BMI Calculator
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By Maria GonzalezReviewed by Dr. James Wu, MD (Internal Medicine)Published: Updated:
References
- [1]World Health Organization, World Health Organization - Obesity and Overweight Fact Sheet. https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight
- [2]CDC, Centers for Disease Control and Prevention - About Adult BMI. https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html
- [3]National Institutes of Health, NIH National Heart, Lung, and Blood Institute - Assessing Your Weight. https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm
How to Use?
- 1
Enter your weight
Type your body weight into the weight field. Use the unit dropdown to switch between kilograms (kg), pounds (lbs), or stone (st).
- 2
Enter your height
Type your height into the height field. Switch between centimetres (cm), metres (m), feet (ft), or inches (in) using the unit dropdown.
- 3
Click Calculate
Press the Calculate button to compute your BMI. The result shows your BMI score, weight category, and a coloured indicator on the BMI scale.
- 4
Review your healthy weight range
The calculator shows the minimum and maximum healthy weight for your height, based on the BMI range 18.5–24.9. Your result displays in the same weight unit you selected.
- 5
Explore the reference tables and visual
Scroll down to review the BMI classification table, ethnicity-adjusted thresholds, disease risk by waist circumference, and body fat percentage categories for context alongside your result.
Worked Examples
1Example 1: Normal weight (70 kg, 1.75 m)
Given Values
weight:70
height:1.75
Results
bmi:22.9
category:Normal weight
healthyMin:56.6
healthyMax:76.3
idealWeight:66.6
2Example 2: Low-normal weight (50 kg, 1.60 m)
Given Values
weight:50
height:1.6
Results
bmi:19.5
category:Normal weight
healthyMin:47.4
healthyMax:63.8
idealWeight:55.7
3Example 3: Overweight (90 kg, 1.80 m)
Given Values
weight:90
height:1.8
Results
bmi:27.8
category:Overweight
healthyMin:59.9
healthyMax:80.6
idealWeight:70.5
4Example 4: Obese class II (120 kg, 1.70 m)
Given Values
weight:120
height:1.7
Results
bmi:41.5
category:Obese
healthyMin:53.5
healthyMax:72
idealWeight:62.9
5Example 5: Underweight (45 kg, 1.65 m)
Given Values
weight:45
height:1.65
Results
bmi:16.5
category:Underweight
healthyMin:50.4
healthyMax:67.8
idealWeight:59.2
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Frequently Asked Questions
BMI (Body Mass Index) is a measure that uses your weight and height to estimate body fat and screen for weight categories that may increase health risk. It is important because it is a simple, standardised, and widely used metric that correlates with disease risk at the population level. Doctors use it as a first screening tool, researchers use it to track obesity trends, and insurers use it in risk assessment. While it has limitations, it remains the most practical starting point for weight-related health screening.
This calculator uses the standard WHO formula and rounds results to one decimal place. It handles unit conversion automatically, so the only possible errors come from incorrect input. For the most accurate result, use a calibrated scale and a stadiometer, weigh yourself at the same time each morning, and avoid estimating your height. The calculator itself is arithmetically exact. Remember that BMI is a screening tool, not a diagnostic test, and individual results should be interpreted in the context of your overall health.
Yes, the calculator supports multiple weight units (kg, lbs, st) and height units (cm, m, ft, in). Select your preferred units from the dropdowns next to each field. The engine converts everything to metric base units before calculating, so you can even mix units — for example, weight in pounds and height in feet and inches. This flexibility means you never need to do manual unit conversions before using the calculator.
BMI often overestimates body fat in athletes and bodybuilders because it cannot distinguish between muscle and fat. Muscle is denser than fat, so muscular individuals may have a high BMI despite very low body fat percentage. An elite rugby player with 12% body fat may have a BMI of 31, which the calculator classifies as obese. For athletes, body fat percentage measured by skinfold calipers, DEXA scan, or bioelectrical impedance provides a more accurate health assessment than BMI alone.
The healthy BMI range for most adults is 18.5 to 24.9 according to the WHO classification. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. However, these thresholds were developed from predominantly Caucasian populations. For people of Asian descent, the healthy range starts at a lower BMI (18.5 to 23), with overweight beginning at 23. Always consult your healthcare provider to determine the most appropriate target range for your individual circumstances, ethnicity, and health profile.
Yes, the relationship between BMI and health risk varies significantly by ethnicity. People of Asian descent tend to have higher body fat and higher cardiometabolic risk at lower BMIs compared to Caucasians. The WHO recommends lower action thresholds for Asian populations: overweight at BMI 23 and obesity at BMI 27.5. For South Asian populations, diabetes risk increases significantly starting at BMI 22. Conversely, people of Black African descent typically have lower body fat at a given BMI, meaning standard cutoffs may overestimate their risk. The ethnicity-adjusted thresholds table on this page provides specific guidance for different populations.
Children and teenagers use BMI percentiles rather than absolute BMI cutoffs. A child with a BMI above the 95th percentile for their age and sex is classified as obese; between the 85th and 95th percentile is overweight; below the 5th percentile is underweight. These percentiles are plotted on CDC or WHO growth charts that account for the rapid changes in height and weight during growth and puberty. This calculator is designed for adults aged 18 and older. For paediatric BMI assessment, consult a healthcare provider or use a dedicated paediatric BMI calculator that references age- and sex-specific growth charts.
Pre-pregnancy BMI determines the recommended weight gain range during pregnancy. A woman with a pre-pregnancy BMI below 18.5 should gain 12.5 to 18 kg, while a woman with a pre-pregnancy BMI above 30 should gain only 5 to 9 kg. During pregnancy itself, BMI is not a useful metric because the weight includes the fetus, placenta, amniotic fluid, and increased blood volume. Pregnant women should follow their obstetric provider’s weight gain recommendations rather than tracking BMI. After delivery, BMI can be used again once the woman has returned to her pre-pregnancy weight baseline.
The standard BMI range of 18.5 to 24.9 may be too restrictive for older adults. As people age, they lose muscle mass (sarcopenia) and gain fat, so a normal BMI can mask unhealthy body composition. Some geriatric guidelines recommend a target BMI of 24 to 27 for adults over 65, as a slightly higher BMI provides metabolic reserve during illness and correlates with better survival outcomes in hospitalised older patients. A low BMI (below 22) in the elderly is a stronger predictor of frailty, falls, and mortality than a moderately elevated BMI. Consult a geriatrician for personalised weight targets in later life.
Lowering your BMI requires reducing body fat through a combination of dietary changes and increased physical activity. Aim for a modest calorie deficit of 300 to 500 calories per day, which produces a sustainable weight loss of 0.5 to 1 kg per week. Focus on whole, unprocessed foods: vegetables, fruit, lean proteins, whole grains, and healthy fats. Include both cardiovascular exercise (walking, cycling, swimming) and resistance training to preserve muscle mass while losing fat. Drink water instead of sugary beverages. Aim for 7 to 9 hours of sleep per night, as poor sleep is linked to weight gain. Avoid crash diets, which typically result in rapid water loss followed by muscle loss and eventual weight regain. Sustainable changes produce lasting results.
BMI estimates body fat indirectly from weight and height. Body fat percentage directly measures what proportion of your weight is fat. BMI cannot distinguish between muscle, bone, and fat, so a muscular person may have a high BMI but low body fat. Conversely, a person with low muscle mass may have a normal BMI but high body fat (sometimes called normal-weight obesity). Body fat percentage is more accurate for individual assessment but requires equipment like skinfold calipers, bioelectrical impedance scales, or DEXA scans. BMI remains the most practical screening tool because it requires only a scale and a measuring tape.
BMI is a useful screening tool but a poor standalone indicator of overall health. A person with a normal BMI can have metabolic syndrome, high blood pressure, and high blood sugar. A person with a BMI in the overweight range can be metabolically healthy with normal lab values and good fitness. BMI is most valuable when combined with other metrics: waist circumference, blood pressure, fasting glucose, cholesterol panel, and lifestyle factors like diet, exercise, smoking, and sleep. Think of BMI as one tile in the mosaic of your health picture, not the full image. Use the reference tables on this page alongside your BMI result for a broader context.
Yes, a BMI below 18.5 is classified as underweight and carries its own set of health risks. Chronic underweight can indicate undernutrition, weakened immune function, osteoporosis, and fertility problems. In older adults, a low BMI is a stronger predictor of frailty, falls, and mortality than a high BMI. If your BMI is below 18.5, consult a healthcare provider to investigate possible underlying causes such as malabsorption, hyperthyroidism, eating disorders, or chronic illness. Gaining weight safely requires a nutrient-dense diet and often the guidance of a registered dietitian.
Checking your BMI once every three to six months is sufficient for most adults. BMI changes slowly because it reflects long-term weight trends rather than day-to-day fluctuations. Monthly checks are reasonable during active weight loss or gain phases, but weekly checks can be misleading due to normal water weight changes of 1–2 kg. For the most consistent tracking, weigh yourself at the same time of day (morning is best), on the same scale, wearing similar clothing. Record your BMI alongside your weight so you can see the trend over time rather than focusing on any single reading.
There is no single best method, but the most evidence-backed approach combines a modest calorie deficit with consistent physical activity. Reducing daily intake by 300 to 500 calories while increasing step count to 8,000–10,000 per day produces sustainable weight loss of 0.5 to 1 kg per week. Resistance training twice per week preserves muscle mass, which keeps your metabolic rate from dropping as you lose fat. Eating more protein (1.6–2.2 g per kg of body weight), drinking water instead of sugary drinks, and getting 7–9 hours of sleep each night all support long-term BMI reduction. Crash diets and extreme exercise programmes rarely produce lasting results.
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