How to Calculate BMI: Simple Formula & Step-by-Step Guide

Calculating your Body Mass Index (BMI) is surprisingly simple - you just need two numbers: your weight and your height. Whether you prefer doing the maths yourself or using our free BMI calculator, understanding the formula helps you grasp what BMI actually measures.

What is the BMI Formula?

BMI is calculated by dividing your weight by the square of your height. The formula looks slightly different depending on whether you're using metric or imperial measurements.

BMI Formula (Metric)

If you measure weight in kilograms and height in metres, the formula is straightforward:

For example, if you weigh 70kg and you're 1.75m tall, you'd calculate: 70 / (1.75 × 1.75) = 22.9

BMI Formula (Imperial)

For pounds and inches, you need to multiply by a conversion factor of 703:

If you weigh 154 pounds and you're 69 inches tall, the calculation is: (154 / (69 × 69)) × 703 = 22.7

Step-by-Step: Calculating BMI in Metric Units

Let's walk through a complete example using kilograms and metres:

1. Measure your weight - Let's say you weigh 82 kilograms
2. Measure your height in metres - Perhaps you're 1.80 metres tall
3. Square your height - Multiply 1.80 by itself: 1.80 × 1.80 = 3.24
4. Divide weight by height squared - 82 / 3.24 = 25.3
5. Your BMI is 25.3 - This falls into the "overweight" category

Step-by-Step: Calculating BMI in Imperial Units

Now let's try the same calculation using pounds and inches:

1. Measure your weight - You weigh 180 pounds
2. Measure your height in inches - You're 5 feet 11 inches, which equals 71 inches total
3. Square your height - 71 × 71 = 5,041
4. Divide weight by height squared - 180 / 5,041 = 0.0357
5. Multiply by 703 - 0.0357 × 703 = 25.1
6. Your BMI is 25.1 - Just into the "overweight" category

Common Calculation Mistakes to Avoid

When calculating BMI manually, watch out for these common errors:

• Mixing units - Don't use pounds with the metric formula or kilograms with the imperial one
• Height in centimetres - If your height is in cm, divide by 100 first to convert to metres
• Forgetting to square - Remember, you multiply height by itself, not by 2
• Rounding too early - Keep a few decimal places until your final answer

Why Use a BMI Calculator Instead?

While the maths isn't complex, there are several good reasons to use our free BMI calculator rather than working it out by hand:

• Speed - Get instant results without doing any arithmetic
• Accuracy - No risk of calculation errors or typos
• Interpretation - Automatically shows which BMI category you're in
• Recommendations - Get personalized health insights based on your result
• Tracking - Calculate regularly to monitor changes over time

What Do BMI Results Mean?

Once you've calculated your BMI, you need to understand what the number means. BMI values are grouped into categories established by the World Health Organization:

• Underweight: BMI less than 18.5
• Healthy weight: BMI 18.5 to 24.9
• Overweight: BMI 25 to 29.9
• Obese: BMI 30 or higher

For a detailed explanation of what each category means for your health, read our guide on understanding BMI categories.

BMI Calculation for Children

The basic BMI formula is the same for children, but interpreting the results is completely different. Children's BMI must be compared to growth charts specific to their age and sex, because healthy body composition changes as kids grow.

If you're calculating BMI for a child under 18, speak to their doctor about what the result means. Adult BMI categories don't apply to children.

When to Calculate Your BMI

There's no "best" time to calculate BMI - it's useful in various situations:

• Starting a new diet or exercise programme
• Your annual health check-up
• Before or after significant weight changes
• When assessing health risks with your doctor
• Every few months to track progress

Ready to Calculate Your BMI?

Now that you understand the formula and calculation process, why not try it for yourself? Our BMI calculator does all the maths instantly and provides personalised recommendations based on your results.

Just enter your height and weight - you'll get your BMI, your category, and helpful guidance on what it means for your health. It's free, takes 10 seconds, and requires no registration.

Important Health Note

BMI is a useful screening tool, but it's not a complete measure of health. It doesn't distinguish between muscle and fat, doesn't measure fitness, and may not be accurate for athletes, elderly people, or certain ethnic groups. Always consult with a healthcare professional about your weight and health, especially before starting any new diet or exercise programme. BMI is one piece of information - your doctor will consider many factors when assessing your health.