Author: Dr. Shirley Cheung | © theshirlshirl.com
How to Use This BMI Calculator
Getting an accurate BMI reading takes less than 10 seconds:
- Choose Your Unit System: Toggle between Metric (cm/kg) or Imperial (ft/in/lbs).
- Enter Your Height: For the most accurate result, measure your height first thing in the morning (spine compresses slightly during the day).
- Enter Your Weight: Use a digital scale on a hard, flat surface. Weigh yourself at the same time of day for consistent tracking.
- Get Instant Results: Your BMI and healthy weight range appear automatically.
The BMI Formula Explained
Body Mass Index is a simple ratio of weight to height squared:
Metric: BMI = Weight (kg) / [Height (m)]²
Imperial: BMI = [Weight (lbs) / Height (in)²] × 703
For example, a person who is 170 cm (1.7 m) tall and weighs 70 kg has a BMI of 70 / (1.7 × 1.7) = 24.2 — within the healthy range.
Understanding Your BMI Category
The World Health Organization (WHO) defines the following standard categories for adults:
- Below 18.5 — Underweight: May indicate insufficient calorie intake or underlying health issues.
- 18.5 – 24.9 — Healthy Weight: Associated with the lowest risk of weight-related health conditions.
- 25.0 – 29.9 — Overweight: Increased risk of cardiovascular disease and type 2 diabetes.
- 30.0 and above — Obese: Higher risk of chronic conditions including hypertension and diabetes.
Limitations of BMI
BMI is a screening tool, not a diagnosis:
- Muscle vs. Fat: BMI doesn’t distinguish between muscle and fat. Athletes often register as “Overweight” despite low body fat.
- Age and Gender: Women naturally carry more body fat. Older adults lose muscle mass, making BMI less accurate.
- Body Fat Distribution: Belly fat is more harmful than hip fat. Waist-to-Hip Ratio gives a clearer picture.
Frequently Asked Questions
What is a good BMI for my age?
The standard 18.5–24.9 range applies to most adults aged 20–65. For older adults (65+), a slightly higher BMI (23–27) may be protective.
How often should I check my BMI?
Once a month is sufficient. Daily weigh-ins fluctuate due to water retention and hormones.
I’m muscular but my BMI says “Overweight.” Should I worry?
Probably not. If your waist circumference is healthy, your BMI may be misleading. Check your Body Fat % instead.
Related Tools & Articles
- Body Fat % Estimator — See your true muscle-to-fat ratio.
- Waist-to-Hip Ratio — Where you carry fat matters.
- Strength Standards — How your lifts compare by age.
- Calorie Deficit Calculator — Fuel your body correctly.
- Peak Physic YouTube Channel — Science-backed fitness videos.
Disclaimer: This tool is for educational purposes only. BMI does not diagnose health conditions. Always consult a qualified healthcare professional for personalized medical advice.