BMI Calculator 2026 — Calculate Body Mass Index for Men & Women (Metric/Imperial)
Check your Body Mass Index in seconds. Metric and imperial units, WHO adult classification, healthy weight range in kilograms.
📚 Official sources
- ↗WHO – Obesity and overweight (fact sheet)
- ↗WHO – BMI classification (Global Database)
- ↗NIH – Calculate Your BMI (Standard BMI Calculator)
- ↗NHS UK – BMI healthy weight calculator
- ↗CDC – About Adult BMI
- ↗Mifflin, M. D. et al. (1990) – BMR predictive equation
- ↗Devine, B. J. (1974) – Ideal body weight formula
- ↗Hodgdon & Beckett (1984) – U.S. Navy body-fat % formula
- ↗Ashwell, M. et al. – Waist-to-height ratio meta-analysis
BMI is a screening tool, not a medical diagnosis. Always discuss weight changes with your doctor.
BMI (Body Mass Index) is a simple weight-for-height ratio widely used to classify adults as underweight, normal, overweight or obese. It's a screening tool — not a diagnosis — but a quick, reliable snapshot of weight status. Enter your height and weight and the calculator applies the standard WHO boundaries and shows the healthy kilogram window for your height.
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How to use it
- Pick your unit system — metric (cm + kg) or imperial (ft/in + lb).
- Enter your height and weight. The result updates instantly as you type.
- Read your BMI value and WHO category (coloured bar shows where you fall).
- The panel below tells you the healthy kg window for your height and how many kg away you are.
Formula
How is BMI calculated?
Body Mass Index is a deceptively simple ratio: in metric units it is BMI = weight in kilograms divided by height in metres squared, and in imperial units it is BMI = (weight in pounds × 703) divided by height in inches squared. The 703 is the conversion factor that lets the imperial formula return the same numerical scale as the metric one, so a person with a metric BMI of 24.5 will also get 24.5 from the imperial calculation. Because height is squared in the denominator, a small mismeasurement of height has roughly twice the impact of the same percentage error in weight — measuring height to the nearest centimetre, in the morning, without shoes, against a flat wall, materially improves the accuracy of the result.
The interpretation of the resulting number follows the World Health Organization classification used worldwide and mirrored by the National Institutes of Health, the Centers for Disease Control, and the UK National Health Service. A BMI below 18.5 is classed as underweight, 18.5 to 24.9 as the healthy or normal range, 25.0 to 29.9 as overweight, 30.0 to 34.9 as obesity class I, 35.0 to 39.9 as obesity class II, and a BMI of 40 or above as obesity class III (sometimes called severe or morbid obesity). These cut-points were chosen because epidemiological studies show all-cause mortality and cardiometabolic risk rise sharply outside the 18.5–24.9 band — not because there is a sudden biological change at the boundary itself.
BMI was never designed as a body-composition tool, and its main limitation is that it cannot tell muscle from fat. A heavily trained rugby player at 95 kg and 178 cm has a BMI of about 30, which the formula labels as obese even though the person carries 10–12% body fat and is in superb health. Conversely, an older sedentary adult with the same BMI of 24 may be classed as healthy yet carry 35% body fat and have very low muscle mass — a pattern researchers call normal-weight obesity, which carries elevated metabolic risk. BMI also does not account for differences in fat distribution: visceral abdominal fat is far more dangerous for the heart and metabolism than subcutaneous fat on the hips and thighs, but BMI sees them identically.
For these reasons, modern clinical practice combines BMI with waist circumference, waist-to-hip ratio (WHR), or waist-to-height ratio. WHO flags raised cardiometabolic risk above 94 cm waist for men and 80 cm for women, with substantially raised risk above 102 cm and 88 cm respectively. Waist-to-height ratio above 0.5 is a simple rule of thumb usable across ages and ethnicities. Direct body-fat measurements — bioelectrical impedance analysis (BIA), dual-energy X-ray absorptiometry (DEXA scan), skinfold calipers, or hydrostatic weighing — are more informative still, with DEXA the gold standard for fat mass, lean mass and bone density. The calculator above complements BMI with a healthy-weight kilogram range based on the same WHO formula.
BMI also varies by age, sex and ethnicity in ways the basic adult cut-points do not capture. Children and teenagers (2–19) should never be evaluated against the adult thresholds — instead, paediatricians use BMI-for-age percentiles from the CDC 2000 growth charts or the WHO child growth standards, where the 5th, 85th and 95th percentiles play roughly the same role as 18.5, 25 and 30 do for adults. The Children BMI tab on this page applies those CDC-2000 percentiles directly. For adults of Asian descent, WHO recommends lower cut-offs of 23 for overweight and 27.5 for obesity because cardiometabolic risk rises at lower BMI values than in European populations. Adults aged 65 and older have the lowest mortality at a BMI of 23–28, slightly higher than the standard healthy range, because in older age a small reserve of body mass protects against acute illness and sarcopenia.
A useful side-calculation is the healthy-weight kilogram range for any given height. Rearranging the formula gives weight = BMI × height². For a height of 175 cm (1.75 m), the lower bound of the healthy range is 18.5 × 1.75² = 18.5 × 3.0625 ≈ 56.6 kg, and the upper bound is 24.9 × 1.75² ≈ 76.3 kg. Anything inside that 56.6–76.3 kg window puts an adult of that height in the WHO healthy category. The calculator on this page reports this band automatically, along with the kilogram distance from the nearest boundary, so you can see exactly how much weight change separates your current BMI from the next category. Use BMI as a screening tool — it is fast, free and surprisingly informative — but pair it with waist circumference, body composition, and clinical judgement for any decision that matters.
💡 Worked example
Metric: height 175 cm · weight 75 kg → BMI = 75 / (1.75)² = 75 / 3.0625 = 24.5 → Normal weight (WHO 18.5–24.9) Imperial: 5'9'' · 165 lb → BMI = 165 × 703 / 69² = 24.4 → Normal weight
Frequently Asked Questions
What is a healthy BMI?
WHO defines a healthy adult BMI as 18.5 to 24.9. Below 18.5 is underweight; 25.0–29.9 is overweight; 30 and above is obese (class I 30–34.9, class II 35–39.9, class III ≥ 40).
Is BMI accurate for athletes?
BMI doesn't distinguish fat from muscle, so very muscular people may land in 'overweight' or 'obese' ranges without any health concern. Pair BMI with waist circumference or body-fat percentage for a more complete picture.
Does BMI apply to children?
No. Children and teens use BMI-for-age percentiles (CDC or WHO growth charts) because body composition changes as they grow. This calculator targets adults aged 20+.
How often should I check my BMI?
Monthly is plenty. Day-to-day weight fluctuations (food, hydration, time of day) swing BMI by 0.5–1.0, so frequent checks add noise without useful signal.
Does BMI work the same for older adults?
Not exactly. Above age 65, a BMI of 23–28 is associated with lower mortality than the 'healthy' 18.5–24.9 range; muscle loss (sarcopenia) also shifts the interpretation. Don't aggressively cut weight in later life based on BMI alone.
Can I use BMI during pregnancy?
No. BMI during pregnancy is meaningless — weight gain is expected and necessary. Pre-pregnancy BMI is used by clinicians to plan target gain (typically 11.5–16 kg for a normal starting BMI), but mid-pregnancy BMI readings should not drive any decisions.
What's the difference between BMI and body fat percentage?
BMI is a ratio of weight to height squared — it doesn't measure fat. Body-fat percentage (via DEXA scan, bioelectrical impedance, or skinfold calipers) measures fat tissue directly and is a better predictor of metabolic risk, especially for athletes and lean people.
Does BMI differ for men and women?
WHO uses the same adult thresholds for both sexes, but women naturally carry more body fat than men at any given BMI. Some researchers argue women should be flagged 1–2 BMI points earlier; clinical practice still uses the unified 18.5–24.9 range.
Should I track BMI or waist circumference?
Track both. Waist circumference (men > 94 cm, women > 80 cm = raised risk per WHO) captures abdominal fat — the dangerous kind for heart and metabolic disease — and catches 'normal-weight obesity' that BMI misses.
Is BMI different across ethnicities?
Yes. Asian populations have higher cardiometabolic risk at lower BMI — WHO recommends an overweight cut-off of 23 (instead of 25) and obesity cut-off of 27.5 for people of Asian descent. Check with your doctor about the best thresholds for you.