how to work out bmi calculator

What is BMI?

BMI stands for Body Mass Index. It is a simple number calculated from your weight and height to estimate whether your body weight is in a healthy range for your height. It is widely used because it is quick, inexpensive, and easy to work out.

BMI is a screening tool, not a diagnosis. In other words, it gives useful guidance, but it does not directly measure body fat, fitness, or overall health.

How to work out BMI manually

Metric formula (kilograms and meters)

If your weight is in kilograms and your height is in meters:

BMI = weight (kg) ÷ [height (m)]²

Example: 70 kg and 1.75 m

• Height squared = 1.75 × 1.75 = 3.0625
• BMI = 70 ÷ 3.0625 = 22.9

Imperial formula (pounds and inches)

If your weight is in pounds and your height is in inches:

BMI = 703 × weight (lb) ÷ [height (in)]²

Example: 165 lb and 69 in

• Height squared = 69 × 69 = 4,761
• BMI = 703 × 165 ÷ 4,761 = 24.4

BMI categories for adults

Most adult BMI calculators use the following ranges:

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

How to use this BMI calculator

• Select your unit system (Metric or Imperial).
• Enter your weight and height values.
• Click Calculate BMI.
• Read your BMI value, category, and estimated healthy weight range.

Why people use BMI

Benefits

• Fast and easy to calculate.
• Useful for tracking trends over time.
• Helpful starting point for health discussions.
• Commonly used in public health research.

Limitations

• Does not distinguish muscle from fat.
• Does not show where fat is stored.
• May not fit all ethnic and body-type differences equally.
• Should be interpreted with other markers (waist size, blood pressure, labs, activity levels).

Practical tips if your BMI is outside the healthy range

If your BMI is high

• Aim for small calorie reductions rather than extreme dieting.
• Prioritize protein, vegetables, whole grains, and hydration.
• Add regular movement: walking, resistance training, and cardio.
• Track progress weekly, not daily.

If your BMI is low

• Increase calories gradually with nutrient-dense foods.
• Include strength training to support lean mass gain.
• Use regular meals plus snacks to raise intake comfortably.
• Discuss underlying causes with a healthcare professional if needed.

Final thoughts

Learning how to work out BMI is a useful skill because it gives you a quick snapshot of weight status. This calculator makes it easy to do the math instantly using either metric or imperial units. For best results, use BMI alongside other health indicators and seek personalized medical advice when needed.