How to Use a BMI Calculator and What Your BMI Actually Means
Body Mass Index (BMI) is a number that relates your weight to your height. It's quick to calculate, widely used by doctors and health researchers, and can be a useful screening tool — but it has significant limitations. Here's everything you need to know about BMI.
How to Calculate BMI
The BMI formula:
Metric: BMI = weight (kg) ÷ height² (m²)
Imperial: BMI = (weight in pounds × 703) ÷ height² (inches²)
Example: A person who is 5'9" (175 cm) and weighs 160 lbs (72.6 kg):
BMI = 72.6 ÷ (1.75 × 1.75) = 72.6 ÷ 3.0625 = 23.7
BMI Categories Explained
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Obese (Class III) |
These categories are set by the World Health Organization (WHO) and are used globally. Canadian health guidelines align with these thresholds.
What Are the Limitations of BMI?
BMI is a blunt instrument. Its most significant limitations:
- Doesn't account for muscle mass — athletes and bodybuilders often have high BMIs despite low body fat
- Doesn't distinguish fat distribution — abdominal fat (around the waist) is more metabolically dangerous than fat elsewhere, but BMI doesn't measure location
- Age and sex differences — older adults and women naturally carry more body fat at the same BMI
- Ethnicity considerations — research suggests Asian populations face higher health risks at lower BMI thresholds; Canadian diabetes guidelines now recommend lower BMI thresholds for some ethnic groups
Better Measurements to Use Alongside BMI
- Waist circumference — high risk: men over 102 cm (40"), women over 88 cm (35")
- Waist-to-hip ratio — divides waist measurement by hip measurement; below 0.9 (men) or 0.85 (women) is lower risk
- Body fat percentage — measured by DEXA scan, bioelectrical impedance, or skinfold calipers; more accurate but harder to measure
BMI for Children and Teens
The standard adult BMI formula doesn't apply to children and teens. For ages 2–19, BMI is interpreted using age- and sex-specific percentile charts. A child with a BMI at the 95th percentile or above for their age group is considered obese.
Frequently Asked Questions
Is BMI 25 overweight in Canada?
Yes — a BMI of 25 to 29.9 falls into the 'overweight' category according to both Canadian and WHO guidelines. However, health risk depends on many factors beyond BMI, including waist circumference, blood pressure, and metabolic markers.
What BMI is considered obese in Canada?
A BMI of 30 or above is classified as obese (Class I) in Canada. Class II starts at 35, and Class III (previously called 'morbid obesity') starts at 40.
Does BMI calculate differently for men and women?
The formula is the same for both sexes. However, healthy body fat percentage differs by sex — women naturally carry more essential fat. Some researchers argue BMI should use different thresholds for men and women.
Can a doctor use BMI to diagnose obesity?
BMI is used as a screening tool, not a diagnosis. Doctors use BMI alongside other assessments — waist circumference, blood tests, blood pressure — to evaluate overall health risk.
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