How to Calculate BMI, and What the Number Actually Tells You
Body Mass Index is two numbers and one division. That simplicity is why every clinic uses it, and also why it gets people wrong. Here is the arithmetic, and the honest limits.
By Rohit Sharma, Founder, SEOShouts
Body Mass Index compares your weight against your height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was studying populations, not patients. That origin explains most of its strengths and nearly all of its weaknesses: it is excellent at describing groups and only roughly useful for describing you.
The formula
BMI = weight (kg) ÷ height (m)²
Height is squared, so it must be in metres, not centimetres. 175 cm becomes 1.75 m.
BMI = 703 × weight (lb) ÷ height (in)²
The 703 converts pounds-per-square-inch into the metric scale so both formulas land on the same number.
A worked example
Take someone who is 175 cm tall and weighs 70 kg. Square the height: 1.75 × 1.75 = 3.0625. Then divide: 70 ÷ 3.0625 = 22.9. That falls in the middle of the healthy range.
The same person in imperial units is 68.9 inches and 154.3 pounds. 68.9² = 4747.2. Then 703 × 154.3 ÷ 4747.2 = 22.9. Identical, as it should be.
The standard categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obese |
These cut-offs are not laws of nature. They are thresholds chosen because, across large populations, health risks begin to rise noticeably around those points. An individual sitting at 25.1 is not meaningfully different from one at 24.9.
Where BMI gets it wrong
BMI measures mass, and it cannot tell what that mass is made of. Muscle is roughly 18 per cent denser than fat, so anyone carrying significant muscle reads high. A rugby forward or a competitive swimmer will frequently be classed as overweight or obese while carrying very little body fat.
The reverse also happens. Someone who has lost muscle through age or inactivity can sit comfortably inside the healthy band while carrying an unhealthy proportion of fat, a pattern sometimes called normal-weight obesity.
- Athletes and heavily muscled people read too high.
- Older adults who have lost lean mass often read too low for their actual risk.
- The standard thresholds were derived largely from European populations; risk rises at lower BMI values in South Asian and some East Asian populations, and several health bodies use a lower overweight cut-off of around 23 for these groups.
- It says nothing about where fat sits, and abdominal fat carries more metabolic risk than the same weight on hips and thighs.
- It is not designed for children, pregnant people, or anyone with a condition that alters fluid retention.
How to use it sensibly
Treat BMI as one reading on a dashboard, not a verdict. It is genuinely useful for tracking a trend in yourself over months, because your height is fixed and your body composition changes slowly. It is far less useful as a single-snapshot judgement, and it was never meant to diagnose anyone.
If your number sits outside the healthy band, the productive next question is not whether the number is right but what else is true: your waist measurement, your blood pressure, your resting heart rate, how you sleep, and what a clinician makes of the combination.
Frequently asked questions
Is BMI accurate for athletes?
Often not. Muscle is denser than fat, so a well-trained athlete can record a BMI in the overweight or obese range while carrying very low body fat. Body-fat percentage or waist-to-height ratio is a better measure for anyone carrying substantial muscle.
What is a healthy BMI?
The standard healthy range is 18.5 to 24.9. Several health authorities apply a lower overweight threshold of about 23 for people of South Asian and some East Asian backgrounds, because metabolic risk rises at a lower BMI in those populations.
Does BMI differ for men and women?
The formula and the thresholds are identical. Women typically carry a higher proportion of body fat at the same BMI, which is one more reason the number is a rough guide rather than a measure of body composition.
Should I use BMI for my child?
Not with the adult categories. Children are assessed against age-and-sex percentile charts, because a growing body changes shape constantly. Ask a paediatrician rather than applying adult cut-offs.
Calculators from this guide
About the author
Rohit Sharma is the founder of SEOShouts, a search consultancy in India, and has worked in technical SEO and content strategy since 2014. He builds and maintains Calcshark.