Free online BMI calculator

Body mass index is a simple height-to-weight ratio. This calculator shows your BMI and whether it falls in the CDC's healthy range for adults.

Units

Your BMI

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Healthy weight range

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For your height (BMI 18.5–24.9)

How to use

Use the Imperial / Metric toggle to choose your preferred units.
Enter your weight (lb or kg) and height (ft + in, or cm).
Your BMI appears instantly with the CDC category label and a gradient bar showing your position.
The second card shows the healthy weight range for your height.

The formula

Metric:

BMI = weight (kg) / height (m)²

Imperial:

BMI = (weight (lb) / height (in)²) × 703

Both produce the same result. The 703 factor is the unit conversion constant (kg/m² to lb/in²).

Worked example

Person weighing 154 lb (70 kg), height 5 ft 7 in (170 cm):

Metric: 70 / 1.70² = 70 / 2.89 ≈ 24.2 (Healthy weight)
Imperial: (154 / 67²) × 703 = (154 / 4489) × 703 ≈ 24.1

Rounding differences are normal — both land in the healthy range.

BMI categories (CDC, adult)

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 – 34.9	Obese (Class I)
35.0 – 39.9	Obese (Class II)
40 and above	Severe obesity (Class III)

What BMI misses

BMI is a screening tool, not a diagnosis. It’s worth knowing when it’s likely to mislead:

Muscle vs. fat — BMI counts pounds, not composition. A 200 lb bodybuilder at 10% body fat and a 200 lb sedentary office worker at 30% body fat have the same BMI, wildly different health profiles.
Older adults — muscle loss (sarcopenia) can leave you in the “healthy” band while actually underweight in lean tissue terms.
Ethnicity — research suggests risk thresholds differ across populations. The WHO suggests Asian adults may carry cardiovascular risk at BMI 23+, below the standard 25 cutoff.
Children and teens use age- and sex-specific percentiles, not the adult scale.

BMI is best read alongside waist circumference, body fat %, and blood markers. Use it as a rough signal, not a verdict.

Notes

BMI categories are the same for men and women on the adult scale.
BMI does not measure body fat percentage directly. A person with the same BMI can have very different body compositions.
Source: CDC Adult BMI — cdc.gov/bmi.

Frequently asked

How is BMI calculated?

BMI = weight (kg) / height (m)². In imperial units: BMI = (weight in lb / height in inches²) × 703. The result is the same number regardless of which unit system you use.

What do the BMI categories mean?

The CDC defines: Below 18.5 = Underweight; 18.5–24.9 = Healthy weight; 25.0–29.9 = Overweight; 30.0–34.9 = Obese (Class I); 35.0–39.9 = Obese (Class II); 40 and above = Severe obesity (Class III). These are population-level categories and don't account for muscle mass, age, or ethnicity.

What is the healthy weight range?

The healthy weight range shown is the range of body weights that produce a BMI between 18.5 and 24.9 for your specific height. It gives a practical lower and upper weight boundary.

Is BMI accurate?

BMI is a screening tool, not a diagnostic measure. Athletes with high muscle mass often register as overweight; older adults may have a healthy BMI with reduced muscle. A doctor evaluates BMI alongside other health indicators.

Does this calculator work for children?

No — BMI for children uses age- and sex-specific growth charts and is not the same as the adult BMI scale. The adult formula used here applies to people 20 and older.

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