And back-of-envelope conversions can be really unreliable. People multiply inches by 2.5 to get their height in centimetres, but its actually 2.54. If you're 6 foot tall, that adds up to over an inch of difference in your converted height. Feed that into a BMI calculator, for example, and it could shift you from 'overweight' to 'obese', with the corresponding medical consequences for treatment.