Have you heard of BMI and that it’s an important indicator in your health? Not sure how it works or what it actually means?

Then you’ve come to the right place!

In this guide, we’ll take a look at BMI and how to calculate it before moving on to look at how accurate BMI is in certain cases.

What Is BMI?

BMI stands for body mass index and it is a calculation that uses your height and weight to estimate how much body fat you have.

This means it’s also a good indicator for how at risk you are of suffering from certain diseases and conditions like breathing problems, type 2 diabetes, certain cancers, high blood pressure, heart disease, and gallstones.

On the whole, BMI is a cost-effective and easy way to ascertain what weight category someone falls in – typically underweight, normal/healthy weight, overweight, and obese.

If someone has a high BMI, this may be an indication that they have high-fat content in their body.

That being said – BMI calculations do have limitations, which is why it’s important to understand these when looking at your BMI (we’ll explore these limits in more detail below). Furthermore, BMI is only a screening tool – it is not a diagnostic tool of an individual’s health or fatness.

How to Calculate BMI

The BMI formula is quite straightforward and can be done using imperial or metric calculations.

For the metric system, take your weight in kilograms and divide this by your height in meters squared. For example, if you weigh 60kg and you’re 150cm tall, you’d divide 60 by (1.5)2 = 26.67.

For imperial measurements, it’s slightly different. Here you’ll need to take your weight in pounds, divide it by your height in inches (squared) and multiply by 703 (this is the conversion factor). For example, if you weigh 132lb and you’re 59 inches tall, you’d divide 132 by (59) 2 x 703 = 26.65.

What do your calculations show?

For adults aged 20 and over, BMI scores are categorized into the following:

  • Underweight – BMI is below 18.5
  • Normal/Healthy Weight – BMI is between 18.5 and 24.9
  • Overweight – BMI is between 25.0 and 29.9
  • Obese – 30.0 and over

But for children and teens, it works a little differently.

How to Calculate BMI in Children and Teens

Although the same formula is used to calculate their BMI, the scores are interpreted in a different way.

A child’s or teen’s BMI needs to be specific to their gender and age because fat differs between sexes and changes as they grow older.

For example, Centers for Disease Control and Prevention has a BMI calculator for those under the age of 19 and the results of this are shown in a percentile ranking. These percentiles have been established based on various data collected over a number of years.

The Limitations of a BMI Calculator

In general, BMI and body fatness are fairly strongly correlated. But even if you have the same BMI as someone else, you may have different levels of body fat.

This is due to various different factors, including:

  • Compared to men, women tend to have more body fat
  • Compared to Whites, Asians have more body fat and Blacks have less body fat
  • Compared to young adults, older people tend to have more body fat
  • Compared to non-athletes, athletes tend to have less body fat

Furthermore, BMI doesn’t apply if you’re pregnant and high results can also be due to high lean body mass (bone and muscle).

Alternatives to BMI

If you don’t feel as though the results you’ve got from the BMI formula are accurate, you may want to confirm your thoughts through other tests.

Measuring your waist or your waist-height or waist-hip ratio may provide more realistic findings.

Waist Measurement

Measure your waist circumference (halfway between the top of your hipbones and the bottom of your ribs), ensuring you’re breathing out when you take the measurement.

  • For men: over 94cm indicates an increased health risk, over 102cm demonstrates a high health risk
  • For women: over 80cm indicates an increased health risk, over 88cm demonstrates a high health risk

Waist to Hip Ratio

To check this, measure your waist in the same way as above. Then measure your hips at the widest part. Divide your waist measurement by your hip measurement to get a ratio.

If you’re a woman and this is more than 0.85, this suggests your risk is greater than average.

If you’re a man and your ratio is over 1.00, this also suggests you’re at a greater than average risk.

Waist to Height Ratio

Using your waist measurement, simply divide this by your height (these can be imperial or metric measurements, so long as they’re both the same).

Whole body obesity is indicated if this ratio is over 0.53 in men or 0.54 in women. Abdominal obesity is 0.59 or more.

Are You a Healthy Weight?

Overall, BMI is a great starting point if you want to figure out whether you or your kids are a healthy weight.

However, these results should be backed up by more accurate assessments with a healthcare practitioner to confirm or disprove your findings.

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