BMI Calculator
Calculate your BMI instantly using height and weight in metric or imperial. Understand your BMI category, what it means, and what to do next.
Disclaimer: This calculator provides educational estimates only. It is not medical advice, diagnosis, contraception guidance, or a substitute for a qualified healthcare professional.
BMI Calculator
Your Body Mass Index (BMI) is a simple number calculated from your height and weight. It gives you a general indication of whether your weight falls into an underweight, healthy, overweight, or obese range for your height. It is widely used by doctors, fitness professionals, and health organisations as a quick screening tool, not a diagnosis.
This calculator gives you your BMI in seconds. Enter your height and weight, choose metric or imperial, and you will see your number alongside the standard BMI category it falls into.
What Is BMI?
BMI stands for Body Mass Index. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has been used in public health research for over a century. The World Health Organisation (WHO) and the US Centers for Disease Control and Prevention (CDC) both use BMI as a standard screening measure for adult weight status.
BMI is not a direct measure of body fat. Two people with the same BMI can have very different body compositions. A muscular athlete may have a high BMI with low body fat, while an older adult may have a normal BMI but high body fat percentage. This is why BMI is always used alongside other assessments, not in isolation.
BMI Formula
Metric (kg and cm):
BMI = weight (kg) ÷ height (m)²
Imperial (lbs and inches):
BMI = (weight (lbs) × 703) ÷ height (inches)²
Both formulas produce the same result. The 703 conversion factor adjusts for the difference between metric and imperial units.
How to Use This Calculator
- Choose your unit system: metric (kg/cm) or imperial (lbs/inches).
- Enter your height accurately. If using feet and inches, convert to total inches when needed.
- Enter your current weight.
- Review your BMI result, category, and healthy-weight range.
For the most accurate result, measure your height without shoes and weigh yourself at the same time of day, ideally in the morning before eating.
What Is a Healthy BMI?
The WHO defines a healthy BMI as 18.5 to 24.9 for adults. Within this range, the risk of weight-related conditions like type 2 diabetes and cardiovascular disease is lower than in the underweight or overweight ranges.
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These ranges apply to adults aged 18 and over. Different thresholds are used for children and teenagers (BMI-for-age percentiles) and some ethnic groups, particularly people of South or East Asian descent, where health risks appear at lower BMI values.
Example BMI Calculation
Example 1 - Metric:
- Height: 175 cm (1.75 m)
- Weight: 70 kg
- BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 -> Healthy weight
Example 2 - Imperial:
- Height: 5 ft 9 in (69 inches)
- Weight: 160 lbs
- BMI = (160 × 703) ÷ (69 × 69) = 112,480 ÷ 4,761 = 23.6 -> Healthy weight
Example 3 - Overweight range:
- Height: 175 cm
- Weight: 90 kg
- BMI = 90 ÷ 3.0625 = 29.4 -> Overweight
What Your BMI Result Means
Underweight (below 18.5) A BMI below 18.5 may indicate insufficient caloric intake, nutrient deficiency, or an underlying health condition. It is associated with weakened immunity, reduced bone density, and fertility issues. If your BMI is in this range, speak with a doctor or registered dietitian.
Healthy weight (18.5 - 24.9) This range is associated with lower risk of weight-related health conditions including type 2 diabetes, cardiovascular disease, and joint problems. Staying within this range through a balanced diet and regular physical activity supports long-term health.
Overweight (25.0 - 29.9) A BMI in the overweight range increases risk of high blood pressure, high cholesterol, type 2 diabetes, and heart disease. However, many people in this range are healthy, especially if they are physically active. Waist circumference and other markers are often more informative at this level.
Obese (30.0 and above) A BMI of 30 or above is associated with significantly increased health risks. The obese category is sometimes divided further into Class I (30-34.9), Class II (35-39.9), and Class III (40+). If your BMI falls here, a conversation with a healthcare provider about weight management options is worthwhile.
Limitations of BMI
BMI is a useful screening tool but it has well-documented limitations:
- It does not measure body fat directly. Muscle weighs more than fat, so athletes can have a high BMI despite low body fat.
- It does not account for fat distribution. Abdominal fat carries higher cardiovascular risk than fat stored elsewhere, and BMI does not distinguish between the two.
- Age and sex affect interpretation. Older adults tend to carry more body fat at a given BMI. Women typically have more body fat than men at the same BMI.
- Ethnicity matters. Research shows that people of Asian descent face higher health risks at BMI levels considered normal by standard WHO thresholds. Some health bodies recommend lower cutoffs for these populations.
For a more complete picture, consider using BMI alongside waist-to-hip ratio, body fat percentage, and blood markers.
Common Mistakes When Using a BMI Calculator
- Entering height in centimetres when the calculator expects metres (for example, entering 175 instead of 1.75 in a metres-only tool).
- Using weight from several months ago rather than a current measurement.
- Comparing BMI across different calculators that use different rounding methods.
- Treating a single BMI reading as a fixed label rather than a data point to track over time.
Related Calculators
- Body Fat Calculator - estimates body fat percentage using measurements
- BMR Calculator - calculates your Basal Metabolic Rate
- TDEE Calculator - estimates your Total Daily Energy Expenditure
- Calorie Calculator - helps set daily calorie targets based on your goal
- Ideal Weight Calculator - shows healthy weight range for your height
FAQs
What is a healthy BMI for an adult?
The WHO defines a healthy BMI as 18.5 to 24.9 for adults. Within this range, the risk of weight-related conditions like type 2 diabetes and cardiovascular disease is lower than in the underweight or overweight ranges.
Is BMI accurate for athletes and muscular people?
No. BMI is less accurate for people with high muscle mass. Muscle is denser than fat, so an athlete can have a high BMI while having a low or normal body fat percentage. In these cases, body fat percentage measurement is more informative.
How often should I check my BMI?
For most adults, checking BMI a few times per year is sufficient. It is more useful as a trend over time than as a one-time snapshot. If you are actively trying to manage your weight, monthly tracking alongside waist measurements gives a clearer picture of progress.
Does BMI differ for men and women?
The same BMI formula and category thresholds are used for both men and women. However, women naturally carry a higher percentage of body fat than men at the same BMI, so the health interpretation can differ. A doctor assessing weight-related risk will typically consider sex alongside BMI.
What BMI is considered obese?
A BMI of 30 or above is classified as obese by the WHO. This is further divided into Class I (30-34.9), Class II (35-39.9), and Class III, sometimes called severe obesity (40 and above).
Can children use this BMI calculator?
This calculator is designed for adults aged 18 and over. For children and teenagers, BMI is interpreted differently using age- and sex-specific percentile charts (BMI-for-age). The CDC provides a separate BMI calculator for children aged 2 to 19.