BMI Calculator

Why We Use BMI

Body Mass Index or BMI can help you understand if you’re a healthy body weight for your height. There’s no “perfect” body weight – and there’s no perfect calculation for a healthy body weight, either – but knowing your BMI can be a useful indicator of whether you’re at risk from weight-related health problems.

It works by calculating a number based on your weight and how tall you are. From this number, you can get an idea of whether you are considered underweight, “normal” weight, overweight, or obese on the BMI scale.

While BMI measurements are imperfect, they’re still widely used for several reasons.

First, BMI is quick and affordable to measure (our tool is completely free) and you can work it out at home with just bathroom scales and a tape measure.

There’s also a strong, proven correlation between BMI and body fat and hundreds of studies that show links between BMI and general health, so it can be a useful place to start to learn more about your body and health.

Understanding BMI Results: Ages 20+

Underweight: Below 18.5

A BMI below 18.5 suggests you could be underweight. There’s a risk that you may not be eating enough, or in some cases, it could be a sign of illness.

Prolonged malnutrition can lead to osteoporosis, fertility issues, anemia, hair and tooth loss, and other problems. A low BMI doesn’t necessarily mean a medical emergency, but you might want to pay a visit to your doctor to understand if there’s an underlying cause.

Normal: 18.5 to 24.9

BMI readings between 18.5 and 24.9 are considered within a healthy range. Those on the lower end of the spectrum may want to make sure they’re getting enough nutrients, while those at the top might take a look at their processed sugar and junk food consumption.

While these numbers are not associated with significantly elevated medical risks, there are other factors to consider. A slim person carrying belly fat may have a “normal” BMI but be at a greater health risk than a fit, muscular person with an “overweight” BMI. Find out more about BMI limitations.

Overweight: 25 to 30

If your BMI is above 25, it could mean that you’re overweight and carrying excess body fat. However, because BMI is a measure of body weight and not body fat, you could also have a high BMI if you’re very muscular.

If you’re not very active, a BMI within this range is a sign you should consider losing weight through a combination of a healthy diet and exercise. For those who are active and lead a healthy lifestyle, a high BMI result may not be a cause for concern.

Obese: Above 30

If your BMI is above 30, you’re almost certainly carrying extra weight. A BMI over 30 has been linked to type 2 diabetes, heart attacks, strokes, arthritis, joint inflammation, depression, sleep apnea, and an elevated risk of cancer.

These dangers are especially elevated for those with a BMI over 40, which is known as “severely obese”. Obesity can lead to serious health and quality-of-life issues. You should visit your doctor to understand the underlying cause and start taking steps to improve your health.

Understanding BMI Results: Ages 2-19

Severely Underweight: Below the 3rd Percentile

A BMI below the 3rd percentile almost certainly means that a child or teen is severely underweight. A medical professional should be consulted on how to support healthy weight gain, while considering possible underlying causes.

Studies on children and young adults in this percentile show that many suffer from protein-energy malnutrition (PEM), which is linked with higher rates of mortality and further illness.

Underweight: From the 3rd to 5th Percentile

A BMI below the 5th percentile suggests a child or teenager could be underweight . It is possible that they simply have a smaller build, so it’s important to clarify this with a doctor or pediatrician.

If a child or teen is underweight for their build, there’s a risk that they’re not eating enough nutritious foods, or in some cases, it could be a symptom of an underlying illness.

Studies show that under nutrition in kids can lead to metabolic disorders, low energy levels, behavioral issues, and learning disabilities.

Normal: From the 5th to the 85th Percentile

BMI readings between the 5th and 85th percentile are within a healthy range. If a teenager or child is on the lower or higher end of the spectrum, confirm with a medical expert to make sure they’re getting enough nutrition and exercise.

In any case, it’s recommended that a child or teen with a healthy BMI still gets support on how to keep their processed sugar and junk food consumption to a minimum for optimal development.

Continuously tracking BMI, especially during puberty, is a helpful way to make sure a child is on track with their weight.

BMI calculations do have their limitations, so it’s best to receive ongoing guidance from a doctor or pediatrician.

Overweight: From the 85th to 95th Percentile

If a child or teenager’s BMI percentile is above 90, it could mean that they are overweight and carrying excess body fat. However, because BMI is a measure of body weight and not body fat, they could also have a high BMI if they have more muscle.

In the case where this child or teenager is not very active, a BMI above the healthy percentile range for their age and gender means they need support in losing weight through a combination of a healthy diet and exercise. Additional guidance from a pediatrician or doctor is recommended for getting a child or teen’s health on track.

Being overweight in childhood often continues on into adulthood, due to genetics and behavioral patterns. This can be detrimental to an individual's emotional health, as well as physical, with increased risks of childhood diabetes in the short term. Long-term effects include varying degrees of illness, so it’s ideal for a child or teen with this result to start getting help now.

For those who are active and lead a healthy lifestyle, a high BMI result may not be a cause for concern, but you should confirm this with a medical expert.

Severely Overweight: Above the 95 Percentile

A BMI above the 95th percentile almost certainly indicates that a child or teenager is carrying more weight than what’s considered healthy. The CDC states that unhealthy eating habits, lack of physical activity, and inconsistent sleeping habits are just some of the factors that contribute to childhood and teenage obesity.

Childhood obesity often leads to obesity in adulthood. An adult BMI over 30 is considered obese, and has been linked to type 2 diabetes, heart attacks, strokes, arthritis, depression, and other serious health and quality-of-life issues.

A child or teen with this result should be assessed by a physician to discover any underlying causes, and clarify what steps to take to improve their health.

Learn more below about how BMI is calculated for children and teens.

How BMI is Calculated

BMI is calculated by dividing your weight in kilograms by the square of your height in meters. If you use imperial measurements you can calculate BMI with the equation:

(Weight in pounds ÷ height² in inches) x 703 = BMI

BMI measurements are taken the same way for men and women. But there are differences in the way this information is interpreted.

BMI for Women

Often due to social stigma, women with higher BMIs report more psychological stress and a lower quality of life than men with high BMIs. Those pressures also cause a higher rate of eating disorders among women.

Women can experience fertility issues and amenorrhea if their BMI goes too far in either direction. Note that if you’re pregnant or nursing, BMI isn’t accurate and shouldn’t be used.

BMI is also not an accurate way to determine the distribution of body fat, in particular, identifying high-risk abdominal body fat. Additional waistline measurements are particularly important for women, who tend to have more body fat than men of the same BMI. Studies show there’s a link between larger BMI and waistline measurements and coronary heart disease in women.

Relying on BMI as the sole indicator for overall health should also be avoided after menopause. This is because the proportions of muscle, fat, and bone change as you age. See more about the limitations of BMI for post-menopausal women.

We added a new customized BMI calculator for women.

BMI for Men

Currently, the average BMI for males is 28. You might think that the healthy weight for a man of the same height as a woman would be higher. But this isn’t the case.

As BMI increases (over 25), so does ill health. A BMI of 28 makes you more than 1,500% more likely to develop diabetes than if your BMI was 23 or lower.

Fortunately, you’ll benefit from dropping even just a few pounds.

BMI for Children and Teenagers

A slightly different method is used to calculate the BMI of children and teenagers aged 2-19 years old.

BMI is still calculated using height and weight but the results are interpreted and measured by percentile. This is where the child or teen’s BMI is compared to others of the same age and sex using data from national surveys.

The CDC is just one of the organizations that monitors the results of these surveys and maintains updated charts of BMI percentiles for boys and girls.

Compared to these nationwide statistics, a child or teen’s BMI is then ranked as follows:

For example, if a child is on the 86th percentile, that means they weigh more than 86 out of 100 children of the same age and sex, and they would fall into the Overweight category.

Limitations of BMI Measurements

Body Fat Distribution

BMI fails to take fat distribution into account. This is an issue because some areas are far more harmful to health than others. Excess abdominal fat has been linked to numerous health issues, such as heart disease and type 2 diabetes. Yet a slim person who carries belly fat might still have a “normal” BMI. You should take your waistline into account, as well as your BMI.

To work out your body fat percentage, ask your health practitioner to do skinfold testing instead.

Athletes and Bodybuilders

BMI is a measurement of body weight, but it can’t differentiate between muscle, fat, or bone mass. Because of this, a bodybuilder with very little body fat but a lot of muscle might still receive a BMI rating of “overweight” or even “obese”, which would be an inaccurate representation of their actual health.

If you consider yourself to be a fit person with a large muscle mass, your BMI won’t be a relevant indicator of your health, so reach out to a medical practitioner for guidance.

Ethnicity

Race and ethnicity factor into BMI readings, as well. The ranges and cut-off points in most general tools are considered international guidelines, although there are some significant variations.

A 20-year study found that at the same BMI, Asian people were twice as likely to develop type 2 diabetes compared to white people. It found that Black, Hispanic, and other ethnic minority groups were also more at risk of diabetes.

Those of Asian origin are also more prone to belly fat than Caucasians. This means they may start to see health problems at a much lower BMI. The World Health Organisation looked into this and suggested that Asian people should class a BMI of 22-25 as overweight (observed risk) and 26-31 as obese (high risk).

Age

BMI calculations are a less accurate indication of a healthy weight for babies and toddlers aged 0 to 2, and for individuals over the age of 60 to 65.

When it comes to infants aged 0 to 2, there are many more factors that contribute to weight. Medical experts trained in this area take measurements such as height and head circumference, and compare those to growth percentiles of children the same age, gender, and height, but they’ll also base their assessment on far more nuanced information.

This is largely based on information you provide, such as general diet, energy, and overall health before determining whether your baby or toddler is at a healthy weight.

As for the limitations in older adults, muscle mass decreases as you get older, while body fat often increases. This means that some older people may fall into a healthy BMI category, while still having a high level of body fat.

If you’re over the age of 60-65, BMI may be an even more unreliable indicator of body fat.

Pregnant/Breastfeeding Women

Given your baby’s weight, and the natural increase in your body fat to accommodate and nurture your baby, BMI is not an accurate tool for you while pregnant or nursing. Instead, the CDC recommends monitoring your weight gain throughout pregnancy and postpartum.

If you know what your BMI was pre-pregnancy, you can compare your current weight with CDC’s recommended weight gain for that BMI. There are separate charts for pregnancy with one baby and pregnancy with twins.

Post-Menopausal Women

BMI can be particularly misleading for postmenopausal women where studies show bone density decreases and body fat increases.

This is means that the Underweight to Obese categories would have to be shifted down to lower weights in order to more accurately determine whether you’re potentially at risk for weight-related health complications. So, whereas a BMI of 30 is the cutoff for obesity, studies show that the cutoff should be lower for post-menopausal women.

Next Steps for Adults

Knowing your BMI is a great starting point for further discussion with your doctor regarding any health concerns or lifestyle goals you may have.

Once you’ve agreed on a health plan with your doctor, there are lots of free resources online to help you stay on track and achieve your health goals—diet and activity trackers, calorie counters, recipe apps etc. Even when it comes to eating healthy, there are tons of meal prep guides to help with planning and cooking with limited time.

Next Steps for Kids

BMI can serve as an initial screening for children and adolescents, but it’s only one aspect of your child’s health and it should be considered as part of the bigger picture factoring in your child’s diet, activity, and family and medical history.

If your child’s weight is outside of the healthy range, you should seek help and advice from your doctor.

Even if your child’s measurement falls within the healthy range, you should continue seeing your child’s healthcare provider and take BMI-by-percentile measurements regularly to track your child's growth pattern and make sure they’re gaining or losing weight at a healthy pace.

Your doctor can help confirm whether weight gain is part of normal development, or something else.

Healthy Eating Habits for Children

Tracking your child’s weight and growth pattern with the support of your doctor is one way of looking out for their health. Nurturing healthy habits from a young age is also important. These habits can include:

• Ensuring your child eats a nutritious diet, including plenty of fresh fruit and veg
• Prioritizing family mealtimes
• Getting your child involved in cooking to foster an interest in food
• Ensuring they drink plenty of water
• Encouraging physical activities and movement each day