onlinecalculator.me

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
—
Healthy weight range
—
For your height (BMI 18.5–24.9)
About this calculator

How to use

  1. Use the Imperial / Metric toggle to choose your preferred units.
  2. Enter your weight (lb or kg) and height (ft + in, or cm).
  3. Your BMI appears instantly with the CDC category label and a gradient bar showing your position.
  4. 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.

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.