BMI Calculator

What is BMI?

The BMI(Body Mass Index) is a index or parameter that shows how a person thin or obese based on person's height and weight. It indicated a person has healthy body or not and it can be categorized in underweight, normal weight, overweight and obese.

BMI was developed by Belgian mathematician adolphe quetelet in the 19th century and gave standards to measure BMI. Later WHO(World Health Organization) has adopted it as a standard screening tool because it is simple to calculate and it also correlates with body fat percentage.

BMI Formula

In metrics unit : weight(kg) ÷ height(m)²

In imperial unit : weight(lbs) ÷ height(inches)² × 703

For example, if you 70kg weight and 1.75m(175cm) height then your BMI can be calculated as 70 ÷ (1.75 × 1.75) = 22.9, this will fall in normal range.

BMI categories(WHO)

Category	Range
Severe thinness	<16
Moderate thinness	16-17
Mild thinness	17-18.5
Normal	18.5-25
Overweight	25-30
obese class 1	30-35
obese class 2	35-40
obese class 3	>40

What is a healthy BMI?

For most of the adults between 18-65 their BMI between 18.5 and 24.9 is considered healthy, because in epidemiological studies people's BMI ranging between 18.5 and 24.9 had lowest mortality risk. But age and ethnicity can impact BMI score so WHO has recommanded lower cutoff for asian populations.

What is BMI Prime?

BMI prime can be obtained by dividing your BMI with 25 which is upper limit of the normal range.

· BMI prime <7.5 means you are underweight

· BMI prime >7.5 - <=1 means you have normal weight.

· BMI prime >1 means you are overweight

What is ponderal index?

The ponderal index can be calculated as weight(kg) ÷ height(m)³. In BMI height was squred and here height is cubed, this can accurately categorize who is very tall or very short. Ponderal index is normal between 11 and 15.

Limitations of BMI

BMI can give you idea of how healthy you are based on your height and weight but it has some liitations as follows:

· It cannot distinguish muscle from fat : Athletes can have high muscle due to their intense activity that is considered healthy but it's BMI can be higher than normal range.

· It does not indicates fat distribution : In our body abdominal fat is more harmful than our subcutaneous fat but BMI does not have and fat classificaltion system it considers all fat as equal.

· Age effect : Older people can have more body fat as same BMI and it can happen of bone loss.

For more clarity you can measure your waist circumference if your waist circumference is >94cm if you are men and if waist circumference >80cm if you are women then it can be risky.

BMI and Cardiovascular helth risk

Framingham heart study and other large cohort studies has conducted a research and they have shown a consistant J-shaped relationship between BMI and cardiovascular mortality. But for cardiovascular disease waist circumference is more stonger predictor than BMI.

BMI and Diavetes risk

If you have obesity or BMI score more than 30 than you have strong chance of type 2 diabetes. If your BMI increases of just 5kg/m² than normal than you have 39% chances of having type 2 diabetes.

This happens primarily from insulin resistance. Excesive fat releases inflammatory cytokines and free fatty acids that can interfere with insulin in liver, muscle, and adipose tissue.

BMI across the life span

Children and adolescents: BMI is calculated the same way as in adults but classificaltion uses age and gender specific percentile charts(CDC growth charts in the US, UK90 charts in the UK) rather than fixed ranges.

Older adults: Some research has found that optimal BMI range shifts slightly upward with age. But unintentional weight loss in older adults is a significant red flag regardless of BMI classificaltion.

When to see a healthcare professional about your BMI

Your BMI number is a starting point, not a diagnosis. Consider consulting with a specialist or dietitian if you have anything as follows:

· Your BMI is <18.5 and particularly if unintentional weightloss is happening.

· Your BMI is >30 especially if you have high blood pressure, high cholesterol, family history of diabetes.

· Your BMI is in normal range but you have high waist circumference.

Related Resources

Related Calculators

• TDEE Calculator - Estimate your total daily energy expenditure based on activity level.
• BMR Calculator - Calculate your basal metabolic rate at rest.

External Authority Resources

• World Health Organization BMI Classification) - Official international BMI guidelines.
• CDC Body Mass Index Information - Centers for Disease Control and Prevention guidelines on BMI.

Sources

• WHO. Global Database on Body Mass Index. who.int)
• Nuttall FQ. Body Mass Index: Obesity, BMI, and Health: A Critical Review. Nutr Today. 2015;50(3):117-128.
• Romero-Corral A et al. Accuracy of body mass index in diagnosing obesity in the adult general population. Int J Obes. 2008;32(6):959-66.
• Guh DP et al. The incidence of co-morbidities related to obesity and overweight. BMC Public Health. 2009;9:88.