Home Education BMI Calculator

BMI Calculator

BMI Calculator


A Body Mass Index (BMI) calculator is a measure of a person's weight and height to determine whether that person is at a healthy weight.  The bmi calculator can give you an idea whether you are overweight or underweight compared to the accepted normal weight for your height and gender. 

BMI calculators are designed to provide a general idea of healthy weight.  It is not designed to provide a complete view of a person's health as a result of their weight.  Instead, it is used mainly by physicians and doctors as a way to help patients with weight problems to understand generally what their healthy weight should be.  

The following steps should be used to determine your body mass index:

1.First, you must collect some basic household items that will be used to obtain the numbers needed to use the body mass index (BMI) calculator. You will need:

– A scale to measure your weight (any standard bathroom scale will work)

– A measuring tool to determine your height such as a measuring tape or yardstick (While a bmi calculator is most accurate with an exact height, you can use an estimate as long as it is within an inch of accuracy)

– Paper and a pencil or pen to record your height and weight

2.  Next you will need to weigh yourself in order to get your current weight.  Place your scale on a smooth hard floor.  In order to get the most accurate bmi calculator results, wear a limited amount of clothes to get your true weight.  Use the scale two or three times to ensure you are getting an accurate reading. 

3. Measure your height, using either the measuring tape or yard stick. Some tips for measuring your height include:

– Find a wall and with your back straight, mark the top of your head on the wall.  Step away from the wall and measure the distance from the floor to your mark. 

– If you are able to, another person can be very helpful in measuring your height.  Have the second person use a measuring tape to accurately measure the bottom of your feet to the top of your head. 

–  If you are unable to get an accurate recording of your height, you may use your best estimate from a previous doctors visit or physical.  Keep in mind that bmi calculators are most accurate when your exact height is provided.  

4. Using the paper and pen or pencil, record your weight and height.  While an online bmi calculator can bes used to calculate your bmi, you can also manually determine your bmi by performing this calculation:

– BMI = Weight (in pounds) / Height (in inches) X Height (in inches).

– Take the above amount and multiply it by 703.  This is your body mass index.  

5. One you have used one of the many bmi calculators, you must next compare your body mass index with the chart that determines whether you are at a healthy weight.  Many bmi calculators not only provide you with your body mass index, but will also tell you whether you are at a health weight, underweight, or overweight, and by how much.  

– Find a body mass index chart.  A healthy body mass index depends greatly on your age, body frame type, and gender, however the following are the general ranges of the body mass index.

– A body mass index of less than 18 is underweight

– A body mass index of between 18 to 18.5 means you are thin for your height

– Body mass index of between 18.6 – 24.9 is the range of a generally healthy weight

– A body mass index of between 25 – 29.9 means you are overweight

– A body mass index of over 30 means you are obese.  If your bmi calculator determines you are obese, it is advised that you consult a doctor immediately in order to help reach a healthy weight. 


1. Keeping your weight within a healthy range is one of the best ways to keep your body in optimal health, which will limit health problems and prolong your life expectancy.  Using a bmi calculator regularly provides you with a good indication of your current weight and overall progress if you are attempting to control your weight.  

2. If you are attempting to use the manual bmi calculator using the above mathematical equation, note that the true multiplier is not just 703, but rather 703 + (4489/64516).  However, the difference is so minimal that it should not affect your results more than one hundredth of a decimal.  Online bmi calculators will use the true multiplier, so keep this in mind if your are comparing your math to the online bmi calculator.  

3. One of the biggest issues with a bmi calculator is the fact that it does not take in to account muscle mass.  Large muscle mass, which is usually healthy, may skew the results of the bmi calculator.  However, bmi calculators are designed to only provide a healthy weight guideline for the average person, it does not provide an exact custom tailored evaluation of your personal healthy weight. 

General Tips While Using the BMI Calculator

1. While body mass index is designed to be used by average people aged 25 to 65, it has many limitation.  The bmi calculator cannot take into account your muscle mass, body type, or frame size.  People with larger frames, larger body types, or above average muscle mass may find that they are a healthy weight despite having a body mass index in the overweight or obese range.  

2. Other forms of body measurement should be used in conjunction with a bmi calculator.  Measuring your waist to hip ratio, body fat calculations, and other measurements should all be used when determining what your personal healthy weight should be.  Consult a physician, nutritionist, or weight loss specialist to get a full picture of your target weight.  

3. Should your weight be a serious concern, it is important that you use much more technical methods of body measurement, which can be accessed through medical facilities or at many gyms and health spas. 

– A skin-fold test used mechanical calipers to measure total body weight from fat.  

– A bio-electrical impedance analysis, which sends a small electrical current through your body, measures your total fat content and amount of weight from fat.

– Both of these methods provide a much more accurate measure of your body fat, muscle content, and should be used to set goals and targets if you have significant health issues stemming from weight problems.  

International Variations

While all bmi calculators use the same formula in all regions of the world, many countries use a different range to determine a healthy weight for their population.  This is mainly due to the differences in body frames and types found in different regions of the world.  Below are some examples of places that use a different scale to determine healthy weights using a bmi calculator.

– In Japan, a normal body mass index is considered between 18.5 to 22.9.  Overweight ranges from 23 – 24.9, and obesity is anything over 25.

– In Singapore, the health body mass index range is the same as Japans, however obesity starts at 27.6 and morbid obesity is considered a body mass index of over 40.  

Age and BMI Calculators

While most bmi calculators do not take into account your age, you should consult a chart that shows you what your body mass index should be compared to others in your age range.  Children's bmi ranges are completely different than those of an adult.  

– Between the ages of 2 and 20, a healthy male child's body mass index should range from a little over 16 at age 2 all the way up to a healthy bmi of 23 at age 20.  These charts take into account normal body growth and the weight associated with maturity and puberty during teenage years.  

– The health body mass index for girls aged 2 to 20 range from a health bmi of 16 at age 2 to a health bmi of 22 at age 20. 

Similarly, people over the age of 65 are not advised to use a bmi calculator, as at this age a health body mass index cannot easily be determined, as many other factors should be considered.  Instead, consult your physician to determine what your own healthy weight should be at this age.  

Gender and BMI Calculators

The most basic bmi calculators and graphs do not take into account the gender of the person, however it is widely accepted that men and women have healthy weight ranges that are different.  

– Generally, due to differences in body type and hormone differences, a woman's healthy body mass index range is slightly higher than a males.  

It is very important that you keep in mind that bmi calculators should only be used as a general indications of your weight and health.  Do not be concerned if your body mass index falls slightly into the underweight or overweight categories unless you believe you are suffering health issues as a result of your weight.  

If you fall within one of the danger zones of body mass index (a bmi of under 18.5 is considered the point of malnutrition, while a bmi of over 30 is considered obese) you should consult a doctor immediately.  Almost all people who fall in these ranges are in danger of imminent health problems which can greatly affect your quality of life and life expectancy.