An S corporation owes the built-in gains tax when it was previously a C corporation (or acquired assets from one on a transferred basis) and sells or otherwise recognizes gain on those assets within five years of the S election taking effect. The tax is a flat 21% imposed at the corporate level under Internal Revenue Code Section 1374, one of the few times an S corporation pays income tax itself rather than passing everything through to shareholders. Some tax discussions refer to an “IRS Schedule S,” but no such standalone form exists for this purpose. The built-in gains (BIG) tax is calculated on Part III of Schedule D (Form 1120-S) and reported on line 23b of Form 1120-S.1Internal Revenue Service. 2025 Schedule D (Form 1120-S)
Which S Corporations Actually Owe It
The tax exists to shut down a specific move: a C corporation loading up on appreciated assets, flipping to S status, and then selling the assets without paying corporate tax on the pre-conversion appreciation.2Office of the Law Revision Counsel. 26 U.S. Code 1374 – Tax Imposed on Certain Built-in Gains Two types of S corporations can be hit:
- Corporations that operated as C corporations and later elected S status. The tax reaches gains on assets the corporation held on the day the S election took effect.
- S corporations that acquired assets from a C corporation in a transaction where the S corporation’s basis in those assets is determined by reference to the C corporation’s basis, such as certain tax-free reorganizations and contributions.3Office of the Law Revision Counsel. 26 U.S. Code 1374 – Tax Imposed on Certain Built-in Gains – Section: (d)(8)
A corporation that was formed as an S corporation from day one and has never acquired assets in a transferred-basis transaction from a C corporation is not subject to the BIG tax at all.
The Five-Year Recognition Period
The BIG tax only applies to gain recognized during the recognition period. Under Section 1374(d)(7), that period runs five years, beginning on the first day of the first tax year the S election is effective.4Office of the Law Revision Counsel. 26 U.S. Code 1374 – Tax Imposed on Certain Built-in Gains – Section: (d)(7) For assets acquired in a transferred-basis transaction, the five-year clock starts on the acquisition date rather than the S election date.
Once the five-year window closes, remaining built-in appreciation in those assets can be sold without any corporate-level BIG tax. That makes timing a real planning lever for corporations that expect to sell appreciated assets after conversion.
How Built-in Gains Are Measured
A built-in gain on any single asset is the amount by which its fair market value exceeded its adjusted tax basis on the first day of the S election (or the acquisition date, for transferred-basis assets). It represents appreciation that accrued during C corporation years and was never taxed at the corporate level.
Net Unrealized Built-in Gain (NUBIG)
The net unrealized built-in gain, or NUBIG, is the corporation’s total exposure. It equals the excess of the fair market value of all the corporation’s assets over their aggregate adjusted bases on the first day of the S election.5Office of the Law Revision Counsel. 26 U.S. Code 1374 – Tax Imposed on Certain Built-in Gains – Section: (d)(1) The NUBIG acts as a cumulative ceiling: the total BIG tax collected across every year of the recognition period can never exceed the tax on this amount.
What Counts Beyond the Obvious Assets
Appreciated real estate, equipment, and inventory are the obvious sources. The tax also reaches items you might not think of as built-in gain. For cash-basis corporations, accounts receivable that had zero basis during C corporation years generate built-in gain when collected after the S election. Under Treasury regulations, any income item recognized during the recognition period is treated as a built-in gain if an accrual-method taxpayer would have included it in gross income before the recognition period began. That sweeps in zero-basis receivables, certain prepaid income, and installment notes carried over from C years.
Built-in losses run the other direction. If an asset’s basis exceeded its fair market value on the conversion date, disposing of it during the recognition period produces a recognized built-in loss that offsets built-in gains for the year.6Internal Revenue Service. 2024 Instructions for Schedule D (Form 1120-S)
Calculating the Tax
Part III of Schedule D walks through the calculation in a specific order. The corporation figures three amounts, then applies the 21% rate to the smallest of them.1Internal Revenue Service. 2025 Schedule D (Form 1120-S)
- Pre-limitation amount (Line 16): recognized built-in gains minus recognized built-in losses for the year, plus any built-in gain carried over from a prior year.
- Taxable income limitation (Line 17): the taxable income the S corporation would have had if it were still a C corporation, worked out on a hypothetical Form 1120.
- NUBIG limitation (Schedule B, Line 8): the original NUBIG minus all net recognized built-in gains from prior years in the recognition period.7eCFR. 26 CFR 1.1374-2 – Net Recognized Built-in Gain
The smallest of the three is the net recognized built-in gain for the year, and the 21% corporate rate applies to that figure.8Office of the Law Revision Counsel. 26 U.S. Code 11 – Tax Imposed If the pre-limitation amount is larger than the taxable income limitation in a given year, the excess carries forward and is treated as a recognized built-in gain the following year, but only if that year is still inside the recognition period.9Internal Revenue Service. 2025 Instructions for Schedule D (Form 1120-S) – Section: Line 18
C Corporation Carryovers That Shrink the Bill
Three categories of C corporation carryovers reduce the BIG tax directly. Missing any of them means overpaying.
Net operating losses. An NOL that arose during C corporation years can be deducted against the net recognized built-in gain before the 21% rate is applied. The deduction appears on Line 19 of Schedule D. Net recognized built-in gain is treated as taxable income for purposes of tracking how much of the NOL carryforward remains available in later years.10Office of the Law Revision Counsel. 26 U.S. Code 1374 – Tax Imposed on Certain Built-in Gains – Section: (b)(2)
Capital losses. Capital loss carryforwards from C years work the same way, but they can only offset the net capital gain portion of the recognized built-in gain.11Internal Revenue Service. 2025 Instructions for Schedule D (Form 1120-S) – Section: Line 19
General business credits. Business credits generated in C corporation years apply against the BIG tax itself on Line 22, after the 21% rate has been applied.12Office of the Law Revision Counsel. 26 U.S. Code 1374 – Tax Imposed on Certain Built-in Gains – Section: (b)(3)
What Shareholders See on Their K-1s
Once the S corporation pays the BIG tax, the amount paid is treated as a loss the corporation sustained during the same tax year. That loss reduces the income passing through to shareholders on Schedule K-1. The character of the loss follows the character of the gain that produced the tax, so a long-term capital gain triggers a long-term capital loss at the shareholder level.13Office of the Law Revision Counsel. 26 USC 1366 – Pass-thru of Items to Shareholders
Shareholders still report the underlying gain on their individual returns. The practical effect: pre-conversion appreciation is taxed twice, once at 21% at the corporate level and again on the pass-through gain (reduced by the BIG tax amount). Appreciation that accrues after conversion passes through only once, the way the S corporation structure normally works.
Where the Tax Is Reported
The calculation lives on Part III of Schedule D (Form 1120-S), and the resulting tax is entered on Form 1120-S, page 1, line 23b.1Internal Revenue Service. 2025 Schedule D (Form 1120-S) The corporation attaches computation statements showing how it arrived at the pre-limitation amount and the taxable income limitation. The BIG tax is paid by the corporation, not the shareholders, and is due when Form 1120-S is filed.
Documenting Asset Values at Conversion
The entire calculation depends on knowing the fair market value of every asset on the day the S election took effect. Without solid contemporaneous records, the corporation ends up arguing values with the IRS after the fact.
A professional appraisal of all tangible and intangible assets at conversion establishes the NUBIG and identifies which assets carry built-in gains and which carry built-in losses. Value assets one by one rather than in a lump sum. Lump-sum valuations leave room for the IRS to reassign values across individual assets in ways that raise the tax.
Documenting later changes in value matters too. If an asset is sold years later for more than its fair market value at conversion, the gain above that conversion-date value is post-conversion appreciation and outside the BIG tax. Without records establishing the conversion-date value, proving that split is difficult. Keep the appraisal and supporting records for at least three years after the recognition period ends.
Common Mistakes and Penalties
Underreporting the BIG tax draws the same accuracy-related penalties as other underpayments: 20% of the underpaid amount when the understatement results from negligence or a substantial understatement of tax.14Internal Revenue Service. Accuracy-related Penalty
The mistakes that tend to show up on audit: failing to identify every asset with built-in gain at conversion, particularly intangibles and cash-basis receivables; relying on stale or poorly documented fair market values; and overlooking C corporation NOLs, capital losses, or business credit carryforwards that would have reduced the tax. A careful appraisal at conversion and a running NUBIG worksheet across the five years are the best defenses.