C-Corp to S-Corp Conversion: The Complete Tax Guide

Converting a C-corp to an S-corp can cut your annual tax bill substantially — but three tax traps can turn that savings into an expensive surprise. This guide covers the mechanics, the math, and the planning moves that determine whether the switch makes sense.

Why owners consider converting a C-corp to S-corp

Most small business owners who incorporated as a C-corp did so for a specific reason: QSBS eligibility, investor expectations, or plain advice from a generic attorney. Once the business is established and those reasons no longer apply, the C-corp structure creates ongoing drag:

• Double taxation on distributions. C-corp pays 21% corporate income tax on profits; shareholders then pay 15%–23.8% on qualified dividends. A business netting $400K can end up with an effective rate north of 38% — even before state taxes.
• No FICA savings on pass-through income. C-corp owners who take a salary can't take tax-efficient distributions the way S-corp owners can. All active income is W-2 and subject to payroll taxes.
• No §199A QBI deduction. The 20% qualified business income deduction is a pass-through-only benefit. C-corps don't get it.

S-corp status turns all three problems around: income passes through to your personal return at ordinary rates, you split between W-2 salary and distributions, and you qualify for §199A. See our S-corp vs LLC tax savings calculator to estimate the FICA savings for your income level.

S-corp eligibility: confirm before you elect

The IRS won't honor an S-corp election if the corporation doesn't qualify. Before filing Form 2553, verify all of these:¹

• 100-shareholder limit. Family members (spouses, lineal descendants, and some trusts) can be treated as a single shareholder for counting purposes.
• One class of stock only. No preferred shares, no different economic rights. Voting vs. non-voting shares are allowed; different economic rights are not.
• Eligible shareholders only. U.S. citizens and permanent residents (individuals), certain trusts (grantor, ESBT, QSST), and estates. No C-corps, S-corps, partnerships, LLCs (as entities), or non-resident aliens as shareholders.
• Domestic corporation. Formed under U.S. or D.C. law. Foreign corporations are ineligible regardless of where they operate.
• Not an ineligible corporation. Financial institutions using reserve method, insurance companies, and domestic international sales corporations (DISCs) cannot elect S-corp status.

For most privately-held small businesses with a handful of individual shareholders, these requirements are easily met. The one-class-of-stock rule creates issues if you have issued convertible notes or SAFEs that could be recharacterized as preferred equity. Resolve these before filing.

Form 2553: how to make the election

The S-corp election is filed on Form 2553, Election by a Small Business Corporation, signed by all shareholders.

Once accepted, the IRS will issue a CP261 Notice confirming S-corp status. The election is binding for all future years unless revoked by shareholders (majority of stock ownership) or terminated by operation of law (e.g., an ineligible shareholder acquires stock).

§1374 Built-In Gains Tax: the main trap

This is the provision that surprises the most owners. When a C-corp converts to S-corp status, any unrealized appreciation in assets owned at the time of conversion doesn't disappear — it follows the asset into the S-corp and remains taxable at corporate rates if recognized within a 5-year window.

How the tax works

If the S-corp sells an asset within the 5-year recognition period beginning on the first day of its first S-corp year, any gain that was built in at the conversion date is taxed at 21% (the corporate income tax rate) at the entity level — before the remaining gain passes through to shareholders and is taxed again at capital gains rates on the personal return.²

The recognition period is 5 years, running from the first day of S-corp status. A C-corp that elects S-corp status effective January 1, 2027 is subject to the BIG tax on any pre-conversion appreciation recognized before January 1, 2032.

What counts as a built-in gain asset

Any asset owned by the C-corp on the conversion date with FMV exceeding tax basis. Common examples:

• Real estate and equipment with large accumulated depreciation deductions
• Accounts receivable on cash-method C-corps (FMV = face value, basis = $0)
• Appreciated investment securities
• Inventory with cost basis below market value
• Goodwill and intangibles: If the business has enterprise goodwill, the IRS can assert it was an asset of the C-corp with $0 basis. This is the subtlest trap — a service business with strong client relationships but few hard assets still has goodwill exposure.

The total §1374 tax is capped by the Net Unrealized Built-In Gain (NUBIG) — the total built-in gain across all assets on the conversion date. A formal appraisal of all assets as of the conversion date establishes the NUBIG and protects you in an audit.

Planning around the 5-year window

• Defer asset sales until after Year 5. If the business can wait, sell appreciated assets after the recognition period expires.
• Use installment sales strategically. Under §453, an installment sale reports gain as payments are received. Gain recognized after the 5-year window is not subject to §1374 even if the sale closed before the window ended — this is a planning opportunity for long-term installment notes.
• Convert when asset basis is high. Right after a major equipment purchase or depreciation catch-up, basis is higher and NUBIG is lower — reducing exposure.
• Consider whether to convert at all if you're planning a near-term sale. If you expect to sell the business within 5 years, the BIG tax will apply to the business sale proceeds. In that scenario, remaining as a C-corp and using an ESOP §1042 structure or QSBS exclusion may be more tax-efficient than converting.

§1375 Passive Income Surtax: a second trap

If the C-corp had accumulated earnings and profits (E&P) at the time of conversion, the S-corp faces an ongoing passive income tax under §1375. The tax applies when both conditions are met:³

1. The S-corp has accumulated E&P from its C-corp years, AND
2. Passive investment income exceeds 25% of gross receipts in the same year

When both are true, the S-corp pays a 21% entity-level tax on the excess passive income (the amount above 25% of gross receipts). If this condition persists for three consecutive tax years, the S-corp election is automatically terminated — reverting the corporation to C-corp status.

Passive income for §1375 purposes includes dividends, interest, royalties, rents, annuities, and gains from the sale of stock or securities. For most operating businesses with minimal investment income, this isn't a concern. But a C-corp that held investment securities, rental property, or collected royalties may have both the E&P and the passive income to trigger it.

The AAA account: tracking distributable income

When a C-corp converts to S-corp status, it establishes an Accumulated Adjustments Account (AAA). This starts at $0 on the conversion date — it does not inherit the C-corp's accumulated earnings. Each subsequent year:

• AAA increases by S-corp income and gains
• AAA decreases by S-corp losses, deductions, and distributions

Distributions come out in this order: first from AAA (return of after-tax S-corp income, tax-free to shareholders), then from C-corp accumulated E&P (taxed as dividends), then from the Other Adjustments Account (OAA), then as a return of basis, then as capital gain. If the E&P was already zeroed out pre-conversion, all distributions come from AAA and OAA — the simplest result.

QSBS implications: the conversion forfeits future eligibility

Under IRC §1202, the Qualified Small Business Stock exclusion — which can shield up to $15M of gain from federal tax under post-OBBBA rules — is available only to stock issued by a C-corporation.⁴ Converting to S-corp has two effects on QSBS:

• Existing qualifying stock retains QSBS treatment. Shares issued while the corporation was a C-corp, held by the original shareholder since issuance, continue to qualify — the conversion doesn't retroactively void qualification for shares that met all requirements.
• No new QSBS stock can be issued. From the moment the S election becomes effective, any newly issued shares are S-corp stock — ineligible for §1202. Employees receiving equity, new investors (if any were permitted), or ESOP shares all lose QSBS eligibility.

Timing risk: if you're within 3 years of the C-corp stock issuance date and haven't yet met the minimum holding period for any exclusion tier (50% at 3 years, 75% at 4 years, 100% at 5 years under OBBBA), converting can forfeit a large exclusion. A $15M gain that was 100% QSBS-excludable at year 5 becomes fully taxable (plus 21% BIG tax on any pre-conversion appreciation) if you convert to S-corp and sell in year 3. Run the §1202 numbers before converting.

See our QSBS guide and QSBS exclusion calculator for the full analysis.

When the conversion pays off

For most asset-light service businesses — consulting, professional services, software — the math often favors conversion because:

• Low NUBIG. No hard assets with large accumulated depreciation. Goodwill risk exists but is manageable if the business isn't planning an imminent sale.
• No accumulated E&P if the C-corp was consistently paying out earnings (or never had them — many startups with operating losses have negative or zero E&P).
• FICA savings are meaningful. At $300K of business income, S-corp election saves roughly $20,000–$25,000 per year in FICA taxes vs. sole prop. Even with $5,000/yr of additional payroll administration costs, the net benefit compounds to $300K+ over 20 years.

When NOT to convert

• You're planning to sell within 5 years AND the business has significant appreciated assets. The §1374 tax on a business sale can easily exceed the cumulative FICA savings from S-corp status.
• You're still within the QSBS holding period. Converting forfeits S-corp eligibility for new shares and could complicate the timing of the §1202 exclusion for existing shares if shareholders change or shares are restructured.
• The C-corp has large net operating loss carryforwards. C-corp NOLs don't carry into S-corp — they're lost on conversion (with very limited exceptions). If you're expecting to use those losses against future C-corp profits, converting eliminates the benefit.
• You need to issue preferred equity. Raising a priced round or issuing convertible instruments that could be preferred stock at conversion is incompatible with the one-class-of-stock requirement.
• Your state doesn't recognize S-corp elections or has material state-level taxes. California charges a minimum 1.5% franchise tax on S-corp net income. New York has its own S election requirements that must be filed separately. Know your state-level cost before relying on federal-only math.

Decision checklist

Before filing Form 2553, work through each of these with your tax advisor:

1. Get an asset appraisal as of the intended conversion date. Establishes NUBIG for §1374 purposes. Also creates a baseline for future §1374 disputes.
2. Calculate accumulated E&P. If it's positive, decide whether to zero it out with a pre-conversion dividend — and model the cost of paying qualified dividend tax now vs. the ongoing §1375 surtax risk.
3. Map your QSBS timeline. How long have current shareholders held the stock? Are you within a holding-period window? Is there active QSBS value at risk?
4. Check your shareholder cap table for eligibility. Foreign shareholders, entity shareholders, preferred stockholders, or convertible note holders can all create ineligibility problems.
5. File Form 2553 before the deadline for the target tax year. Missing the 2-month-15-day window means the election doesn't take effect until the following year — another year of double taxation.
6. Notify your state tax authority. Most states require a separate state-level S-corp election. California requires Form 3560; New York requires Form CT-6. Failing to file the state election means paying state corporate tax while filing as an S-corp federally.

The conversion decision sits at the intersection of tax planning, exit planning, and entity structuring. A fee-only advisor who specializes in business owner planning will model the actual after-tax impact across the likely scenarios for your specific situation — asset composition, expected sale timeline, income level, and state — before committing to the switch.

Sources

1. IRS — S Corporations. Eligibility requirements: 100-shareholder limit, one class of stock, eligible shareholders (U.S. individuals, certain trusts and estates). See also About Form 2553 for election requirements, deadline, and consent requirements.
2. IRC §1374 — Tax Imposed on Certain Built-In Gains, via Cornell LII. Recognition period: 5 years beginning with the first day of the first S-corp taxable year (as amended; prior 10-year period reduced by statute). Tax rate equals the highest rate imposed under §11, currently 21% (TCJA). NUBIG cap: net unrealized built-in gain determined on conversion date.
3. IRC §1375 — Tax Imposed on Passive Investment Income When C-Corp E&P Exists, via Cornell LII. Entity-level tax at highest §11 rate (21%) on excess passive income when passive income exceeds 25% of gross receipts and accumulated C-corp E&P is present. Sustained violation for 3 consecutive years terminates S election under §1362(d)(3).
4. IRC §1202 — Partial Exclusion for Gain from Certain Small Business Stock, via Cornell LII. QSBS exclusion requires stock issued by a domestic C corporation. OBBBA (July 2025) raised exclusion cap to $15M per issuer, tiered 50%/75%/100% exclusion at 3/4/5-year holding periods. S-corp stock is not eligible for §1202 treatment regardless of holding period.

Tax rates and thresholds cited are for federal tax year 2026. §1374 BIG tax rate is 21% per TCJA (flat corporate rate). §1375 and §1374 statutory references via law.cornell.edu. Form 2553 deadline rules per IRS Instructions for Form 2553 (Rev. December 2020). QSBS rules reflect OBBBA as enacted July 4, 2025. State-level S-corp recognition varies; confirm with a qualified tax advisor in your state.