Many of my valued clients ask me, “How long should my book be?”

My answer is, “Your book is like your legs. Your legs should be long enough to reach the ground. Your book is no different.”

They think for moment, then reply, “Don’t be a wise guy. How long should my book be?”

Then I ask them to sit back, get comfortable, and I’ll explain it to them.

The basic rule is this: Your book should be the exact length needed to tell your reader everything they need to know, or are capable of knowing, about your chosen subject. This means that book length depends upon three factors.

1. Your reader. You should always write to your audience. This is true whether you’re writing a non-fiction self-help book or a novel. You cannot deliver more or less than your audience is likely to want or accept.

For example, if you’re writing a middle grade novel (ages 8-12), you’ll generally plan on 20,000 to 50,000 words. For a young adult novel (ages 13-18), the typical word count range is 60,000 to 90,000 words. If you’re writing a self-help book to teach college students how to dress appropriately for an interview and while on the job, you should keep it concise at no more than 40,000 words. It could even be a free 5,000-word ebook. If you’re writing an adult science fiction fantasy novel, you could exceed 100,000 words with no problem.

Time matters, too. If you think your reader has many hours to spend devouring every word of your masterpiece, then you can make it longer. If you think your reader is a busy person with little spare time, then your book should be on the short side.

2. Your depth. By this I mean the degree to which you’ll dig into your subject. I always tell my clients that you can write about any topic endlessly. It’s up to you to decide how deep you want to go, which is also a factor of how much your reader can absorb.

You could design a financial planning book for seniors to be a 10,000-word ebook. The same book could be expanded to 30,000 words for a printed paperback. If you want to be truly comprehensive and target ultra-high-net-worth readers, you could publish a 60,000-word hardcover.

You could write a 1,000-word ghost story as flash fiction. You could expand the same story to a 40,000-word novella. The next step would be a novel, which is anything over 50,000 words.

For example, the classic horror novel Frankenstein began in 1816 as a hurried short story written Mary Shelley when she was just eighteen years old. Encouraged by her husband, Percy Bysshe Shelley, she kept working on it, giving it more depth, and two years later she published it, anonymously, as a full-length novel of roughly 70,000 words. Then in 1831 she published a revised edition, under her own name; this is the one we most often read today.

In the world of fiction, stories like this are very common; works evolve over time. In extreme cases, full-length feature films can be based on the slimmest of short stories. “The Curious Case of Benjamin Button,” by F. Scott Fitzgerald, was originally a 26-page short story published in Collier’s in 1922. In 2006 it was released as a 166-minute feature film starring Brad Pitt. And how about Stanley Kubrick’s 1968 film, 2001: A Space Odyssey, which grew from a super-short 8-page story by Arthur C. Clark entitled “The Sentinel,” which he wrote in 1948 to enter into a BBC contest.

3. Your budget. Yes, money matters! On one end of the spectrum, if you want to write a book, you could do it yourself for free. Just start writing! Then you could get a literary agent for free, and land a publishing deal that could earn you millions of dollars. This is what happened to Stephen King with Carrie, J.K. Rowling with Harry Potter and the Philosopher’s Stone, and many others. It could happen to you!

But many authors are not, in fact, writers. They can’t or won’t take on the challenge, so they hire a ghostwriter. Nothing wrong with that! But ghostwriters charge by the word, so the longer your book, the more you pay. Good ghostwriters charge anywhere from $5,000 for a short book to $100,000 or more for a prestige memoir. What you’ll pay depends on the length and the complexity of the project. And if you self-publish, or use a subsidy publisher, you’ll need to get out your credit card, because book manufacturing and promotion can cost you more than you spent writing it.

So to get back to the original question… How long should your book be? Weigh the three factors carefully and set your target word count. But don’t worry—you can always change it!