So let’s say you got a book deal. Yay! Have that glass of wine or virgin daiquiri — and then it’s time to get down to business. How much can you actually earn from this venture?

If you’re new to publishing, you may not know how you’ll earn a check from a book deal. With the advent of self-publishing and ebooks, there are many different models. Still, many publishers depend on the tried and true advance-royalty model. But how does this work?

First the writer gets an advance. This is a lump sum — sometimes paid in installments — that is paid to the writer before the book is published. This figure can vary widely, from $0 to a seven-figure value. It’s based on the author’s expertise, reader demand and expected sales. That means J.K. Rowling is going to earn a higher advance than a funny, charming math geek.

The advance is exactly what it sounds like — an upfront fee based on what the publisher thinks the book will earn. So you’ll get paid for your time writing the book, even though the publisher isn’t earning anything yet.

Royalties are what you’ll earn from the book sales — usually a percent of the price of the book. But there are lots of things to consider here, like how much a bookstore actually pays for the book.

Here’s the catch: you’ll need to earn out your advance, before you start getting royalties. In other words, you won’t see a dime from your publisher until after your royalties equal the advance you were paid. (And sadly, some books never do this.)

Whew! And you thought all you’d have to do is write the darned thing!

Any experienced writer will warn you: read your contract carefully before signing. I’ll go one step further: do the math. (Surprise!)

What you want to know is the number of books you have to sell before you actually earn some cash. If that number is in the stratsophere — or your advance is really low — you may want to shop around. (Then again, there’s something to be said for taking a risk or enjoying a labor of love.)

Mathematically speaking, there are several variables: the amount of your advance, price of the book, royalty percent and the number of books you’ll need to sell to earn out. The advance, book price and royalty percent are set. What you need to know is the number of books you need to sell.  Let’s look at a simple example:

June has been shopping around Physics for Grownups for several years. She’s convinced that she has a great idea, if she can only find the right publisher. Lo and behold, on her 17th try, she finds a publisher who is interested. The contract offers a $5,000 advance and 5% royalty. The book is priced at $12.99.

Is this a good deal?

June is no dummy. She knows that a physics book isn’t likely to be a best seller. Still, she doesn’t want to spend weeks and weeks writing if she’s not likely to see any profit. Based on the contract, how many books would she need to sell to earn out her advance?

She will earn 5% on each $12.99 book. (That’s assuming that all books are purchased at the cover price.)

$12.99 x 0.05 = $0.65

In other words, June will earn $0.65 per book. To find out her break-even point, she needs to divide her advance by the amount she’ll earn per book:

$5,000 ÷ $0.65 = 7592.3

So, June will need to sell 7,593 books before she will even get a royalty check. That’s a lot of books.

Only June can decide if this is worth it. Some books are like tortoises — they make slow and steady progress, while other books are flashes in the pan. If June plays her cards right, she could do just fine over a long period of time. The math can only give her the cold, hard details.

Have you published a book? If so, what went into your decision to take the leap? Did you do the math or just decide it was worth it regardless? Share your ideas in the comments section.