BMI Calculator
Calculate your Body Mass Index instantly using metric or imperial units. Find out whether your weight is underweight, healthy, overweight, or obese.
Free Instant Results Metric & Imperial No Sign-up
BMI Calculator Tool
Your BMI Result
-- Body Mass Index
—
Healthy Range —
Ideal Weight —
Weight Status —
To Healthy Weight —
BMI Formula Explained
The Body Mass Index formula was developed by Belgian mathematician Adolphe Quetelet in the 19th century and has since been adopted worldwide as a standard screening tool by health organisations including the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC).
BMI = kg ÷ m²
Divide your weight in kilograms by your height in metres squared. For example, a person who is 70 kg and 1.75 m tall has a BMI of 70 ÷ (1.75²) = 22.9.
BMI = 703 × lbs ÷ in²
Multiply your weight in pounds by 703, then divide by your height in inches squared. The factor 703 converts the result to match the metric scale.
BMI Categories & Weight Ranges
The following table shows the standard adult BMI classification used by the WHO. These ranges apply to adults aged 20 and over of all genders and ethnicities, though some health organisations recommend adjusted thresholds for certain populations.
| Category | BMI Range | Health Risk |
|---|---|---|
| Underweight | Below 18.5 | Increased (nutritional deficiency, immunity issues) |
| Healthy Weight | 18.5 – 24.9 | Lowest risk of weight-related health problems |
| Overweight | 25.0 – 29.9 | Increased risk of chronic disease |
| Obesity Class I | 30.0 – 34.9 | High risk |
| Obesity Class II | 35.0 – 39.9 | Very high risk |
| Obesity Class III | 40.0 and above | Extremely high risk |
Understanding Your Body Mass Index
BMI, or Body Mass Index, is one of the most widely used methods for assessing whether a person has a healthy body weight relative to their height. It is a simple, non-invasive, and cost-free measurement that can be calculated in seconds — making it invaluable as an initial health screening tool for populations and individuals alike.
Despite its simplicity, BMI provides a meaningful snapshot of your overall weight status. Research consistently shows correlations between BMI values outside the healthy range and an increased risk of developing serious chronic conditions including type 2 diabetes, cardiovascular disease, hypertension, stroke, certain cancers, and musculoskeletal disorders.
How BMI Works in Practice
Your BMI is calculated from just two measurements: your height and your weight. Because taller individuals naturally weigh more, dividing weight by the square of height normalises the value across different body sizes. The result is a single dimensionless number that falls on a universal scale, allowing straightforward comparison and categorisation regardless of whether you are 5 feet or 6 feet 5 inches tall.
Health professionals use BMI as a starting point for conversations about weight and health. If your BMI falls outside the healthy range, it signals that further investigation may be worthwhile — not that you are definitively unhealthy. Additional assessments such as waist circumference, body fat percentage, blood lipid panels, and blood glucose tests provide a far more complete picture of your metabolic health.
Benefits of Tracking Your BMI
- Provides a quick, objective benchmark for your weight status
- Helps identify early weight-related health risks before symptoms appear
- Motivates healthy lifestyle changes by giving you a measurable goal
- Universally understood by healthcare professionals worldwide
- Free to calculate — no equipment or medical appointment required
Limitations of BMI
While BMI is a valuable screening tool, it is important to recognise its limitations:
- Muscle vs Fat: BMI cannot distinguish between lean muscle mass and body fat. Athletes and bodybuilders may have a high BMI due to muscle despite having very low body fat.
- Age: Older adults tend to carry more body fat than younger adults at the same BMI, and lose muscle mass as they age — meaning BMI may underestimate health risk in the elderly.
- Sex: Women naturally carry a higher percentage of body fat than men at the same BMI, due to hormonal differences and reproductive biology.
- Ethnicity: Research suggests that people of South Asian, East Asian, and some other ethnic backgrounds may face elevated health risks at lower BMI values compared to white European populations.
- Fat Distribution: Where you carry fat matters enormously. Visceral fat (around the abdomen) is far more metabolically dangerous than subcutaneous fat. BMI does not account for fat distribution at all.
How to Improve Your BMI
Whether your BMI is too high or too low, sustainable lifestyle changes are the most effective path to a healthier weight. Crash diets and extreme exercise regimens rarely produce lasting results. Instead, focus on gradual, evidence-based changes:
- Adopt a balanced diet rich in vegetables, lean proteins, whole grains, and healthy fats
- Reduce ultra-processed foods, added sugars, and excessive saturated fats
- Aim for at least 150 minutes of moderate aerobic activity per week
- Incorporate resistance training to build and preserve lean muscle mass
- Prioritise quality sleep — poor sleep disrupts hunger hormones and promotes weight gain
- Manage stress, which can trigger emotional eating and hormonal imbalances
- Stay adequately hydrated — often confused with hunger, dehydration can lead to overeating
- Work with your doctor or a registered dietitian for personalised guidance
Frequently Asked Questions
Everything you need to know about BMI and how to use our calculator.
What is BMI?
BMI (Body Mass Index) is a numerical value derived from a person's height and weight. It is a widely used screening tool to categorise individuals as underweight, healthy weight, overweight, or obese. BMI is calculated by dividing weight in kilograms by the square of height in metres. It was developed by Belgian statistician Adolphe Quetelet in the 1830s and was adopted by the medical community in the 1970s as a population-level health screening metric.
Is BMI accurate for everyone?
BMI is a useful starting point but has notable limitations for individuals. It does not account for muscle mass, bone density, age, sex, or how fat is distributed throughout the body. Athletes often have a high BMI due to muscle rather than excess fat, while older adults may have a normal BMI despite unhealthy visceral fat levels. It works best as a population-level screening tool rather than an individual diagnostic measure.
How do I calculate BMI?
There are two formulas depending on your preferred units. For metric, BMI = weight (kg) ÷ height (m)². For imperial, BMI = 703 × weight (lbs) ÷ height (inches)². Our free BMI Calculator handles both automatically — just enter your height and weight and click Calculate BMI.
What is a healthy BMI for adults?
For most adults aged 20 and over, a BMI between 18.5 and 24.9 is considered healthy. Values below 18.5 indicate underweight, 25.0–29.9 indicates overweight, and 30.0 or above indicates obesity. Some health organisations recommend lower thresholds for certain ethnic groups, particularly South Asian and East Asian populations.
Can athletes have a high BMI?
Yes, absolutely. Athletes with high muscle mass frequently have elevated BMI values despite having very low body fat. Since muscle is denser than fat, a muscular person weighs more at a given height — pushing their BMI upward. This is one of the most commonly cited limitations of BMI as a health metric. Athletes should consider additional measurements like body fat percentage to accurately assess their body composition.
Is BMI calculated differently for children?
Yes. For children and adolescents (ages 2–19), BMI is interpreted using age- and sex-specific growth charts rather than fixed thresholds. The result is expressed as a percentile. A BMI at or above the 85th percentile is considered overweight, at or above the 95th percentile is obese, and below the 5th percentile is underweight. Our BMI Calculator is designed for adults; please consult a paediatrician for child-specific assessments.
What BMI range is considered overweight?
According to WHO guidelines, a BMI of 25.0 to 29.9 is classified as overweight for adults. Being overweight is associated with a moderately increased risk of developing chronic conditions including type 2 diabetes, heart disease, and high blood pressure. Even a modest weight loss of 5–10% of body weight can significantly reduce these risks.
What BMI is considered obese?
A BMI of 30.0 or above is classified as obese. Obesity is further divided into three classes: Class I (30.0–34.9), Class II (35.0–39.9), and Class III (40.0 and above, also called severe or morbid obesity). Each class carries progressively higher risks of serious health complications. If your BMI is in the obese range, speaking with a healthcare provider about a safe and sustainable weight management plan is strongly recommended.
Can BMI predict my health risks?
At the population level, yes — higher BMI is statistically associated with increased risk of type 2 diabetes, cardiovascular disease, certain cancers, sleep apnoea, and joint problems. However, at the individual level, BMI is an imperfect predictor. A person with a BMI in the overweight range who exercises regularly, eats well, and has excellent blood markers may be far healthier than someone with a BMI in the healthy range who is sedentary and has poor metabolic health.
Should I rely solely on BMI for my health assessment?
No. BMI is a useful screening tool, but it should always be considered alongside other health indicators including waist circumference, waist-to-hip ratio, body fat percentage, blood pressure, fasting glucose, and cholesterol levels. A comprehensive health assessment from a qualified doctor or registered dietitian provides a far more accurate picture of your overall health status. Use BMI as a starting point, not a final verdict.