How to Calculate BMI (Body Mass Index): Complete Guide with Table and Examples

The Body Mass Index (BMI) is the most used health screening metric in the world. Adopted by the World Health Organization (WHO) since the 1980s, BMI allows you to quickly classify an adult's nutritional status based on the relationship between weight and height — without the need for laboratory tests or specialized equipment.

But what exactly does your number mean? What are the index limits? And how can the result guide health decisions? Use our BMI Calculator to calculate your result, and read this guide to interpret it correctly and completely.

The BMI Formula

The BMI calculation is mathematically simple and uses only weight (in kilograms) and height (in meters):

BMI = Weight (kg) ÷ [Height (m)]²

In other words: you divide your weight in kilograms by the square of your height in meters.

Step-by-Step Calculation Examples

Example 1: Person weighing 70 kg and 1.75 m tall

1. Square the height: 1.75 × 1.75 = 3.0625
2. Divide the weight by the result: 70 ÷ 3.0625 = 22.86 kg/m²
3. Classification: Normal Weight (18.5 to 24.9)

Example 2: Person weighing 95 kg and 1.68 m tall

1. Square the height: 1.68 × 1.68 = 2.8224
2. Divide: 95 ÷ 2.8224 = 33.66 kg/m²
3. Classification: Grade I Obesity (30.0 to 34.9)

Example 3: Person weighing 55 kg and 1.80 m tall

1. Square the height: 1.80 × 1.80 = 3.24
2. Divide: 55 ÷ 3.24 = 16.98 kg/m²
3. Classification: Underweight (below 18.5)

BMI Classification Table — WHO (Adults)

The World Health Organization establishes the following ranges for adults (over 18 years of age):

BMI table for the elderly (over 65 years old)

For the elderly, the Pan American Health Organization (PAHO) and the Brazilian Society of Geriatrics and Gerontology (SBGG) recommend slightly different BMI ranges, as natural aging causes loss of muscle and bone mass (sarcopenia), which distorts the standard BMI:

Note that a BMI of 25 in a young adult is "overweight", while for an elderly person it is still within the appropriate weight. Use our BMI Calculator for Seniors for age-specific classification.

Calculation of Ideal Weight by BMI

A very common question is: "What should my ideal weight be?" Based on BMI, the ideal weight is any value that places the index within the healthy range (18.5 to 24.9).

Formula for ideal weight:

Ideal Minimum Weight = 18.5 × Height² Ideal Maximum Weight = 24.9 × Height²

Example for 1.70 m:

• Minimum weight: 18.5 × (1.70)² = 18.5 × 2.89 = 53.5 kg
• Maximum weight: 24.9 × (1.70)² = 24.9 × 2.89 = 71.9 kg

For a 1.70 m person, the healthy weight range is 53.5 kg to 71.9 kg.

Ideal Weight by Height Table (BMI between 18.5 and 24.9)

When BMI Is Not Enough: Important Limitations

BMI is a population screening tool — not an individual diagnosis. It has significant limitations that you need to be aware of:

1. Doesn't Distinguish between Fat and Muscle

A high-performance athlete weighing 95 kg and 1.75 m would have a BMI of 31 — classified as "Grade I Obesity". But your body fat percentage may be as little as 8%, far below the risk threshold. BMI does not differentiate body mass composition.

2. Does not indicate fat distribution

Visceral fat (accumulated in the abdominal region, around the organs) is much more dangerous for cardiovascular health than subcutaneous fat (below the skin). Two people with the same BMI can have completely different risk profiles depending on where the fat is distributed.

Complementary indicators to BMI:

• Abdominal Circumference: Increased risk above 80 cm (women) and 94 cm (men), according to the WHO
• Waist-Hip Ratio (WHR): Values above 0.85 (women) and 0.90 (men) indicate high metabolic risk
• Body Fat Percentage: Measured by bioimpedance, DEXA or skinfolds

3. Variations by Ethnicity

Studies show that Asian populations have increased cardiovascular and metabolic risk with BMIs lower than the WHO standard ranges. For populations of Asian origin, the cutoff point for overweight is often considered to be 23 kg/m².

4. Does not apply to children and adolescents

In children under 18 years of age, BMI should be assessed using age- and sex-specific growth percentiles (WHO/WHO growth curves). Use our Child BMI Calculator for correct pediatric assessment.

5. Pregnant women

During pregnancy, pre-pregnancy BMI can guide recommended weight gain, but gestational BMI is not an adequate nutritional assessment tool.

BMI and Health Risks: What Science Says

Large-scale population studies confirm associations between high BMI and several conditions:

On the other hand, very low BMI (thinness) is also associated with:

• Osteoporosis and bone fractures
• Anemia and nutritional deficiencies
• Immunosuppression
• Fertility problems
• Greater risk in surgeries and infections

Common Mistakes When Calculating BMI

1. Use weight in grams or height in centimeters without converting: BMI requires weight in kilograms and height in meters. 175 cm must be entered as 1.75.

2. Confusing adult BMI with child BMI: Children and adolescents should be evaluated with percentile tables for age and sex, not with the WHO ranges for adults.

3. Use BMI as the sole diagnosis: BMI is a screening tool, not a diagnostic one. A doctor must consider other tests and clinical factors.

4. Not updating the calculation regularly: Weight and body composition change over time. Recalculating your BMI periodically (every 3-6 months) helps you track trends.

Frequently Asked Questions (FAQ)

1. What is considered a normal BMI for an adult? According to the World Health Organization (WHO), the normal BMI for adults over 18 years of age is between 18.5 and 24.9 kg/m². Values ​​below 18.5 indicate underweight; values ​​from 25 to 29.9 indicate overweight; and values ​​equal to or above 30 indicate obesity.

2. Is a BMI of 25 already a concern? A BMI of 25 to 29.9 is classified as overweight, which indicates a moderately increased risk of some metabolic conditions. However, a value of 25 may be within the normal range for people with greater muscle mass. The most important thing is to evaluate your waist circumference and health history with a doctor.

3. Is it possible to have a normal BMI and still have excess body fat? Yes, this phenomenon is called "normal weight obesity" or TOFI (Thin Outside, Fat Inside). People with low muscle mass and a high proportion of visceral fat may have a BMI within the normal range, but still present elevated metabolic risks. Waist circumference and bioimpedance are important complementary assessments.

4. What is the ideal BMI for women? The WHO table is the same for adult men and women: the healthy range is 18.5 to 24.9. However, women naturally have a higher percentage of body fat than men of the same BMI, which is physiologically normal. For a more accurate assessment of body composition by gender, also consider the specific female BMI available in our Female BMI Calculator.

5. How is BMI affected by age? With natural aging, there is a progressive loss of muscle and bone mass (sarcopenia and osteopenia). Therefore, seniors over 65 are assessed with slightly different BMI ranges — the healthy range starts at 22 kg/m², not 18.5. A BMI of 22-26.9 is considered appropriate for this age group.

6. What to do if my BMI indicates overweight or obesity? The diagnosis of overweight or obesity must always be confirmed and treated by a healthcare team (doctor, nutritionist, physical educator). In general, recommendations include: dietary changes with a moderate calorie deficit, regular exercise (150-300 min/week of moderate intensity), improved sleep and stress management. Losing 5-10% of body weight already has significant benefits across most health markers.

7. What is the BMI formula in pounds and inches? For the imperial system: BMI = [Weight (lb) × 703] ÷ [Height (inches)]². For example, for 154 lb and 5'9" (69 inches): BMI = (154 × 703) ÷ 69² = 108,262 ÷ 4,761 ≈ 22.7 kg/m² (normal weight).

8. Is low BMI as dangerous as high BMI? Yes. Severe thinness (BMI below 16) is associated with a high risk of mortality, immunodeficiency, organ failure and fertility problems. Eating disorders such as anorexia nervosa (which occur with very low BMI) have one of the highest mortality rates among psychiatric disorders. See a doctor immediately if your BMI is below 17.5.

Calculate Your BMI Now

Ready to know your number? Our calculator automatically applies the WHO formula, displays the exact rating, calculates your ideal weight and shows the healthy range for your height.

Access our free BMI Calculator — instant results without registration.

Related calculators:

• Female BMI Calculator — classification by sex
• Male BMI Calculator — classification by sex
• Child BMI Calculator — percentiles by age and sex
• Calorie Calculator (TDEE) — calculate your daily calorie requirement