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Body Mass Index is calculated by dividing your weight in kilograms by the square of your height in metres (BMI = kg/m²); in imperial units it is 703 times your weight in pounds divided by the square of your height in inches. The World Health Organization groups the result into ranges — underweight below 18.5, healthy 18.5 to 24.9, overweight 25 to 29.9, and obese 30 and above — as a screening estimate of body fatness, not a direct measurement.

BMI Calculator

Based on a height of 1.7 m and weight of 70 kg.

BMI24.2BMI — Healthy weight
BMI Prime
0.97
Ponderal Index
14.2
Healthy weight (min)
53.5
Healthy weight (max)
72.3

How it's calculated

  1. Divide weight by height squaredBMI=mh2\text{BMI} = \frac{m}{h^2}
    m
    = 70
    h
    = 1.7
    24.22
BMI24.2

How it works

Body Mass Index compares your weight to your height. The metric formula is BMI = weight (kg) ÷ height (m)²; the imperial formula multiplies by 703 to convert pounds and inches. The World Health Organization uses the same thresholds worldwide to classify the result.

Worked example

For an adult 5 ft 9 in (1.753 m) tall weighing 168 lb (76.2 kg): BMI = 76.2 ÷ (1.753 × 1.753) = 76.2 ÷ 3.073 = 24.8. The CDC lists 168 lb as the top of the healthy-weight range at 5 ft 9 in (125–168 lb), and BMI 24.8 sits just under the 25 overweight threshold — consistent with that classification.

Frequently asked questions

How is BMI calculated?

BMI divides your weight in kilograms by the square of your height in metres. In imperial units, multiply weight in pounds by 703 and divide by height in inches squared. The result is a single number compared against fixed thresholds.

What is a healthy BMI?

The World Health Organization classifies a BMI of 18.5 to 24.9 as a healthy weight, 25 to 29.9 as overweight, and 30 or above as obese; below 18.5 is underweight. The CDC further divides obesity into Class 1 (30 to under 35), Class 2 (35 to under 40), and Class 3 (40 and above).

What is my healthy weight range?

Your healthy-weight range is the weight that puts your BMI between 18.5 and 25 for your height — calculated as 18.5 × height² to 25 × height² in kilograms.

What are BMI Prime and the Ponderal Index?

BMI Prime is your BMI divided by 25 (the upper healthy limit), so a value below 1 is within range. The Ponderal Index divides weight by height cubed, which scales more consistently across very tall or very short people.

Does BMI differ by age or sex?

For adults, BMI is computed the same way regardless of age or sex. For children and teens aged 2–20, BMI is interpreted against age- and sex-specific growth percentiles rather than the adult thresholds.

Does the unit system change the result?

No. Metric and imperial give the same BMI; the 703 factor only converts pounds and inches into the equivalent of kilograms and metres.

What are the limitations of BMI?

BMI is a screening estimate from height and weight only. It does not distinguish muscle from fat or measure body-fat percentage directly, so it can overestimate fatness in very muscular people and underestimate it in others. It is not a diagnosis — treat it as a starting point, not a verdict.

How we know this is right

Last reviewed
Jul 18, 2026
Precision
Rounded to 1 decimal place.
Read our methodology

Sources