How to Calculate BMI Correctly and Read It Honestly
BMI is a rough screening tool, not a health verdict. Calculating it right and interpreting it with context matters more than the number. Body Mass Index is one of the most common health numbers people calculate, and also one of the most misunderstood. It is a ratio of weight to height, designed as a population-level screening tool, not a diagnosis of any individual health. Calculating it correctly is straightforward. Interpreting it honestly takes a little more care. I have tracked my BMI for years alongside other metrics, and the nuance matters more than most people realize. The Formula and the Unit Trap BMI equals weight in kilograms divided by height in meters squared. For someone who weighs 70 kilograms and is 1.75 meters tall, the calculation is 70 divided by 1.75 squared, which is 70 divided by 3.0625, or about 22.9. That falls in the normal range. The trap is units. Most calculators accept either metric or imperial input, and the imperial formula is different: weight in pounds times 703, divided by height in inches squared. Mixing units, or using the metric formula with imperial inputs, produces nonsense numbers. When calculating by hand, confirm which formula the calculator expects. When building a calculator, accept one unit system and convert at the boundary so the core math stays in metric.