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Qualified Small Business Stock (QSBS) Exclusion Calculator

Calculate the potential capital gains tax exclusion on qualified small business stock under Section 1202.

Result

Excludable Gain
$5,000,000.00
Estimated Tax Saved
$1,000,000.00
Remaining Taxable Gain
$0.00
Tax Owed on Remaining Gain
$0.00

Assumes stock acquired after September 27, 2010 (qualifying for 100% exclusion) and held more than 5 years. The exclusion is capped at the greater of $10 million or 10x your cost basis. Qualifying requires the stock be from a C-corporation with under $50 million in gross assets at issuance, among other requirements - consult a tax professional to confirm QSBS eligibility.

About the QSBS Exclusion Calculator

This calculator estimates the federal capital gains tax exclusion available under Section 1202 for Qualified Small Business Stock (QSBS). It is meant for founders, early employees, or investors who acquired stock directly from a qualifying C-corporation and want to see how much of a future sale gain could escape federal capital gains tax entirely.

How It Works

You enter the capital gain on the sale, your original cost basis, and your long-term capital gains rate. The tool caps the excludable gain at the greater of $10 million or 10 times your cost basis, treats anything above that cap as ordinary taxable gain, and then multiplies each portion by your entered capital gains rate to show tax saved versus tax still owed.

exclusionCap = max(costBasis x 10, $10,000,000); excludedGain = min(gain, exclusionCap); taxableGain = max(gain - excludedGain, 0); taxSaved = excludedGain x capGainsRate; taxOwed = taxableGain x capGainsRate.

Formula & Methodology

Start by comparing your cost basis multiplied by 10 against the flat $10 million floor, and take whichever is larger as your exclusion ceiling. Compare your total gain to that ceiling and exclude the smaller of the two. Any leftover gain above the ceiling is taxed normally at your entered rate, while the excluded portion generates the 'tax saved' figure by applying that same rate to the amount you no longer have to pay tax on.

Examples

Startup founder with a $5 million gain

A founder sells qualifying stock with a $5,000,000 gain, a $100,000 cost basis, and a 20% capital gains rate. Since 10x the cost basis ($1,000,000) is below the $10 million floor, the exclusion cap is $10,000,000, so the entire $5,000,000 gain is excluded and roughly $1,000,000 in tax is saved, with no remaining taxable gain.

Large gain exceeding the cap

An early investor with a $250,000 cost basis realizes a $15,000,000 gain. Ten times the cost basis is $2,500,000, still below the $10 million floor, so the exclusion cap remains $10,000,000. That leaves $5,000,000 of taxable gain, which at a 20% rate produces $1,000,000 of tax owed alongside $2,000,000 in tax saved on the excluded portion.

Advantages

  • Quickly separates a large stock-sale gain into an excluded portion and a still-taxable portion using the actual 10x-basis-or-$10M rule rather than a rough estimate.
  • Shows both the tax saved and the tax still owed side by side, useful for comparing outcomes across different cost-basis or gain scenarios.
  • Lets a user test how a higher cost basis (through additional stock purchases or exercised options) changes the effective exclusion ceiling.

Common Mistakes

  • Assuming the exclusion is unlimited: the cap always applies, so gains well above $10 million or 10x basis remain partly taxable.
  • Overlooking the five-year holding period and post-2010 acquisition date requirements, which this calculator assumes are already satisfied.
  • Forgetting that the exclusion percentage can be less than 100% for stock acquired before September 28, 2010, which this tool does not model.

Edge Cases to Watch For

  • The calculator assumes 100% exclusion eligibility, which under current law requires stock acquired after September 27, 2010 and held more than 5 years; stock acquired earlier only qualifies for a 50% or 75% exclusion, not modeled here.
  • A very low cost basis (for example, near-zero founder stock) still benefits from the $10 million floor, since the cap uses whichever of the two thresholds is larger.
  • The tool does not verify the underlying eligibility tests, such as the issuing company being a domestic C-corporation with gross assets under $50 million at the time the stock was issued.
  • Because the exclusion cap and tax figures are calculated per this single sale, stacking QSBS gains across multiple tax years or entities is not reflected in a single calculation.

Common Use Cases

  • Startup founders estimating after-tax proceeds before a planned exit or acquisition.
  • Early employees with exercised stock options evaluating a potential sale of C-corp shares.
  • Angel investors and venture backers comparing the tax outcome of a large exit against their original investment size.
Written & fact-checked by the Calculateus TeamLast updated August 5, 2026How we verify our formulas

Frequently asked questions

Why is QSBS considered one of the most valuable tax breaks for startup investors and founders?

A 100% federal capital gains exclusion (up to the cap) on qualifying startup stock held over 5 years is an extraordinarily generous tax benefit compared to standard capital gains treatment, effectively letting successful startup founders and early investors keep their entire gain tax-free up to the exclusion limit - this is a major reason QSBS planning is a standard consideration when structuring or investing in an eligible C-corp startup.

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Conclusion

The QSBS Exclusion Calculator applies the actual Section 1202 cap formula to show how much of a stock sale gain could be excluded from federal capital gains tax. Because real QSBS eligibility involves company-level and holding-period requirements the calculator does not verify, it works best as a planning estimate rather than a final determination.

References