BMI Calculator — Free Body Mass Index Tool

Q: How often should I check my BMI?

For most adults, checking once every 3 to 6 months is sufficient, unless you are on a specific weight management program. More frequent checks can be useful when tracking progress toward a health goal.

Q: What is the BMI formula in imperial units?

The imperial BMI formula is: BMI = 703 × weight in pounds ÷ (height in inches)². For example, someone weighing 160 lbs at 5 feet 7 inches (67 inches) has a BMI of 703 × 160 ÷ (67 × 67) = 25.1.

What Is BMI?

Body Mass Index (BMI) is a numerical screening tool used by healthcare providers worldwide to identify potential weight-related health risks. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and has been adopted by the World Health Organization (WHO) as a standard population-level screening measure.

BMI provides a simple, inexpensive snapshot of whether your weight falls within a range associated with good health for your height. While it is not a diagnostic tool, it is an excellent starting point for conversations with your doctor about your health.

How Is BMI Calculated?

BMI is calculated by dividing your weight by the square of your height. The formula differs slightly depending on the unit system you use:

Metric: BMI = weight (kg) ÷ height (m)²

Imperial: BMI = 703 × weight (lb) ÷ height (in)²

Worked example (metric): A person who weighs 75 kg and stands 175 cm (1.75 m) tall has a BMI of 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5 — which falls in the Normal weight range.

Worked example (imperial): A person who weighs 165 lbs and stands 5 feet 9 inches (69 inches) has a BMI of 703 × 165 ÷ (69 × 69) = 115,995 ÷ 4,761 = 24.4 — also Normal weight.

2026 BMI Categories (WHO & CDC Guidelines)

The following classification table reflects the latest WHO and CDC adult BMI guidelines as of 2026:

BMI Range	Category	Status
Below 18.5	Underweight	Underweight
18.5 – 24.9	Normal weight	Healthy
25.0 – 29.9	Overweight	Overweight
30.0 – 34.9	Obesity Class I	Obese I
35.0 – 39.9	Obesity Class II	Obese II
40.0 and above	Severe Obesity (Class III)	Obese III

Why BMI Isn't the Full Picture

BMI is an excellent first-step screening tool, but it has well-documented limitations that every user should understand:

Muscle mass: Athletes and highly muscular individuals may register as Overweight or Obese despite having very low body fat. BMI cannot distinguish between fat and muscle tissue.
Ethnicity: Research shows that people of Asian descent face higher risks of type 2 diabetes and cardiovascular disease at BMIs below the standard 25.0 threshold. The WHO has proposed lower cut-offs for Asian populations (23.0 for overweight).
Age: Older adults often benefit from a slightly higher BMI (up to 26–27) to protect against frailty, osteoporosis, and poor recovery from illness.
Sex differences: Women naturally carry more body fat than men at the same BMI. BMI does not account for these hormonal and physiological differences.
Waist circumference: Visceral fat stored around the abdomen is often a stronger predictor of cardiovascular risk than BMI. A waist circumference above 88 cm (35 in) for women or 102 cm (40 in) for men indicates elevated risk, regardless of BMI.

For a more complete picture of your health, consider discussing waist circumference, blood pressure, fasting blood glucose, and cholesterol levels with your healthcare provider.

BMI Guidance for Canada, UK, US, and Australia

BMI classification is internationally standardised, but local health systems offer different resources and support:

Canada: Health Canada follows WHO BMI guidelines. The Canadian Obesity Network and your family physician are excellent resources for weight management support.
United Kingdom: The NHS uses the same adult BMI categories and provides a free NHS Weight Loss Plan app for people in the Overweight or Obese range.
United States: The CDC and American Heart Association use WHO-aligned categories. Many employer health plans cover weight management counselling.
Australia: The Australian Institute of Health and Welfare (AIHW) reports on national BMI trends. The My Health Record system allows you to track your BMI over time.

Frequently Asked Questions

What is a healthy BMI range?

A healthy BMI range for most adults is 18.5 to 24.9, according to WHO guidelines. This range is associated with the lowest risk of weight-related health conditions. Remember, BMI is a screening tool — your overall health picture involves much more than a single number.

Is a BMI of 27 unhealthy?

A BMI of 27 falls in the Overweight category (25.0–29.9). However, if you are highly muscular or have a small waist circumference, your actual health risk may be lower than the number suggests. It is always best to discuss your result with a healthcare professional who knows your full health history.

How is BMI calculated?

BMI is calculated by dividing your weight in kilograms by your height in metres squared (kg/m²). In imperial units: BMI = 703 × weight in pounds ÷ height in inches². Our calculator handles both systems automatically.

Does BMI apply to children?

No. Children and teens use BMI-for-age percentiles, which compare their growth against others of the same age and sex. The standard adult BMI categories do not apply to anyone under 18. Ask your paediatrician to calculate your child's BMI-for-age percentile.

How often should I check my BMI?

For most adults, checking every 3 to 6 months is sufficient unless you are actively managing your weight. If you are on a structured diet or exercise programme, monthly checks can help you track progress meaningfully.

What are the limitations of BMI?

BMI does not account for muscle mass, bone density, age, sex, or ethnicity. Athletes may have a high BMI due to muscle, not excess fat. People of Asian descent may face higher health risks at BMI values below the standard thresholds. Waist circumference and body composition scans provide a more complete assessment.

Can BMI predict health outcomes?

BMI is a useful population-level screening tool but is a poor predictor of individual health outcomes on its own. Waist circumference, blood pressure, fasting blood glucose, and cholesterol levels together provide a far more complete picture of cardiovascular and metabolic health.

What is the BMI formula in imperial units?

The imperial BMI formula is: BMI = 703 × weight (lbs) ÷ height (inches)². For example, someone weighing 160 lbs at 5 feet 7 inches (67 inches) has a BMI of 703 × 160 ÷ (67²) = 25.1, placing them just into the Overweight range.

Medical Disclaimer: This BMI calculator is provided for informational and educational purposes only. BMI is a screening tool, not a diagnostic instrument. Results should not be used to self-diagnose or replace professional medical advice. Always consult a qualified healthcare provider regarding your weight, health, and any medical concerns.

Last updated: March 2026

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