Calculate Your BMI

Getting an Accurate Measurement

Small errors in either measurement move the result more than people expect, and at the edge of a category they can move you into the wrong one.

1. Measure your height without shoes, standing straight with your heels against a wall.
2. Weigh yourself in the morning, before you have eaten, wearing light clothing.
3. Enter both numbers in whichever units you think in, metric or imperial.
4. Add your waist measurement if a tape measure is within reach, because it sharpens the result considerably.
5. Read the category rather than the decimal, since the band is what clinicians work from.

A Formula Older Than the Lightbulb

Adolphe Quetelet worked out this formula in 1832. He was a Belgian astronomer who spent much of his career studying populations rather than patients, and while sifting through height and weight data he found that weight scales with the square of height. He was explicit that his index described the average man as a statistical composite, and he never suggested using it on the person in front of you.

For 140 years nobody did. Demographers used the index, social scientists used it, and doctors assessing an individual patient reached for insurance company height and weight tables instead. Then in 1972 the American physiologist Ancel Keys went looking for a practical stand-in for body fat, judged Quetelet's index the best of a limited field, and renamed it the body mass index. Medicine adopted it within a generation.

That history explains every limitation further down this page. A measure built to describe crowds is now asked to describe you.

What the Categories Mean

Health Canada updated its classification in 2003 and aligned it with the World Health Organization bands used internationally, so your number carries the same meaning here as it would anywhere else. You can read the full system in the Canadian Guidelines for Body Weight Classification in Adults.

What those bands describe is how risk behaves across a population, which means they describe a probability and never a diagnosis. Health Canada says as much directly, treating weight classification as one part of a fuller assessment. A reading outside the normal band is a reason to start a conversation, not a verdict on your health.

Words You Will Hear

Waist circumference. The distance around your middle at its narrowest point, which estimates how much fat you carry in the abdomen specifically.

Visceral fat. The fat packed around the liver, pancreas and intestines, and the type most closely tied to metabolic problems.

Subcutaneous fat. The fat just under the skin that you can pinch, which carries noticeably less risk than the visceral kind.

Body composition. How much of your weight is fat, muscle, bone and water. BMI cannot tell them apart, because a kilogram is a kilogram as far as the formula is concerned.

BMI class. The band rather than the decimal, since a reading of 27.4 and one of 27.9 mean the same thing in practice.

Why Waist Measurement Matters

Waist size forms the second half of the Canadian system, and it answers the question BMI cannot reach, which is where your body puts what it stores. Health Canada sets the threshold for increased risk at 102 centimetres for men and 88 centimetres for women. Measure at the narrowest point of your torso, tape snug, breath relaxed rather than held.

Those thresholds are closer to the ordinary Canadian than almost anyone expects. When Statistics Canada measured adults aged 18 to 79 directly rather than asking them, the average waist came in at 95.1 centimetres among men and 87.3 among women. Twenty-nine percent of men and 41 percent of women were already at or above the threshold, so it is not a distant marker for other people.

When BMI Gets It Wrong

Health Canada names the groups whose risk the number overstates or understates, and the list runs longer than you would expect from a measurement this widely used.

• Pregnant and breastfeeding women fall outside the classification system altogether.
• Muscular adults read high, because the formula counts a kilogram of muscle exactly as it counts a kilogram of fat.
• Naturally lean adults, and young adults who are still growing, tend to read lower than their risk warrants.
• Adults over 65 lose muscle as they age, so an identical reading describes a different body.
• Several ethnic groups carry weight-related risk at thresholds below the standard bands.
• Children and teenagers need age and sex percentile charts rather than the adult categories.

None of that makes the measurement useless. It makes it a screening tool, and nothing more.

Same Number, Thirteen Different Answers

Your BMI means the same thing in Iqaluit as it does in Halifax, but what happens next depends entirely on which province issued your health card. Free dietitian advice is the widest gap, and the least known. British Columbia staffs registered dietitians on its 8-1-1 line at no cost and with no referral, and Ontario runs an equivalent service through Health811 that does not ask for an OHIP number. Most provinces route something similar through their own 811 line.

Private dietitians work differently, charging a fee that provincial plans rarely cover and extended health benefits usually do, so it is worth checking your workplace plan before paying out of pocket. Most provinces also fund bariatric surgery publicly, including both procedures under British Columbia's MSP, generally at a BMI of 40 and above, or 35 and above alongside a weight-related condition. Waits run into years and differ a great deal between provinces, so ask the programme in your own region.