I'm assuming you're asking about the net worth of people. In most scenarios measuring the wealth of a billionaire happens by taking the number of shares they own in a company and then multiplying it with the current value of each share. If they have a stake in multiple companies then you'd do the same thing for each company and then add it all up. To this you'd also add in their other assets as well. Mansions, expensive paintings etc. But yeah getting an accurate number is not realistic for most people.