2+2 may not always equal 4. 

Depending on the type of plan, events may transpire post petition which could increase the amount necessary to satisfy the Chapter 13 obligations.  There may be additional administrative claims (for legal fees on application to the Court) or post petition creditor claims which could increase the amount necessary to fund the plan.

13 Myths About Chapter 13 Bankruptcy

So much is written and put out on the internet about the “Individual Repayment” option contained in the Bankruptcy Code under Chapter 13 that it probably is not surprising that much of it is either skewed for one reason or another - or is outright misinformation. This article includes some of the fairly constant refrains we as bankruptcy lawyers hear from clients to explain their hesitancy to consider Chapter 13. It is further an attempt to clarify and/or reassure individuals who may be considering Chapter 13 or desperately need its enhanced protections under the Bankruptcy Code.

Most people have heard of the basic Chapters of Bankruptcy filing; Chapter 7, 11 or 13.   Some are even aware of the less used provisions of Chapter 12 (for farmers or fishermen) or Chapter 9 (municipal entities) or even of late Chapter 15 (for foreign corporations).  But a hybrid approach called "Chapter 20" is getting a lot more attention.