What Is a Long-Term Capital Gains Tax Estimator?

A long-term capital gains tax estimator works out what a sale actually costs in tax. Selling an asset held more than a year does not produce a single flat rate: the gain is stacked on top of your ordinary income and sliced across the 0%, 15% and 20% brackets, then a 3.8% net investment income tax can apply above the statutory income thresholds. State tax may take another cut on top. The estimator turns those rules into a dollar figure so you can decide whether to sell now, split the sale, or hold.

It is built for the common case: a stock, fund or property held for years and sold at a profit, with the 2026 federal thresholds hard-coded from the IRS inflation-adjustment release and a flat state rate you supply. That makes it useful for planning real decisions — exercising concentrated stock, selling a rental, or harvesting gains at the end of a low-income year.

How It Works

The gain is your sale proceeds minus cost basis. It is then stacked on top of ordinary taxable income, which is your income after the standard or itemized deduction. Any part of the gain that fits under the 0% ceiling — $49,450 for single filers, $98,900 for married couples filing jointly, $66,200 for heads of household in 2026 — is tax-free. The next slice up to the 15% ceiling of $545,500, $613,700 or $579,600 is taxed at 15%, and anything above that at 20%.

The 3.8% net investment income tax is computed separately. It applies to the lesser of your net investment income and the amount by which modified adjusted gross income exceeds $200,000 for single and head-of-household filers, $250,000 for joint filers, or $125,000 for married filing separately. Because those thresholds are fixed in statute rather than indexed, a high-income seller in the 20% bracket faces 23.8% overall before state tax.

The tool also annualises the after-tax result using the holding period, which converts a lump-sum tax bill into a comparable rate of return. A gain that looks large in dollars may represent a modest annual return once you account for how long the capital was tied up and what tax was paid to release it.

Understanding Your Results

The primary card shows the total tax with the effective rate on the gain, which is almost always lower than the marginal rate because the first slice is often taxed at 0% or 15%. The three statistics beside it isolate the federal component, the marginal rate you reached, and the after-tax proceeds you would actually receive.

The bracket table shows exactly how the gain was sliced, with the amount and tax in each band plus the NIIT and state tax lines. Summary Statistics adds the cost basis, the gain as a percentage of basis, the tax per $1,000 of gain and the estimated MAGI used for the NIIT test. A useful experiment: raise the ordinary income field and watch the 0% slice vanish, then try splitting the sale across two years by halving the proceeds and doubling the years held — the annualised return usually improves, which is the arithmetic behind gain harvesting.

Data vintage: 2026 thresholds per IRS Rev. Proc. 2025-32 (single: $49,450 / $545,500; joint: $98,900 / $613,700; head of household: $66,200 / $579,600; married filing separately: $49,450 / $306,850). NIIT thresholds are fixed by statute. Re-check the figures each October when the IRS announces the following year's adjustments.