When One Stock Hits 40% of Your Portfolio: The Tax Bill of Trimming vs. the Risk of Holding

A stock that grows into 40% of a portfolio creates an awkward money problem. Selling can create a capital-gains tax bill, while holding leaves a large share tied to one company. Neither choice exists in a vacuum, because the tax cost depends on the shares sold, their cost basis, holding period, income and account type.

The decision is less about predicting the stock’s next move and more about comparing two costs. One appears on a tax return. The other can show up in the portfolio if that company takes a major hit.

The Tax Bill Comes from The Gain, Not the Sale

Suppose a taxable brokerage account holds $500,000, with one stock representing $200,000. If the investor sells $100,000 of that stock, the IRS does not automatically treat the entire $100,000 as taxable income. The taxable capital gain generally reflects the difference between the sale proceeds and the adjusted basis of the shares sold.

If those shares cost $25,000 and sell for $100,000, the gain equals $75,000 before other adjustments. Federal tax then depends on the holding period and taxable income. The IRS generally treats stock held more than one year as long-term, while stock held one year or less produces a short-term gain taxed at ordinary income rates.

For 2026, most long-term capital gains fall into the 0%, 15% or 20% federal rate structure. The IRS lists maximum 0% thresholds of $49,450 for single filers and $98,900 for married couples filing jointly. The 15% thresholds extend to $545,500 and $613,700, respectively.

A Partial Sale Can Change the Equation

An investor does not have to choose between keeping the entire position and selling everything. Selling enough shares to bring the stock from 40% of the portfolio to 30%, for example, creates a smaller taxable event than eliminating the position.

The share lots matter, too. Investors who bought the same stock at different prices may have very different gains attached to different shares. The IRS allows specific identification when the investor properly identifies shares with the broker. Without adequate identification, the IRS generally applies first-in, first-out treatment to stock bought at different times and prices.

A brokerage account may also contain losses elsewhere. Capital losses can offset capital gains, and an individual generally can deduct up to $3,000 of excess net capital losses against other income in a year, with unused losses carried forward under the tax rules. So the tax impact of trimming one winner cannot always be calculated by looking at that stock alone.

Concentration Risk Keeps Running While the Tax Question Sits Still

The uncomfortable part of concentration risk does not require a dramatic market collapse. If a portfolio holds $500,000 and one stock represents $200,000, a 25% drop in that stock would reduce the portfolio by $50,000 before considering other investments.

The reverse can happen, too. If the stock keeps climbing while everything else stays roughly flat, its portfolio share can grow beyond 40%. A position that once looked manageable can become an even larger bet without another purchase. Waiting does not freeze the choice.

Taxes therefore represent only one side of the calculation. A large unrealized gain creates a future tax obligation if the investor sells, but the concentrated position creates market exposure today. Both numbers can move.

The Account Type Can Change the Tax Question

The calculation looks different inside a retirement account. Traditional and Roth IRAs generally do not tax investment earnings while those earnings remain inside the account. Traditional IRA distributions generally become taxable when withdrawn, while qualified Roth IRA distributions can be tax-free under the applicable rules.

That means someone considering a concentrated stock position inside an IRA does not face the same immediate capital-gains calculation as someone selling shares in a taxable brokerage account. The investment risk remains, but the tax mechanics differ.

The Tax Bill Does Not Have to Be an All-Or-Nothing Event

Timing can matter. Someone close to a one-year holding period may face different federal treatment after the shares qualify for long-term status. Waiting also creates market risk, though, because the stock can fall or rise before the tax distinction matters.

Other planning tools can affect the calculation. Investors sometimes coordinate appreciated stock sales with losses elsewhere or consider charitable giving of eligible appreciated assets. Each approach has its own tax rules, so a strategy that sounds simple can become complicated once the actual shares and tax return enter the picture.

Before selling, inspect the brokerage account rather than relying on an estimate. Check purchase dates, cost basis, unrealized gain, account type and losses elsewhere. Then calculate how a partial sale would change both the portfolio percentage and the tax result.

The Decision Involves Two Different Kinds of Risk

A 40% position creates a tradeoff that a diversified portfolio usually avoids. Holding preserves the full upside of that company but leaves a large portion of the portfolio exposed to one security. Trimming can reduce that exposure, but a taxable account may turn some of the paper gain into a current tax obligation.

The useful question is not simply how much tax a sale creates. It is also how much risk the current concentration creates. A tax bill and a portfolio loss are different events, and neither can be predicted with certainty.

Would you rather pay a tax bill to reduce a 40% stock position, or keep the shares and accept the concentration risk? Share your thoughts in the comments.