# BMI Calculator by Age and Gender

Body mass index (BMI) is a value derived from the mass (weight) and height of a person. The BMI is defined as the body mass divided by the square of the body height and is universally expressed in units of kg/m2, resulting from mass in kilograms and height in meters.

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The BMI may be determined using a table or chart which displays BMI as a function of mass and height using contour lines or colors for different BMI categories, and which may use other units of measurement (converted to metric units for the calculation).

The BMI is a convenient rule of thumb used to broadly categorize a person as underweight, normal weight, overweight, or obese based on tissue mass (muscle, fat, and bone) and height. That categorization is the subject of some debate about where on the BMI scale the dividing lines between categories should be placed. Commonly accepted BMI ranges are underweight (under 18.5 kg/m2), normal weight (18.5 to 25), overweight (25 to 30), and obese (over 30).

BMIs under 20 and over 25 have been associated with higher all-cause mortality, with the risk increasing with distance from the 20–25 range.

## BMI formula

The BMI (Body Mass Index) in (kg/m2) is equal to the mass in kilograms (kg) divided by the square height in meters (m):

BMI(kg/m2) = mass(kg) / height2(m)

The BMI (Body Mass Index) in (kg/m2) is equal to the mass in pounds (lbs) divided by the square height in inches (in) times 703:

BMI(kg/m2) = mass(lb) / height2(in) × 703

BMIs under 20 and over 25 have been associated with higher all-causes mortality, with the risk increasing with distance from the 20–25 range.

## BMI Calculator by Age and Gender

### BMI in Children: aged 2 to 20

MI is used differently for children. It is calculated in the same way as for adults but then compared to typical values for other children of the same age. Instead of comparison against fixed thresholds for underweight and overweight, the BMI is compared against the percentiles for children of the same sex and age.

A BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is considered obese. Children with a BMI between the 85th and 95th percentile are considered to be overweight.

Recent studies in Britain have indicated that females between the ages of 12 and 16 have a higher BMI than males of the same age by 1.0 kg/m2 on average.

### International variations

These recommended distinctions along the linear scale may vary from time to time and country to country, making global, longitudinal surveys problematic. People from different ethnic groups, populations, and descent have different associations between BMI, percentage of body fat, and health risks, with a higher risk of type 2 diabetes mellitus and atherosclerotic cardiovascular disease at BMIs lower than the WHO cut-off point for overweight, 25 kg/m2, although the cut-off for observed risk varies among different populations. The cut-off for observed risk varies based on populations and subpopulations both in Europe and Asia.

### United States

In 1998, the U.S. National Institutes of Health and the Centers for Disease Control and Prevention brought U.S. definitions in line with World Health Organization guidelines, lowering the normal/overweight cut-off from BMI 27.8 to BMI 25. This had the effect of redefining approximately 29 million Americans, previously healthy, to overweight.

This can partially explain the increase in the overweight diagnosis in the past 20 years and the increase in sales of weight loss products during the same time. WHO also recommends lowering the normal/overweight threshold for South East Asian body types to around BMI 23, and expects further revisions to emerge from clinical studies of different body types.

The U.S. National Health and Nutrition Examination Survey of 2015-2016 showed that 71.6% of American men and women had BMIs over 25. Obesity—a BMI of 30 or more—was found in 39.8% of US adults. A survey in 2007 showed 63% of Americans are overweight or obese, with 26% in the obese category (a BMI of 30 or more). As of 2014, 37.7% of adults in the United States were obese, categorized as 35.0% for men and 40.4% for women; class 3 obesity (BMI over 40) values were 7.7% for men and 9.9% for women.

By entering an individual’s weight and height, the above BMI calculator will determine the body mass index or BMI. You can enter the weight in pounds, stones, or kilograms. You can enter height in inches, feet, meters, or centimeters.

### Underweight

If you’re underweight, you might not be eating enough or you may be sick. A doctor can help if you’re underweight.

### Healthy weight

You’re doing a great job! We have food and diet and fitness sections with tips on maintaining a healthy weight.

### Overweight

Diet and exercise are the most effective ways to lose weight if you’re overweight.

With the BMI calculator, you can calculate your personal calorie allowance to reach a healthy weight.

### Obese

When you are obese, diet and exercise, as well as medicine, maybe the best way to lose weight. Consult your doctor for guidance.

### Black, Asian, and other minority ethnic groups

Blacks, Asians, and other minorities are more likely to suffer chronic diseases such as type 2 diabetes than other ethnic groups.

The following adults have a BMI of:

• If you are 23 or older, you are at increased risk
• For those ages 27 and older, you are at increased risk

## Why waist size also matters

The waist circumference is a good way to examine whether you carry too much fat in your abdomen, which can increase your risk of heart disease, type 2 diabetes, and stroke.

Having excess tummy fat even with a healthy BMI puts you at risk for these conditions.

Here are the steps for measuring your waist:

1. Identify the bottom of your ribs and the top of your hips.
2. Measure your waist halfway between these two points.
3. Make sure you exhale naturally as you measure.

Your waistline should be the following regardless of your height or body mass index:

• 94cm (37ins) or more for men
• 80cm (31.5ins) or more for women

You’re at very high risk and should contact a GP if your waist is:

• 102cm (40ins) or more for men
• 88cm (34ins) or more for women

## Children’s BMI

BMI calculators for children and young people between the ages of 2 and 18 account for age, gender, height, and weight.

It is thought that overweight children are at greater risk of a variety of health conditions, as well as having greater chances of becoming overweight as adults.

Using the BMI calculator, you can calculate how fat a child or young person is:

• Underweight – below the 2nd centile
• Healthy weight – 2nd to 91st centiles
• Overweight – 91st centile or above
• very overweight – 98th centile or above

As part of national surveys, a child’s BMI is expressed as a “centile” to compare how their BMI compared with other children.

One such example is a female at the 75th centile who is heavier than 75 other women her age.

It is not advisable to measure waist size for children since it does not take their height into account.

Seeing a GP if you’re worried about your child’s weight is a good idea. Perhaps they can refer you to a healthy lifestyle program for children, young people, and families in your area.

### Mens & Womens BMI Chart by age

#### BMI History

Adolphe Quetelet, a Belgian astronomer, mathematician, statistician, and sociologist, devised the basis of the BMI between 1830 and 1850 as he developed what he called “social physics”. The modern term “body mass index” (BMI) for the ratio of human body weight to squared height was coined in a paper published in the July 1972 edition of the Journal of Chronic Diseases by Ancel Keys and others. In this paper, Keys argued that what he termed the BMI was “…if not fully satisfactory, at least as good as any other relative weight index as an indicator of relative obesity”.

The interest in an index that measures body fat came with observed increasing obesity in prosperous Western societies. Keys explicitly judged BMI as appropriate for population studies and inappropriate for individual evaluation. Nevertheless, due to its simplicity, it has come to be widely used for preliminary diagnoses. Additional metrics, such as waist circumference, can be more useful.

The BMI is universally expressed in kg/m2, resulting from mass in kilograms and height in metres. If pounds and inches are used, a conversion factor of 703 (kg/m2)/(lb/in2) must be applied. When the term BMI is used informally, the units are usually omitted.

BMI provides a simple numeric measure of a person’s thickness or thinness, allowing health professionals to discuss weight problems more objectively with their patients. BMI was designed to be used as a simple means of classifying average sedentary (physically inactive) populations, with average body composition. For such individuals, the value recommendations as of 2014 are as follows: a BMI from 18.5 up to 25 kg/m2 may indicate the optimal weight, a BMI lower than 18.5 suggests the person is underweight, a number from 25 up to 30 may indicate the person is overweight, and a number from 30 upwards suggests the person is obese. Lean male athletes often have a high muscle-to-fat ratio and therefore a BMI that is misleadingly high relative to their body-fat percentage.

## What are the health consequences of having a high or low BMI?

Obesity carries significant health hazards, whereas maintaining a healthy weight is a preventative measure against illnesses and cardiovascular difficulties. People with a BMI of more than 30 are more likely to have problems such as:

• hypertension
• diabetes type 2