how to calculate bmi

What is BMI?

BMI stands for Body Mass Index. It is a quick screening number that compares your weight to your height. BMI can help identify whether your weight is in a range that may increase health risk, but it does not directly measure body fat.

In short: BMI is useful as a starting point, not a complete diagnosis.

The BMI formula

Metric formula (kilograms and meters)

BMI = weight (kg) / [height (m)]²

If your height is in centimeters, convert it to meters first by dividing by 100. Example: 175 cm = 1.75 m.

Imperial formula (pounds and inches)

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

For height in feet and inches, convert everything to inches first. Example: 5 ft 10 in = 70 inches.

Step-by-step examples

Example 1: Metric

• Weight = 70 kg
• Height = 175 cm = 1.75 m
• BMI = 70 / (1.75 × 1.75) = 22.9

This BMI falls in the healthy-weight range for most adults.

Example 2: Imperial

• Weight = 180 lb
• Height = 5 ft 10 in = 70 in
• BMI = 703 × 180 / (70 × 70) = 25.8

This BMI falls in the overweight range.

BMI categories for adults

• Below 18.5: Underweight
• 18.5 to 24.9: Healthy weight
• 25.0 to 29.9: Overweight
• 30.0 to 34.9: Obesity (Class 1)
• 35.0 to 39.9: Obesity (Class 2)
• 40.0 and above: Obesity (Class 3)

Common mistakes when calculating BMI

• Using centimeters in the metric formula without converting to meters.
• For imperial units, forgetting to convert total height to inches.
• Entering incorrect decimal values (for example, 1.75 m as 175 m).
• Relying on BMI alone and ignoring waist size, fitness, and medical history.

How to use BMI the smart way

BMI is best used together with other health markers. To get a more complete view, also check:

• Waist circumference
• Blood pressure
• Blood sugar and cholesterol
• Physical activity level
• Sleep quality and stress management

Athletes and highly muscular people may have a higher BMI without excess body fat. Older adults, pregnant women, and children also require different interpretation standards.

If your BMI is outside the healthy range

Practical next steps

• Focus on sustainable habits, not crash diets.
• Prioritize protein, fiber, fruits, vegetables, and whole foods.
• Aim for regular exercise: strength training + cardio.
• Track progress over months, not days.
• Speak with a healthcare professional for personalized guidance.

Quick FAQ

Is BMI accurate for everyone?

No. It is a population-level screening tool and can be less accurate for certain individuals.

What is a good BMI?

For most adults, 18.5 to 24.9 is considered healthy. Individual goals may differ.

How often should I check BMI?

Monthly or every few months is usually enough. Daily changes are not meaningful.

Note: This page is for educational use and not a substitute for medical advice.