Body mass index from height and weight.
Body Mass Index is a single number derived from your height and weight that places you on a continuum from underweight to severely obese. It is the most common screening tool for weight-related health risk in primary care, and it is used by the World Health Organization, the U.S. CDC, life insurance underwriters, military fitness programs, and just about every health app on a phone. The reason it has stuck despite being a crude measure is brutal practicality: anyone can compute it in seconds with two numbers, and at the population level it correlates well enough with cardiovascular risk, type 2 diabetes incidence, and overall mortality to flag people for closer evaluation. It is not a diagnostic tool. A BMI in the "overweight" range does not, by itself, mean you are unhealthy, and a BMI in the "normal" range does not exempt anyone from those same conditions. Treat it as a starting line, not a verdict.
BMI is computed in metric units as:
BMI = weight (kg) / height (m)²
In imperial units, the formula is:
BMI = 703 × weight (lb) / height (in)²
The constant 703 reconciles the unit difference. The result is interpreted against the WHO's adult thresholds:
Pediatric BMI uses age- and sex-specific percentile curves rather than these flat cutoffs, because children's body composition changes with growth.
The panel takes two inputs: your height and your weight. Toggle between metric (cm and kg) and imperial (feet/inches and pounds) using the unit switch. The calculator instantly returns your BMI value rounded to one decimal, the named category (e.g. "normal weight"), and the corresponding ideal-weight range for your height — the lower and upper bounds of the 18.5 to 24.9 band. That range is shown both as a span ("70 to 90 kg") and as the two endpoints of the formula re-arranged for height.
A 30-year-old man stands 5 feet 10 inches tall (about 178 cm) and weighs 185 pounds (about 84 kg). In metric, his BMI is 84 / 1.78² ≈ 84 / 3.17 ≈ 26.5. In imperial, 703 × 185 / 70² = 703 × 185 / 4900 ≈ 26.5. Both methods agree, as they must. He falls in the overweight category. To reach the upper edge of the normal weight band at the same height, he would need to weigh about 24.9 × 1.78² ≈ 79 kg or about 174 lb — roughly 11 pounds less. His ideal-weight range is therefore approximately 59 kg to 79 kg, or 130 lb to 174 lb. A second scenario: a 45-year-old woman of 165 cm (5 ft 5 in) weighing 72 kg (159 lb) has a BMI of 72 / 1.65² ≈ 26.4 — also overweight, with a normal-weight band of approximately 50 kg to 68 kg. Note that both individuals are barely above the threshold; a 4 kg loss would put either of them comfortably back in the normal-weight range, illustrating how narrow the categorical boundaries actually are.
BMI fails on edge cases that are not actually edge cases at all. First, it does not distinguish between muscle and fat. A bodybuilder with 8 % body fat and a sedentary office worker with 35 % body fat can have identical BMIs because muscle and fat weigh nearly the same per liter of volume. Second, it ignores fat distribution. Visceral fat around the abdomen is far more dangerous than subcutaneous fat on the thighs or hips, but BMI sees only the total. Third, it underestimates risk for South Asian, East Asian, and some Pacific Islander populations, where adverse cardiometabolic markers appear at lower BMIs. The WHO has issued lower screening thresholds (23 for overweight, 27.5 for obesity) for these groups. Fourth, it overestimates risk in older adults, whose mortality curve actually flattens or rises slightly with mild overweight. Fifth, it is not validated for pregnant or lactating women.
Several alternatives address BMI's blind spots. Waist-to-height ratio (waist circumference divided by height, with a cutoff at 0.5) is increasingly recommended by the U.K. NICE guidelines as a better signal of metabolic risk because it captures abdominal fat. Waist-to-hip ratio serves the same purpose with a different geometry. Body fat percentage, measured by skinfold calipers, bioelectrical impedance, or DEXA scan, is more accurate but requires equipment. Lean body mass index subtracts estimated fat mass before normalizing by height, which solves the muscular-build problem. The A Body Shape Index (ABSI), published in 2012, combines BMI with waist circumference and outperforms either metric for predicting all-cause mortality. None of these have replaced BMI in practice because none match its simplicity. For a clinical decision, BMI is a triage signal — if it lands in a concerning band, the next step is a conversation with a doctor and one of the more detailed measurements above, not a diet plan from an app.