The tax gap is the difference between what taxpayers should pay and what they actually pay due to not filing tax returns, not paying their reported tax liability on time, or failing to report their correct tax liability. The tax gap, about $450.0 billion based on updated fiscal year 2006 estimates, represents the amount of noncompliance with the tax laws. Underreporting of income tax, employment taxes, and other taxes represents 84 percent of the gross tax gap. The IRS remains committed to finding ways to increase compliance and reduce the tax gap, while minimizing the burden on the vast majority of taxpayers who pay their taxes accurately and on time.
The tax gap is the aggregate amount of tax (i.e., excluding interest and penalties) that is imposed by the tax laws for any given tax year but is not paid voluntarily and timely. The tax gap arises from three types of noncompliance: not filing required tax returns on time or at all (the nonfiling gap), underreporting the correct amount of tax on timely filed returns (the underreporting gap), and not paying on time the full amount reported on timely filed returns (the underpayment gap). Of these three components, only the underpayment gap is observed; the nonfiling gap and the underreporting gap must be estimated. Each instance of noncompliance by a taxpayer contributes to the tax gap, whether or not the IRS detects it, and whether or not the taxpayer is even aware of the noncompliance. Obviously, some of the tax gap arises from intentional (willful) noncompliance, and some of it arises from unintentional mistakes.
The collection gap is the cumulative amount of assessed tax, penalties, and interest that has been assessed over many years, but has not been paid by a certain point in time and which the IRS expects to remain uncollectible. In essence, it represents the difference between the total balance of unpaid assessments and the net taxes receivable reported on the IRS' balance sheet. The tax gap and the collection gap are related and overlapping concepts, but they have significant differences. The collection gap is a cumulative balance sheet concept for a particular point in time, while the tax gap is like an income statement item for a single year. Moreover, the tax gap estimates include all noncompliance, while the collection gap includes only amounts that have been assessed (a small portion of all noncompliance).