BMI Calculator: How to Calculate BMI Correctly — Practical Guide
bmi calculator: Calculate BMI from height and weight, convert units correctly, and understand why BMI is a screening measure rather than a diagnosis.
Body mass index (BMI) compares weight with height using a simple ratio. Adults can calculate it from kilograms and meters or pounds and inches, but the result is only a screening measure. It does not directly measure body fat or distinguish muscle from fat. Use a BMI calculator to check the arithmetic, then consider age, health history, and clinical guidance before drawing conclusions. BMI categories are interpreted differently for children and teens, where age and sex percentiles are used.
Enter weight and height in matching units
For metric units, BMI equals weight in kilograms divided by height in meters squared. For U.S. customary units, multiply weight in pounds by 703, then divide by height in inches squared. Enter the units exactly as shown and avoid mixing centimeters with meters. Rounding height too early can shift the result, so use a reasonably precise measurement. A calculator can prevent arithmetic slips, but it cannot confirm that the measurement itself is accurate.
Read adult ranges with context
Common adult BMI categories use thresholds such as 18.5, 25, and 30, but they are population-level screening ranges rather than a diagnosis for an individual. Body composition, pregnancy, fluid retention, age, and some health conditions affect interpretation. A muscular person may have a higher BMI without excess body fat, while a person can have health risks at a BMI within a typical range. Discuss a concerning result with a qualified clinician.
Use a different method for children
For people ages 2 through 19, BMI is interpreted using a percentile that accounts for age and sex, not the adult categories. A standard adult calculator cannot provide that clinical interpretation. For children, use guidance from a pediatric healthcare professional or an official child growth chart. Do not use a single number to label a child’s health, diet, or body image. Seek urgent care for acute symptoms regardless of a calculated BMI value.
Worked example
Metric example:
Weight = 70 kg
Height = 1.75 m
BMI = 70 / (1.75 × 1.75) = 22.86Is BMI the same as body fat percentage?
No. BMI uses height and weight. Body-fat percentage estimates the share of body mass made up of fat and requires different measurements.
Reference and further reading
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