Estimating Index Size

One of the best ways to cut down on editing time is to make sure your index is the right size in the first place. Not all clients will give you a length requirement to fill, but when they do, it’s easier to have an index that’s the right size than to have to edit a long one down, or pad a short one.

Of course, there are as many ways of approaching this as there are indexers. After all, our motto is, “It depends.”

Hans Wellisch, in Indexing A to Z, spends two pages describing the methods of calculating the length of an index using lines per page, including a handy(ish) table for common trim sizes. Then, using the table, you can figure out how many total index lines you have. Then,

total index lines
———————–    =    entries per page (assuming one locator per entry)
text pages

Then, subtract 20% for turnover lines. You still will be able to have multiple locators per heading, so it usually comes out more or less in the wash.

Chicago Manual of Style is much simpler: 5 entries per page = an index 1/50th the length of the text. 15+ entries = 1/20th the length of the text.

index pages      1
—————– =     ——
text pages        x

“x” will likely be between 50-20, and you can estimate number of entries from CMS’s range of 5-15 entries per page.

This works great if you know the number of pages allotted for the index. If you’re given the number of lines instead, you can either look up the table in Wellisch, or just go to another book by the same publisher and manually count index lines. Amazon’s Look Inside feature or Google Books can save you a trip to the library. Then, do the math and figure how many pages you have for the index.

But the simplest—saved for last—is Nancy Mulvaney‘s recommendation. Divide the index pages by text pages as above, then multiply by 100 to arrive at a percentage for the index. The lower the “percent index,” as you may hear it called, the fewer entries per page. It correlates roughly 1:1, plus or minus one entry per page.

Trade books are lighter—2-5% (or 3–5 entries per page)—while textbooks, reference, and scholarly books run around 7-8%. Technical manuals go higher, depending on their complexity (legal and government are among the highest, naturally).

So, now what?

Confused? It’s ok. Go with Mulvaney’s recommendation, and keep an eye on your number of entries per page. In my experience, scholarly introductions are likely to not be indicative of the rest of the book’s density, so you may not get an accurate picture until you’ve done the first couple of chapters and can get an average.

The point is to be close enough to have something to work with when it comes down to editing. Changing an index length by 1 or 2% is wholly different from changing it by 50%, and can mean the difference between making or missing a deadline.