 150 × 703 | 105,450 |
| 3. Divide | 105,450 ÷ 4,489 | 23.5 |
This person's BMI is approximately 23.5, which falls into the Normal category on the standard scale.
Try it yourself
BMI Calculator
Enter your height and weight in metric or imperial units to get your BMI, category, and healthy weight range instantly, with a visual gauge showing exactly where your result falls.
BMI Categories: What Does Your Number Mean?
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal |
| 25 to 29.9 | Overweight |
| 30 and above | Obese |
These categories are the standard reference ranges used in most general health contexts, though some healthcare providers apply adjusted thresholds for specific populations or age groups.
BMI for Women vs Men: Are There Differences?
The BMI formula itself is identical regardless of sex, calculated the same way for everyone using only height and weight. Research indicates that body composition often differs between men and women on average, such as typical differences in muscle mass and fat distribution, which is part of why some health professionals interpret a given BMI number somewhat differently depending on sex and other individual factors. The calculation does not adjust for this automatically, so the number itself is identical, even if its interpretation in a broader health context sometimes is not.
Limitations of BMI as a Health Measure
- It cannot distinguish muscle from fat. A muscular, athletic person can register a high BMI despite having low body fat, since the formula has no way to separate the two.
- It does not account for fat distribution. Studies suggest that where fat is carried on the body can matter for health risk, a distinction BMI cannot capture.
- It uses fixed categories for a highly varied population. Age, frame size, and individual variation all affect what a "healthy" weight actually looks like for a given person, beyond what one formula can capture.
- It was not originally designed as an individual diagnostic tool. BMI has historically been used more as a population-level screening measure than a precise individual health verdict.
None of this makes BMI useless. It remains a fast, free, and widely understood starting point, as long as it is treated as one data point among several rather than a complete verdict on someone's health.
Frequently asked questions
Is BMI the same formula for adults of any age?
The formula itself does not change based on age, but BMI categories are generally designed for adults. Children and teenagers are typically assessed using age- and sex-specific growth charts and percentiles rather than the same fixed categories used for adults.
Why do two people with the same BMI sometimes look very different?
BMI only uses height and weight, so it cannot distinguish between weight from muscle, bone, and fat. Two people with very different body compositions, such as a muscular athlete and a sedentary individual of similar height and weight, can land on the exact same BMI number despite looking and being built very differently.
What should I do if my BMI falls outside the normal range?
Treat the number as a general screening signal rather than a diagnosis. Research indicates that a fuller health picture usually requires additional context, such as waist circumference, body composition, family history, and a conversation with a healthcare provider, rather than relying on BMI in isolation.
Does BMI account for where fat is distributed on the body?
No. BMI is a single number based only on overall height and weight, and it does not distinguish between fat carried around the waist versus the hips or limbs. Studies suggest that fat distribution can be a meaningful factor in health risk assessment, which is one of the reasons BMI is generally treated as a starting point rather than a complete picture.
Now that you know how to calculate BMI using both metric and imperial units, and what the resulting category actually represents, you have enough context to use the number sensibly. Use the BMI Calculator above to get your BMI, category, and healthy weight range instantly, with a visual gauge and metric or imperial toggle built in.
For other health-adjacent calculations, see how to convert Celsius to Fahrenheit , or for tracking age-related milestones, see how to calculate age from a date of birth.