How Long Should the Appendices of a Technical Document Be?

How long should the appendix or the appendices of a technical document be?

This is an interesting question that I haven’t seen discussed earlier anywhere.

The question is especially pertinent when the document suffers from “mission creep“, that is, when additional documentation goals get added gradually to the original documentation plan.

In such cases, the document starts as a simple Something-Guide, let’s say a User’s Guide.

Then, as the product, software or service gets adjusted and modified according to market conditions, it becomes apparent that more information need to be delivered to the customers. If such new information is reference data, it more  often than not gets appended to the end of the main document as an Appendix. Thus the “tail” starts to get fatter and longer.

The more the product is modified, the more the number of such appendices start to go up.
Finally, there comes a point when you can’t help but ask yourself: “How long my appendices need to be?”

This is the point where the length of the appendices start to take over the length of the main document itself — which of course contradicts and obviates the whole notion of an “appendix.”

What starts to emerge is analogous to a “cat” with a tail longer than its body.

At that point you start to wonder whether what you’ve got is a “cat”, a “fox”, or some other animal all together.

So what the ratio of the appendices to the other parts of the document should be?
Since I haven’t see any well-established universal rule about this, I’ll go ahead and propose one myself:
The appendix should not be longer than 25% of the document.

Yes, it sounds arbitrary and it is to a certain extent. But it’s better than none. It is based on my 15 years of experience as a technical writer. That’s why I feel comfortable with this criterion.

If you have another length rule I’d like to hear about that too. Please feel free to share your opinion on this question with us.