C Corp to S Corp 5-Year Rule: BIG Tax, Losses, and Installment Traps

The C corp to S corp five-year rule is the built-in gains tax recognition period: for the first five years after an S election takes effect, the corporation still owes a corporate-level tax at 21% on any appreciation that existed in its assets on the conversion date, if those assets are sold inside that window.¹ Wait out the five years and that same sale escapes the extra corporate layer entirely.

When the Clock Starts and Stops

The recognition period begins on the first day the S election is effective and runs for exactly five years. A corporation that converts on January 1, 2026 is inside the window through December 31, 2030. A sale on January 2, 2031 is outside it.

The period was originally ten years. Congress shortened it temporarily several times before the PATH Act of 2015 fixed it permanently at five years for tax years beginning on or after January 1, 2015.

The rule exists to keep C corp appreciation from escaping corporate tax through a quick election-and-sell maneuver. Without it, a C corp holding heavily appreciated assets could flip to S status, liquidate, and push the gains straight through to shareholders at individual rates.

What Counts as a Built-In Gain

A built-in gain is the spread between an asset’s fair market value and its tax basis on the day the S election kicks in. A building worth $2 million with a $500,000 basis on conversion day carries a $1.5 million built-in gain. Sell it inside five years and the 21% tax applies to that $1.5 million.¹

Appreciation that occurs after the conversion date is not subject to this tax. Only the pre-conversion piece is caught. That distinction is the reason a Day 1 professional appraisal matters so much: it establishes fair market value and adjusted basis for every asset on the exact date the election takes effect, and it is the evidence the corporation will rely on if the IRS challenges a later sale.

How the Tax Is Calculated

The 21% rate is applied to the smallest of three figures, computed each year during the recognition period.¹ The lowest number controls, which effectively caps what the IRS can collect for the year:

Net recognized built-in gain for the year: gains on sales of assets held on conversion day, reduced by losses on sales of assets that had a built-in loss on that same day. Winners and losers sold in the same year offset each other.
Remaining net unrealized built-in gain: the total built-in gain pool from conversion day, reduced by net recognized built-in gains from all prior S corp years. This balance shrinks over time; once it reaches zero, no further BIG tax is possible even if the five-year period hasn’t ended.
Hypothetical C corp taxable income: what the corporation would owe tax on if it were still a C corp for the year, computed without the dividends-received deduction or any NOL carryforwards generated in S corp years.

Whatever gain is taxed at the corporate level reduces the amount that flows through to shareholders on their personal returns, so the same dollar isn’t taxed twice.²

Using C Corp Losses to Shrink the Bill

Net operating loss carryforwards from the corporation’s C corp years can be deducted against net recognized built-in gains, and capital loss carryforwards from those years get the same treatment.¹ This is an exception to the general rule that S corporations can’t use C corp NOLs. For a business that entered the S election with appreciated assets and accumulated losses on the books, those carryforwards can meaningfully reduce, or in some cases wipe out, the BIG tax on a sale during the recognition period.

The Installment Sale Trap

A common assumption is that a sale in Year 3 or Year 4 can be structured on the installment method to push cash receipts past the five-year mark and out of the BIG tax. It doesn’t work. If the sale itself happens inside the recognition period, every installment payment is governed by the rules for the year of sale, no matter when the cash actually arrives.¹ Payments collected in Year 10 on a Year 4 sale are still BIG tax payments. The only reliable way to avoid the tax is to hold the asset until after the five-year window has closed, then sell.

Who Has to Prove What

When the IRS looks at a gain realized during the recognition period, the S corporation carries the burden of proof. To keep the gain out of the BIG calculation, the corporation has to show that the asset was acquired after conversion or that the recognized gain exceeds the pre-conversion appreciation. If it can’t prove either, the entire gain is presumed to be built-in.¹ That’s why the Day 1 appraisal is not optional in practice. Without contemporaneous valuations and basis records, the corporation loses these fights on paperwork alone.

A Different Five-Year Rule to Not Confuse This With

There is a separate five-year rule tied to S elections that has nothing to do with built-in gains. If an S election is terminated or revoked for any reason, the corporation generally cannot re-elect S status for five tax years without IRS consent.³ The count begins with the first tax year the termination is effective, and it applies whether the termination came from an eligibility violation, three consecutive years of excessive passive investment income while carrying C corp earnings and profits, or a voluntary revocation. Early re-election is possible with IRS consent, but only if the corporation can show the cause of the original termination has been fixed and is unlikely to recur.

The built-in gains five-year rule and the re-election five-year rule are frequently mixed up because they share a number. They are unrelated: one is a tax exposure period after conversion into S status, the other is a waiting period after leaving it.

1
Office of the Law Revision Counsel. 26 U.S. Code 1374 – Tax Imposed on Certain Built-In Gains
2
Office of the Law Revision Counsel. 26 U.S. Code 11 – Tax Imposed
3
Office of the Law Revision Counsel. 26 U.S. Code 1362 – Election; Revocation; Termination