How Much Capital Gains Tax Will I Pay on $300,000?

Long-term capital gains fill bracket space after your other ordinary income is counted first, not independently. Picture your other taxable income as already occupying the bottom of the bracket ladder; the capital gain stacks on top of it and gets taxed at whatever rate applies to that next slice of income, 0%, 15%, or 20% depending on where the total lands.

Using the 2026 married-filing-jointly thresholds ($0-$98,900 at 0%, $98,901-$613,700 at 15%, above $613,700 at 20%), a couple with $80,000 of other taxable income has $18,900 of room left in the 0% bracket before any capital gain is added. A $300,000 gain uses that room first, then fills the 15% bracket for the remaining $281,100, well short of the $613,700 threshold that would push any of it into the 20% bracket.

The Net Investment Income Tax is the layer most people forget when they quote a capital gains rate from memory. It's a flat 3.8% surtax that applies to the lesser of your net investment income or the amount your modified AGI exceeds $200,000 (single) or $250,000 (married filing jointly), thresholds that are fixed by statute and not adjusted for inflation. In the example above, the couple's $380,000 MAGI exceeds the $250,000 threshold by $130,000, so NIIT applies to that $130,000, not the full gain.

State tax is a separate variable that can swing the total dramatically. Arizona taxes capital gains as ordinary income at its flat 2.5% rate, adding roughly $7,500 on a $300,000 gain in this example. States with no income tax add nothing. High-tax states can add a meaningfully larger amount on top of the federal number, sometimes exceeding the federal tax itself on a gain this size.

A $300,000 capital gain is not the same thing as $300,000 in sale proceeds. Gain is what's left after subtracting your cost basis (and selling costs, for real estate) from what you received. Confusing the two, assuming a $300,000 sale price means a $300,000 taxable gain, dramatically overstates the tax owed for anyone with meaningful basis in the asset.

For a gain of this size that isn't forced by a specific event, the same strategies that apply to smaller gains scale up: spreading the sale across two tax years to reduce how much stacks into higher brackets, harvesting the sale in a lower-income year, offsetting it with tax-loss harvesting elsewhere in the portfolio, or reducing other stacked income (a QCD instead of a taxable IRA withdrawal, for example) to preserve more room in the lower brackets before the gain is layered on top.

Don't estimate the tax on a large gain by applying a single percentage from memory. Run the actual stacking calculation against your specific other income, filing status, and state, the difference between a rough guess and the real number is often five figures.

If the timing of a large gain is discretionary, model whether splitting it across two tax years, or pairing it with other income-reducing moves in the same year, meaningfully changes how much lands in the 20% bracket or triggers NIIT.

• Assuming one flat percentage applies to the entire gain, rather than understanding that it fills bracket space progressively on top of other income.
• Forgetting the 3.8% Net Investment Income Tax, which isn't part of the headline capital gains rate but applies on top of it above the MAGI thresholds.
• Ignoring state tax entirely when estimating the total bill on a large gain, the swing between states can be tens of thousands of dollars.
• Confusing the gross sale amount with the taxable gain. They're only the same number if your cost basis is zero.