Body Mass Index (BMI) is a simple screening measurement that relates a person's weight to their height. It's used by clinicians, researchers, and public health organizations worldwide as a fast, low-cost way to flag whether someone's weight falls in a range associated with higher health risk. This calculator computes BMI using either metric (kilograms and centimeters) or imperial (pounds and inches) units, then places the result in one of the standard BMI categories.
How BMI is calculated
BMI compares weight to the square of height, which normalizes the comparison across people of different heights. The metric and imperial formulas produce the same result; they simply use different starting units.
For example, a person weighing 70 kg who is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9. Using imperial units, someone weighing 154 lb at 69 inches tall gets 703 × 154 ÷ (69 × 69) ≈ 22.7 — the small difference comes from rounding during unit conversion.
Standard BMI categories for adults
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These thresholds come from World Health Organization guidance for adults and are the most widely used reference ranges internationally, though some national health bodies apply adjusted thresholds for specific populations.
Step-by-step: calculating BMI by hand
- Convert your height to meters (metric) or inches (imperial) if it isn't already.
- Square the height value.
- Divide your weight by that squared height (metric), or multiply weight by 703 first, then divide (imperial).
- Compare the result to the category table above.
Real-life use cases
- Routine health checks: Many clinics record BMI as a quick baseline alongside blood pressure and other vitals.
- Tracking trends over time: Recalculating BMI periodically can help you and a healthcare provider notice gradual weight trends.
- Insurance and wellness programs: Some employer or insurer wellness programs use BMI as one input among several health metrics.
- Sports and fitness screening: Coaches sometimes use BMI as an initial, low-cost screening tool before more detailed body composition testing.
Common mistakes and limitations
- Treating BMI as a diagnosis. BMI is a screening tool, not a diagnostic measurement — it doesn't directly measure body fat percentage.
- Ignoring muscle mass. Very muscular individuals, such as athletes, can have a high BMI while carrying low body fat, because BMI cannot distinguish muscle from fat.
- Applying adult ranges to children. Children and teens should be assessed with age- and sex-specific percentile charts, not the fixed adult categories.
- Mixing unit systems. Entering height in inches while weight is in kilograms (or vice versa) produces a meaningless result — always keep units consistent for the system you've selected.
Tips
- Weigh yourself at a similar time of day for more consistent tracking, since body weight naturally fluctuates throughout the day.
- Pair BMI with other measurements — such as waist circumference or body fat percentage — for a fuller picture of health.
- If your BMI falls outside the "normal" range, treat it as a prompt to talk with a healthcare provider rather than a final verdict.
Frequently asked questions
What is a healthy BMI range?
For most adults, the World Health Organization defines 18.5 to 24.9 as the healthy weight range, under 18.5 as underweight, 25 to 29.9 as overweight, and 30 or above as obese.
Is BMI accurate for everyone?
BMI is a useful population-level screening tool, but it does not distinguish between muscle and fat mass. It can overestimate body fat in muscular individuals and underestimate it in older adults who have lost muscle mass, so it should be considered alongside other measurements.
Does BMI apply the same way to children?
No. Children and teenagers are assessed using age- and sex-specific BMI percentile charts rather than the fixed adult categories, because healthy body composition changes significantly during growth.
What formula does the BMI calculator use?
BMI equals weight in kilograms divided by height in meters squared. For imperial units, BMI equals weight in pounds divided by height in inches squared, multiplied by 703.
References
- World Health Organization — BMI classification thresholds for adults
- Centers for Disease Control and Prevention — BMI background and healthy weight guidance