Behind the dollars to knuts calculator

NEW YORK (CNNMoney.com) -- In Harry Potter and the Sorcerer's Stone we read that dragon liver sells for about 17 sickles an ounce. Nice to know, but it leaves you wondering (if you wonder about things like this) "Just how much is 17 sickles, anyway?" Now, thanks to the magic of computer programming and arithmetic, we have the answer. In U.S. currency, dragon liver costs $4.82 an ounce. That's actually not too bad compared to the price of, say, foie gras.

You may wonder how we could possible know this. Harry Potter's wizard money is, after all, completely fictional. Dragon liver is, for that matter, completely fictional. To find the answer, you have to look at some of the lesser-known Harry Potter books. Scholastic, the publishers of the Harry Potter series, also published some slim paperback versions of what are, ostensibly, the textbooks used by Harry Potter and his friends at Hogwarts. The secret to our calculator is on the back of those little books, right under the UPC symbol. The price of the book, it says, is "$3.99 (14 sickles, 3 knuts)."

That information, combined with the knowledge that there are 29 knuts in a sickle and 17 sickles in a galleon (which we learned from more careful study of Harry Potter books), told us all we needed to know to build our Harry Potter wizard money calculator.

We used a "lowest common denominator" approach in creating our calculator. For example, when going from U.S. dollars to wizard money, the first step is to figure out how many knuts one would get for the amount of dollars and cents given. The next step is to figure out how many galleons we could get out of the number of knuts we have.

Once we've changed as many of the knuts into galleons as possible, there will usually be some left over. The next question, then, is how many of those knuts can be turned into sickles. If there are any knuts left over after that step, those just stay knuts. Since knuts don't divide evenly into dollars, there may be decimal points left. Since knuts are the smallest unit of wizard money there is, we just round that amount off. (We round up if it's .5 or greater, down if it's less than .5.) Because of rounding, different approaches to converting dollars to wizard money may yield slightly different results.

So now we can find out the price of dragon livers and silver unicorn horns ($139.82). You can also calculate the price of a Harry Potter action figure (2 galleons, 1 sickle, 8 knuts) or a McDonald's Happy Meal (10 sickles, 6 knuts).

This story was originally published in Ocober, 2001