The built-in gains (BIG) tax under IRC Section 1374 is a corporate-level tax that applies when a C corporation converts to an S corporation and then sells assets that had already appreciated on the day of the conversion. It exists to close an obvious loophole: without it, a C corporation sitting on an appreciated building, a customer list, or a decade of receivables could elect S status the week before a sale and skip the corporate layer of tax on all of that appreciation. Section 1374 keeps the double tax alive for a limited window, five years, on the gain that economically accrued during the C years.
An S corporation that has been an S corporation from its first day never owes this tax. It also reaches assets an S corporation acquires from a C corporation with a carried-over basis, such as in a tax-free merger, so a clean S history can still pick up BIG exposure by acquisition.
How the tax is computed
On the conversion date, the corporation measures its net unrealized built-in gain (NUBIG): roughly, the gain it would have recognized if it had sold everything at fair market value that day, netted against built-in losses. That snapshot is the lifetime cap on the tax. Nothing that appreciates after the S election takes effect is ever touched.
When the company sells a built-in gain asset during the recognition period, the recognized built-in gain is taxed at the corporation itself at the highest corporate rate, currently 21%. Three limits soften the hit in a given year. Recognized built-in losses on other assets sold the same year net against the gains. The year's taxable income, computed as if the company were still a C corporation, caps the amount subject to tax, with any excess carried forward within the recognition period rather than forgiven. And net operating loss and credit carryforwards left over from the C years, which are otherwise frozen while the S election is in effect, can be used against the built-in gain.
The tax the corporation pays is not the end of the story, but it is not wasted either: the gain still flows through to shareholders on Schedule K-1 (Form 1120-S), reduced by the BIG tax paid, which passes through as a loss. The combined result approximates what the old double tax would have been, which is exactly the point.
The five-year recognition period
The recognition period is the five years beginning on the first day the S election is effective, made permanent at five years by 2015 legislation after bouncing between ten, seven, and five in earlier law. Sell a built-in gain asset in year six and there is no corporate-level tax at all, only the shareholder-level tax on the flow-through gain.
The five-year clock makes the conversion-date valuation the single most valuable document in the whole exercise. A contemporaneous appraisal of the business and its major assets fixes the NUBIG ceiling and fixes each asset's built-in gain, so appreciation after the election date is provably exempt. Reconstructing a valuation years later, with the IRS free to argue the appreciation was already baked in at conversion, is a much worse position.
Cash-method companies get caught by the least obvious assets: receivables and work in progress. A consulting or service firm that converts with $400,000 of uncollected receivables has $400,000 of built-in gain items that will be recognized in the first few months of the S election, no matter how patient the company is about selling buildings. Inventory and depreciation recapture on equipment behave the same way.
Planning around it
The cleanest plan is patience: hold appreciated assets past the five-year mark before selling. Where a sale cannot wait, the levers are the annual taxable income cap (a low-income year absorbs less tax now, though the excess carries forward), harvesting built-in losses in the same year, and spending frozen C-year NOLs against the gain. Installment sales need care: gain from a sale made during the recognition period stays subject to the tax even when the installment payments arrive after the period ends.
For a creator or owner deciding whether to convert an existing C corporation, the BIG tax is a modeling exercise, not a reason to stay put. Most creator businesses hold little appreciated property beyond goodwill and receivables, and the five years often pass before any exit is on the table. The mistake is converting without the valuation, not converting at all.
Mistakes that cost money
Skipping the conversion-date appraisal: without a fixed snapshot, every later sale becomes an argument with the IRS about how much of the gain predated the election, and the corporation carries the burden of proof.
Forgetting receivables: cash-method service companies recognize built-in gain the moment old invoices are collected, producing a surprise corporate tax bill in the first S year.
Assuming the taxable income limit forgives the tax: gain that escapes tax in a low-income year carries forward within the recognition period. It is a deferral, not an exemption.
Missing the acquisition trap: an S corporation that absorbs a C corporation in a carryover-basis transaction inherits a fresh recognition period on the acquired assets, even if its own election is decades old.
Selling in month 59: the recognition period is measured to the day. A sale that could have closed a few months later can carry a six-figure corporate tax that patience would have erased entirely.