BMI Calculator

Calculate your Body Mass Index instantly using metric or imperial units. Free, highly accurate, and 100% private.

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What Is BMI and How Is It Calculated?

BMI — Body Mass Index — is a numerical value calculated from your height and weight. It was developed in the 1830s as a simple population-level screening tool. Today it is used by doctors and health organisations worldwide as an initial indicator of whether a person's weight may be putting their health at risk.

Unit SystemFormulaExample
MetricBMI = weight (kg) ÷ height² (m)70kg ÷ (1.75 × 1.75) = 22.9
ImperialBMI = (weight (lbs) ÷ height² (in)) × 703(154 ÷ 4761) × 703 = 22.7
Privacy note: All calculations happen entirely in your browser. Your weight, height, and BMI result are never stored, shared, or sent to any server.

BMI Limitations — What It Doesn't Tell You

BMI is a highly useful starting point, but it has well-documented limitations. Understanding these helps you interpret your result correctly.

GroupWhy BMI May Be MisleadingBetter Measure
AthletesHigh muscle mass pushes BMI into "overweight" despite being very fitBody fat percentage
Elderly peopleMuscle loss with age means BMI can read "normal" while body fat is actually highWaist circumference + fat %
Pregnant womenWeight gain during pregnancy is normal — BMI is not a useful indicatorGestational weight charts
ChildrenStandard adult BMI categories do not apply to children — growth stage must be consideredBMI-for-age percentile charts
Important: BMI is a screening tool, not a diagnosis. If your result falls outside the normal range, speak to a qualified healthcare professional before making drastic changes to your diet or exercise habits.

Frequently Asked Questions

What is a healthy BMI for adults?
For most adults, a BMI between 18.5 and 24.9 is considered healthy by the World Health Organisation. However, optimal ranges can vary by age, sex, and ethnicity. Always consult a healthcare provider for personalised guidance.
Is BMI accurate for everyone?
BMI is a useful population-level screening tool but has limitations. It does not distinguish between muscle and fat mass, so athletes often score in the "overweight" range despite being in excellent health. It is best used as one data point alongside other health measures.
What BMI range is considered obese?
A BMI of 30.0 or above is classified as obese, further divided into: Class I (30–34.9), Class II (35–39.9), and Class III or severe obesity (40 and above). If your BMI falls in this range, speaking to a doctor is highly recommended.
Can I use this calculator for children?
Standard adult BMI categories do not apply to children and teenagers. Children's BMI must be interpreted using age and sex-specific growth charts (BMI-for-age percentile). This calculator is designed strictly for adults aged 18 and over.
How often should I check my BMI?
There is no strict rule, but checking monthly is reasonable if you are actively managing your weight. For general wellness tracking, quarterly checks are sufficient. BMI alone is not a day-to-day health metric — focus on trends over time rather than individual daily readings.

Health Disclaimer: The BMI calculator on ToolsNest is provided for informational purposes only. BMI is a screening tool, not a medical diagnosis. Results should not be used as a substitute for professional medical advice. Always consult a qualified healthcare provider regarding any health concerns.

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