How to Work Out BMI – Step-by-Step Guide, Calculator & Ranges

Knowing how to work out your BMI is a quick way to check if your weight is in a healthy range for your height. The calculation itself is straightforward, but understanding what the number means—and its limitations—is just as important.

Body Mass Index is a screening tool used by health professionals worldwide. It is calculated using a simple formula, and the result places you into a category that can help indicate potential health risks. While it does not measure body fat directly, it serves as a useful starting point for a broader health conversation.

This guide walks through the step-by-step calculation using both metric and imperial units, explains the standard weight categories, and outlines where BMI is accurate and where it falls short.

How Do I Calculate My BMI? (Step-by-Step Formula)

To calculate your BMI, you need your weight and height. The formula changes depending on whether you use metric kilograms and centimetres or imperial pounds and inches. The core principle remains the same: divide weight by the square of height.

• BMI is a simple screening tool, not a diagnostic. It does not measure body fat percentage or distribution. Cleveland Clinic / NHS
• The formula is identical for men and women, but healthy ranges are the same; however, muscle mass can skew results. NHS / CDC
• For children and teens, BMI is age- and sex-specific (percentile charts). NHS Child BMI page
• Recent debates question BMI’s accuracy for diverse ethnicities and older adults. Research references

How to Work Out BMI Using kg and cm (Metric)

For metric calculations, measure your weight in kilograms and your height in metres. Square your height first, then divide your weight by that number. The NHS provides a worked example: if you weigh 70 kg and are 1.70 m tall, square the height: 1.70 × 1.70 = 2.89. Then divide weight by that result: 70 ÷ 2.89 = 24.22, which rounds to 24.2. Source

How to Calculate BMI in Pounds and Inches (Imperial)

If you prefer imperial units, use this formula: BMI = 703 × weight (lb) / height (in)². A person who weighs 180 lb and is 65 inches tall has a BMI of 703 × 180 ÷ 65² = 29.95, rounding to 29.9. Source This places them in the overweight category. The same formula from the Cleveland Clinic and Ochsner Health is used consistently across medical sites.

What Is a Healthy BMI Range? (Chart & Interpretation)

Once you have calculated your BMI, interpreting the number means checking it against standardised categories. These categories are used by the NHS, NHLBI, and the Heart Foundation to classify weight ranges in adults. Source

Common adult BMI categories are:

• Underweight: below 18.5
• Healthy weight: 18.5 to 24.9
• Overweight: 25.0 to 29.9
• Obesity: 30.0 and above

This chart applies to most adults. However, the CDC and NHS note that children and teens use a different BMI approach that accounts for age and sex. Source

How to Use the NHS BMI Calculator (UK Guidance)

The official NHS BMI calculator is the most authoritative tool for UK adults. It is hosted on the NHS website and uses the same formula described above. The calculator provides an instant result and places it within the standard healthy weight range. Source

The tool is designed for adults (typically 20+ in the CDC tool and adults generally in NHS guidance). It does not require age input for adults. For children, the NHS provides a separate child BMI calculator that uses percentile charts.

Beyond the NHS, tools such as the Heart Foundation BMI calculator and the CDC BMI calculator offer similar functionality with slight variations in presentation and additional risk information.

Is BMI Accurate? Limitations and Adjustments

BMI is a screening tool, not a direct measure of body fat or overall health. The Cleveland Clinic, Diabetes Canada, and CDC all note it can miss differences in body composition. Source

Does BMI Work for Athletes?

For athletes with high muscle mass, the standard BMI formula often overestimates body fat. The Cleveland Clinic advises that athletes should not rely solely on BMI for health assessment.

Is BMI Accurate for Women?

The formula is the same for men and women, but hormonal changes during pregnancy and menopause can affect weight distribution and body composition. The NHS does not provide separate BMI thresholds for women.

How Does Age Affect BMI Interpretation?

For adults over 60, some experts suggest that a slightly higher BMI (up to 27) may be associated with better survival outcomes, although the standard range of 18.5–24.9 remains the official guideline.

How to Calculate BMI for Children and Teens

A child’s BMI must be interpreted differently from an adult’s. The calculation uses the same formula, but the result is plotted on a percentile chart that accounts for the child’s age and sex. The NHS child BMI calculator and the CDC both provide this percentile-based approach.

The CDC notes that children and teens use a different BMI approach than adults, which is essential for tracking growth patterns. Source

History of BMI

1. 1832 – Adolphe Quetelet develops the Quetelet Index (later BMI) as part of social physics.
2. 1972 – Ancel Keys coins the term “body mass index” and demonstrates its utility.
3. 1990s – WHO adopts BMI as the standard for classifying obesity.
4. 2020s – Growing calls to adjust BMI thresholds for different ethnic groups and incorporate waist circumference.

What BMI Can and Cannot Tell You

Context for Using BMI Today

While BMI remains the most widely used weight-classification tool, health professionals recommend combining it with other measurements such as waist circumference, body fat percentage, and individual risk factors. For personal health, the NHS calculator provides a quick starting point, but a full health assessment should include blood pressure, cholesterol, and lifestyle evaluation. BMI is not a final verdict on health.

Sources and Expert Views

BMI is a useful population-level health measure, but it should be interpreted with caution on an individual level.

World Health Organization (WHO)

The BMI calculator gives you a quick idea of whether you’re a healthy weight for your height, but it’s not a complete health assessment.

NHS website

It’s important to remember that BMI is not a diagnostic tool. It’s a screening tool that can provide a starting point for a conversation with your doctor.

Cleveland Clinic

The WHO BMI Classification provides the global standard definitions and thresholds.

The Cleveland Clinic also offers a detailed medical overview with limitations and risks, available at Cleveland Clinic – Body Mass Index (BMI).

The Heart Foundation of Australia provides an Australian perspective with heart health context via their BMI Calculator.

Next Steps After Calculating Your BMI

If your BMI is outside the healthy range, consult your GP or a dietitian. Consider measuring your waist circumference for additional risk assessment. Incorporate a balanced diet and regular physical activity to manage weight. For parents, use the NHS child BMI calculator to track growth percentiles. Staying active is essential, and you can find guidance on daily step targets in How Many Steps Should I Walk a Day – Science-Backed Goals by Age.

Common Questions About BMI Calculation (FAQ)