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BMI Calculator

The Body Mass Index (or BMI) is a calculation of the ratio between your weight and your height. The formula is used to determine the amount of body fat you carry. The formula is your weight divided by your height in meters, squared (BMI = kg/m2). BMI can be used to indicate if you are overweight, obese, underweight or normal.

Proposed classification of weight by BMI in different adult ethnic groups. (2007 Clinical Handbook – New Zealand Dietetic Association, eighth edition - Revised table)

Classification	NZ European People	Pacific Island & Maori People	Asian & Indian People	Risk of obesity related diseases
Underweight	Less than 18.5	Less than 18.5	Less than 18.5	Low
Healthy weight	18.5 – 25	18.5 – 26	18.5 – 23	Normal
Overweight	25 – 30	26 – 32	23 – 25	High
Obese	Greater than 30	Greater than 32	Greater than 25	Very High

Being overweight or obese puts you at increased risk of developing certain medical conditions associated with obesity.These include:

• Hypertension (high blood pressure)
• Cardiovascular Disease (heart disease, stroke etc)
• Adult-Onset Diabetes (Type II Diabetes)
• Sleep Apnoea (difficulty breathing during sleep)
• Osteoarthritis (degenerative arthritis or degenerative joint disease)
• Female Infertility
• Depression, low self esteem and other psychological conditions
• Some types of cancers

BMI is meant to broadly categorise groups of people but has it’s limitations for use with individuals. As such, BMI is not appropriate for use with:

• Children or adolescents under the age of 18 years
• Very muscular people
• People who have lost muscle mass, for example the elderly
• Pregnant women
• People who are very short (less than 150cm) or very tall (greater than 190cm)

Combining your BMI with your waist circumference gives a better indicator of whether you are carrying too much weight and if the extra weight you may be carrying is accumulating around your “middle” (which is associated with higher risk of some health problems like heart disease, diabetes, stroke, high blood pressure and gall bladder disease).

Measuring your waist

To measure your waist circumference, stand with your feet 25-30cm apart with your feet flat on the ground. Your waist measurement should be taken from the narrowest point width-wise, which is usually just above the belly button. Make sure the tape measure is level around your waist then tighten it without depressing the skin. Now take a “normal” breath in. Compare your waist measurement with the table below.

Maximum Acceptable Waist Circumference in different adult ethnic groups
(2007 Clinical Handbook – New Zealand Dietetic Association, eighth edition)

	Men	Women
New Zealand European People	Less than 102cm	Less than 88cm
Pacific Island and Maori People	Less than 102cm	Less than 88cm
Asian and Indian People	Less than 90cm	Less than 80cm

Body Shapes

You may have heard the terms “apple” or “pear” body shapes. An “apple body shape” is when body fat is stored around the middle or abdomen and surrounds internal organs, such as the heart. The “apple shape” is linked with health problems such as coronary heart disease, diabetes, stroke, high blood pressure and gall bladder disease. Men are more likely to have “apple” body shapes, although women are more prone to develop an apple shape in mid-life, particularly after menopause.

People with a “pear body shape” have hips wider than their shoulders as they store fat - on their hips and thighs. “Pear shaped” bodies carry extra weight below the waistline, and do not seem to have as high a risk of developing health problems like diabetes, heart disease, stroke, high blood pressure etc when compared to people of an “apple” body shape.

Measuring your waist circumference is a good measure of whether you are carrying too much weight around your middle and helps you determine if you may be an “apple” or “pear” shape. When combined with your BMI this is a more accurate indicator of your individual situation and the implications your weight and body shape might have on your health.

Please remember these measures are only two of the many possible ways to assess your weight. If you have any concerns about your weight, please discuss them with a dietitian or registered nutritionist who is in a position (unlike a BMI calculator for example), to address your specific individual situation.