How to Calculate BMI and What Your Number Means

What Is BMI and Why Should You Calculate It?

Learning how to calculate BMI takes less than two minutes, and the result gives you an instant benchmark for where your weight stands relative to your height. Body Mass Index is the number the World Health Organization uses to classify weight status across populations — and it remains the most widely used screening tool in clinical and public health settings worldwide.

Two formulas exist depending on which unit system you use. Neither requires anything beyond a scale and a tape measure.

Calculate your BMI instantly with the free BMI Calculator

Step 1: Measure Your Weight and Height Accurately

Before you run any calculation, accurate inputs matter.

• Weight: Weigh yourself in the morning, after using the bathroom and before eating. Use the same scale each time.
• Height: Stand barefoot against a flat wall, heels together. Mark the highest point of your head and measure from the floor.

If you measure in centimetres and kilograms, use the metric formula. If you measure in inches and pounds, use the imperial formula below.

Step 2: Apply the BMI Formula

Metric formula:

BMI = weight (kg) ÷ height (m)²

Imperial formula:

BMI = 703 × weight (lbs) ÷ height (inches)²

Worked example — metric: A person weighs 78 kg and stands 175 cm (1.75 m) tall.

BMI = 78 ÷ (1.75)² = 78 ÷ 3.0625 = 25.5

Worked example — imperial: The same person in imperial units: 172 lbs, 5 ft 9 in (69 inches).

BMI = 703 × 172 ÷ 69² = 120,916 ÷ 4,761 = 25.4

Both results round to the same category: Overweight.

Step 3: Look Up Your Category on the WHO Scale

The World Health Organization defines six adult BMI categories. These thresholds apply to adults aged 18 and over, regardless of sex:

Our example result of 25.5 falls in the Overweight category — above normal, but not yet in the obese range.

Step 4: Find Your Healthy Weight Range

Once you know your height, you can reverse the formula to find exactly which weights place you in the "Normal" band (BMI 18.5–24.9).

Formula: Healthy weight = BMI target × height (m)²

For a height of 175 cm (1.75 m):

• Lower bound: 18.5 × (1.75)² = 18.5 × 3.0625 = 56.7 kg (125 lbs)
• Upper bound: 24.9 × (1.75)² = 24.9 × 3.0625 = 76.3 kg (168 lbs)

The table below shows healthy and adjacent ranges for common heights:

Step 5: Understand What to Do With Your Result

BMI is a screening number, not a diagnosis. Here is how to act on it:

• Normal (18.5–24.9): Maintain current habits. Focus on exercise, sleep, and nutrient quality.
• Overweight (25–29.9): A modest calorie deficit of 300–500 kcal/day can move you back into the normal range over 3–6 months. Pair this with understanding your daily calorie needs.
• Underweight (below 18.5): Speak with a doctor. Increasing calorie-dense, nutritious foods and resistance training is the usual path.
• Obese (30+): BMI alone does not determine health risk — but sustained elevation above 30 is associated with type 2 diabetes, cardiovascular disease, and joint problems. A structured plan with a healthcare provider is advisable.

BMI does not account for muscle mass, bone density, or fat distribution. Athletes often show a BMI above 25 with low body fat. For a more precise measurement, consider checking your body fat percentage, which distinguishes lean mass from fat mass directly.

Conclusion

Key takeaways:

• The metric formula is BMI = kg ÷ m²; the imperial formula is BMI = 703 × lbs ÷ inches²
• The WHO "Normal" range is 18.5 to 24.9
• At 175 cm, a healthy weight falls between approximately 57 kg and 76 kg
• BMI is a useful starting point, but always combine it with other metrics like body fat percentage
• Use the free BMI Calculator to get your result instantly without doing the arithmetic yourself