BMI Calculator: How to Calculate Body Mass Index and What It Means
A practical guide to BMI — the formula, healthy ranges, limitations, and how to use the calculator for metric and imperial units.
BMI is one of the most widely used numbers in health screening, yet most people have only a vague idea of what it means or how it’s calculated. Here’s a plain explanation of the formula, the categories, what the number actually tells you — and where it falls short.
What is BMI?
BMI (Body Mass Index) is a single number derived from your height and weight. It gives a rough indication of whether your body weight is in a healthy range for your height.
The formula:
BMI = weight (kg) ÷ height² (m)
For someone who is 1.75 m tall and weighs 75 kg: 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 ≈ 24.5
That falls in the Normal weight category.
BMI categories (WHO)
The World Health Organization defines four main ranges:
- Underweight — BMI below 18.5
- Normal weight — BMI 18.5 to 24.9
- Overweight — BMI 25 to 29.9
- Obese — BMI 30 or above
These thresholds apply to adults regardless of age or sex. Some health authorities recommend adjusted ranges for specific ethnic groups — more on that below.
How to calculate BMI in imperial units
If you work in feet, inches, and pounds:
BMI = (weight in lbs × 703) ÷ height in inches²
For someone who is 5 ft 9 in (69 inches) and weighs 160 lbs: (160 × 703) ÷ (69 × 69) = 112,480 ÷ 4,761 ≈ 23.6
The BMI Calculator handles both unit systems — switch between Metric and Imperial and enter your numbers.
What your BMI result actually means
Your result places you in a category, but it’s most useful alongside your healthy weight range — the span of weights at which your BMI would fall between 18.5 and 24.9 for your height.
If your BMI is in the Normal range, your weight is already within that zone. If it’s in the Underweight range, gaining a modest amount and discussing nutrition with a professional may reduce health risks. If it’s Overweight or Obese, even a 5–10% reduction in body weight tends to produce measurable results: lower blood pressure, improved cholesterol, reduced strain on joints.
Where BMI falls short
BMI is a population screening tool. It was designed for comparing large groups, not for diagnosing individuals. Its limitations are well documented:
Muscle mass. BMI can’t distinguish fat from muscle. A well-trained athlete may have a BMI of 28 while carrying very little body fat. An untrained person can have a BMI of 22 with high fat percentage — what’s sometimes called “skinny fat.”
Body fat distribution. Where fat is stored matters. Abdominal fat carries more cardiovascular risk than fat around the hips and thighs. BMI doesn’t tell you which you have.
Age and sex. Older adults and women naturally carry more body fat at the same BMI as younger men. The categories don’t adjust for this.
Ethnic differences. Research suggests that health risks at a given BMI differ across ethnic groups. Several health bodies recommend lower thresholds for people of Asian descent — in the UK, for example, the NHS uses 23 as the Overweight cutoff for South Asians rather than 25.
For a fuller picture, your doctor may also measure waist circumference, blood pressure, cholesterol, and blood glucose.
How to use BMI sensibly
Track trends, not snapshots. A single reading is less informative than watching how your BMI changes over months. A trend in either direction tells you more than any one number.
Combine it with other measures. Waist-to-height ratio — your waist circumference divided by your height, ideally below 0.5 — is a useful complement that captures abdominal fat specifically.
Don’t use it for children. BMI for under-18s is calculated using age- and sex-specific percentile charts, not the adult thresholds above.
Use the free BMI Calculator to find your number and healthy weight range — no signup, no data sent to any server.