How Can We Calculate BMI? Your Questions Answered + A Simpler Tool

Understanding your body weight and its implications for your health has never been more important. With obesity rates reaching epidemic proportions worldwide, knowing how can we calculate BMI is an essential first step toward a healthier life. In this comprehensive, up-to-date guide, we’ll walk you through everything you need to know about Body Mass Index — from the basic formula to the latest 2025–2026 medical guidelines that are reshaping how we think about BMI and obesity.

1. What Is BMI?

21 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 expressed in units of kg/m², resulting from mass in kilograms (kg) and height in metres (m).

In simpler terms, if you’ve ever wondered how can we calculate BMI, it’s essentially a mathematical ratio that compares your weight to your height to give you a number. 21The BMI is a convenient rule of thumb used to broadly categorize a person based on tissue mass (muscle, fat, and bone) and height.

2 BMI is one tool used to tell whether a person is at a healthy weight. BMI is often used to help tell if your weight might put you at risk for health problems such as heart disease, diabetes, and cancer.

2. Why Is BMI Important in 2026?

Understanding how can we calculate BMI is more relevant than ever in 2026. The global obesity crisis has reached staggering levels:

• 12 As of 2025, approximately 16% of the global adult population is classified as obese (BMI ≥30), representing over 1 billion people worldwide.
• 11 The World Obesity Federation’s 2025 Atlas projects that the total number of adults living with obesity will increase by more than 115% between 2010 and 2030, from 524 million to 1.13 billion.
• 12 The United States has an obesity rate of 42.4% as of 2025, showing a 1.8% increase from 2024.
• 19 Current trends suggest that by 2030, 50% of adult men and women will be living with high BMI.

These alarming statistics make learning how can we calculate BMI a vital health skill for everyone in 2026.

3. How Can We Calculate BMI? (Step-by-Step)

Calculating your BMI is straightforward. Here’s a simple step-by-step process:

Step 1: Measure Your Weight

Use a calibrated digital scale. Weigh yourself in the morning before eating, wearing minimal clothing. Record your weight in kilograms (kg) or pounds (lbs).

Step 2: Measure Your Height

Stand upright against a wall without shoes. Use a tape measure or stadiometer. Record your height in meters (m) or inches (in).

Step 3: Apply the BMI Formula

9 BMI can be calculated using Metric Units: BMI = Weight (kg) / (Height (m) x Height (m)).

Or in Imperial Units: BMI = [Weight (lbs) ÷ Height (in)²] × 703

Step 4: Compare Your Result to BMI Categories

Once you have your number, compare it to the standard WHO BMI categories (listed below).

4. BMI Formula: Metric and Imperial Units

Here are the two main ways to answer how can we calculate BMI:

Metric Formula

BMI = Weight (kg) ÷ [Height (m)]²

Example: A person weighing 70 kg and standing 1.75 m tall: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.86

Imperial Formula

BMI = [Weight (lbs) ÷ Height (in)²] × 703

Example: A person weighing 154 lbs and standing 69 inches (5’9″) tall: BMI = [154 ÷ (69 × 69)] × 703 = [154 ÷ 4761] × 703 = 0.03234 × 703 = 22.74

2 In adults, BMI is calculated the same way for both men and women.

5. BMI Categories According to WHO

Once you know how can we calculate BMI, you need to understand what your number means. 21Major adult BMI classifications are underweight (under 18.5 kg/m²), normal weight (18.5 to 24.9), overweight (25 to 29.9), and obese (30 or more).

Here’s the detailed breakdown:

5 For the past 5 decades, obesity has been primarily defined by body mass index (BMI, calculated as weight in kilograms divided by height in meters squared). In this classification, obesity is categorized as a BMI of 30 or higher, with the following subcategories: class I (BMI of 30–34.9), class II (BMI of 35–39.9), and class III (BMI of ≥40).

BMI Categories for Asian Populations

It’s important to note that BMI thresholds vary across ethnic groups. 21In addition to the principal, international WHO BMI cut-off points (16, 17, 18.5, 25, 30, 35 and 40), four additional cut-off points for at-risk Asians were identified (23, 27.5, 32.5 and 37.5).

9 Asian populations may experience lowest mortality rates starting at a BMI of 23 to 24 kg/m². This means that in many Asian countries, a BMI of 23 or above already signals overweight status.

6. BMI Chart: Quick Reference Table

If you prefer not to do the math when wondering how can we calculate BMI, you can use a BMI chart. 2Charts are another way to find your BMI. To use the table below, find your height on the left side of the chart, then go across to the weight that is closest to yours.

Note: This is a simplified reference. For precise results, always use the BMI formula or an online BMI calculator.

7. How Can We Calculate BMI for Children and Teens?

Understanding how can we calculate BMI for younger populations requires a slightly different approach than for adults.

21 BMI is used differently for people aged 2 to 20. It is calculated in the same way as for adults but then compared to typical values for other children or youth 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.

Here’s how child/teen BMI percentiles break down:

23 Obesity in children and teens is defined as a BMI at or above the 95th percentile for sex and age. 2 An easy way to determine your child’s BMI percentile is to use the CDC’s online BMI percentile calculator.

Why Child BMI Matters in 2026

The World Obesity Atlas 2026, released on World Obesity Day (March 4, 2026), revealed alarming data about childhood obesity:

• 17 Worldwide, the atlas found that more than one in five (20.7%) in the 5–19 age group are living with obesity or are overweight, an increase of 14.6% since 2010.
• 12 Childhood obesity (ages 5–19) affects approximately 8–10% globally as of 2025, representing 340–390 million children.

8. BMI Calculation Examples

Let’s work through some practical scenarios so you truly understand how can we calculate BMI:

Example 1: Metric Calculation

• Weight: 85 kg
• Height: 1.80 m
• BMI = 85 ÷ (1.80 × 1.80) = 85 ÷ 3.24 = 26.23
• Category: Overweight

Example 2: Imperial Calculation

• Weight: 200 lbs
• Height: 5’10” (70 inches)
• BMI = [200 ÷ (70 × 70)] × 703 = [200 ÷ 4900] × 703 = 28.69
• Category: Overweight

Example 3: A Person in the Healthy Range

• Weight: 62 kg
• Height: 1.68 m
• BMI = 62 ÷ (1.68 × 1.68) = 62 ÷ 2.8224 = 21.97
• Category: Normal Weight

Example 4: A Person in the Obese Range

• Weight: 110 kg
• Height: 1.70 m
• BMI = 110 ÷ (1.70 × 1.70) = 110 ÷ 2.89 = 38.06
• Category: Obesity Class II

9. New 2025–2026 Obesity Guidelines: Beyond BMI

While understanding how can we calculate BMI remains important, the medical community has made significant advances in 2025–2026 that go beyond BMI alone.

The Lancet Commission’s New Framework (2025)

4 The latest guidelines for measuring obesity — created by 58 medical experts, endorsed by 76 organizations, and published in the Lancet Diabetes & Endocrinology journal in early 2025 — are shifting away from solely using your BMI to measure obesity. 8 The Lancet Diabetes & Endocrinology Commission — which is endorsed by 75 medical organizations around the world — has set out a new way to diagnose obesity to use objective measures of illness based on an individual’s risk factors. The commission also has introduced two novel diagnostic categories of obesity: clinical obesity and pre-clinical obesity, which delineate if and how an individual’s excess body fat affects their organ health and daily functioning.

What’s New?

Under the new framework:

10 A person is classified as having obesity if they have a high BMI plus at least one elevated anthropometric measure (a condition the authors term “BMI-plus-anthropometric obesity”), or if they have a normal BMI and at least two elevated anthropometric measures (a condition termed “anthropometric-only obesity”).

This is groundbreaking because 1using a new definition that includes waist-based measurements, researchers found that more than 75% of adults may meet criteria for obesity compared to 40% when using BMI alone.

Clinical Obesity vs. Pre-Clinical Obesity

8 Clinical obesity is defined in the new guidelines as a chronic disease with specific signs and symptoms of ongoing organ dysfunction due to obesity alone. 8 Pre-clinical obesity is defined in the new guidelines as being associated with variable level of health risk, but no ongoing health complications due to excess body fat.

The Impact of the New Definition

10 Obesity prevalence was 68.6 percent with the new definition, versus 42.9 percent under the traditional BMI-based definition. 1 Nearly four in 10 adults with a “normal” BMI were found to have excess body fat when waist-based measures were also applied.

This means that while knowing how can we calculate BMI is still essential as a starting point, you should also incorporate additional measurements for a complete health picture.

10. Limitations of BMI

While BMI is a useful screening tool, it’s important to understand its limitations. 6Your BMI is just one piece of the puzzle. It’s based on height and weight but doesn’t take into account your muscle mass, bone density, or body composition.

Key Limitations Include:

1. Doesn’t Differentiate Muscle from Fat

2 Bodybuilders or other very muscular people can have a high BMI because of their muscle mass, even though they’re not necessarily in the overweight range for BMI.

2. Doesn’t Account for Age-Related Changes

2 BMI can also underestimate body fat in people who have lost muscle mass, such as some older people.

3. Doesn’t Measure Fat Distribution

1 BMI alone does not account for adipose tissue, which stores energy, insulates organs and produces hormones that regulate appetite.

4. Ethnic Variations

9 BMI and its associated disease and mortality risk appear to vary among ethnic subgroups. 26 Several studies have shown that Asian populations exhibit a higher overall body fat and increased cardiovascular risk factors at lower BMI values than white populations.

5. Not Ideal for Individual Diagnosis

24 As a measure of size not health, it is useful as a screening tool at the individual level and for estimating overweight and obesity at a population level. It is not recommended that it be used in isolation as a diagnostic tool in a clinical setting.

11. Alternatives and Complementary Measurements to BMI

Now that you understand how can we calculate BMI and its limitations, here are some complementary measurements healthcare professionals are recommending in 2026:

1. Waist Circumference (WC)

Measures abdominal fat, which is closely linked to metabolic diseases. A waist circumference of more than 102 cm (40 inches) for men and 88 cm (35 inches) for women signals increased health risk.

2. Waist-to-Hip Ratio (WHR)

Divide your waist measurement by your hip measurement. 22Participants were classified as having obesity if they had a BMI equal to or over 30 kg/m² in combination with at least one measure of excess adiposity (waist circumference >88 cm for women, >102 cm for men; WHR >0.85 for women, >0.90 for men; or WHtR >0.50 for all).

3. Waist-to-Height Ratio (WHtR)

This simple ratio (waist ÷ height) should ideally be under 0.50 for optimal health.

4. Body Fat Percentage

Measured using tools like DEXA scans, bioelectrical impedance scales, or skinfold calipers — this gives a more precise picture of fat vs. lean mass.

5. The Edmonton Obesity Staging System (EOSS)

5 EOSS is a strong predictor of mortality, cardiovascular events, and health care use. It stages obesity by its functional consequences, not just weight. 2 The provider might use other factors such as skinfold thickness (a measure of body fat), waist size, evaluations of diet and family health problems, and other factors to find out if someone’s weight might pose health risks.

12. How to Use Your BMI Results

After learning how can we calculate BMI, here’s what to do with your results:

If Your BMI Is Below 18.5 (Underweight):

• Consult a healthcare provider to rule out nutritional deficiencies or underlying conditions
• Focus on nutrient-dense, calorie-rich foods
• Consider strength training to build healthy muscle mass

If Your BMI Is 18.5–24.9 (Normal Weight):

• Maintain a balanced diet and regular exercise
• Continue monitoring your BMI annually
• Consider additional measurements (waist circumference) for a complete picture

If Your BMI Is 25–29.9 (Overweight):

• Evaluate your diet and physical activity levels
• Aim for 150–300 minutes of moderate exercise per week
• Consider consulting a nutritionist or dietitian
• Monitor for signs of metabolic conditions

If Your BMI Is 30 or Above (Obese):

• Seek medical advice promptly
• Your doctor may recommend a comprehensive health evaluation
• 14 In December 2025, WHO published the guideline on the use of glucagon-like peptide-1 (GLP-1) therapies for the treatment of obesity in adults. This guideline aims to support the safe, equitable and appropriate inclusion of pharmacological therapy for adults as part of comprehensive obesity chronic care programmes.
• Lifestyle modifications, medication, and in some cases surgical options may be discussed

14. Conclusion

Understanding how can we calculate BMI is a fundamental health skill that everyone should master in 2026. Whether you’re using the simple metric formula (weight ÷ height²) or the imperial version (weight ÷ height² × 703), BMI provides a quick, accessible snapshot of your weight status relative to your height.

However, as we’ve explored, the health landscape in 2025–2026 has evolved significantly. 4While BMI has been a long-standing tool to assess your weight in relation to height, it doesn’t account for other factors like body composition, fat distribution, or health risks. The new Lancet Commission guidelines encourage everyone — doctors and patients alike — to look beyond BMI by incorporating waist circumference, waist-to-hip ratio, and functional health assessments.

15 It is estimated that overweight and obesity are associated with 1.6 million annual premature deaths from diabetes, cancer, heart disease, and stroke. With stakes this high, taking the time to calculate your BMI and take action is one of the most important health decisions you can make.

Start today: Calculate your BMI, measure your waist circumference, and schedule a health check-up. Your future self will thank you.