Selling at a Loss on Purpose — and the Rule That Can Undo It
How tax-loss harvesting actually works, the wash sale rule that quietly disallows it, and the $3,000 cap most people misunderstand.
Most investors think of tax-loss harvesting as a December scramble: scan the portfolio, sell whatever is red, take the deduction. The instinct is right. The mechanics matter more than the instinct does.
Handled carelessly, a loss sale can be disallowed entirely by a rule most people have heard of but few fully understand. Handled well, it can reduce a tax bill without meaningfully changing what your portfolio is invested in.
What a Harvested Loss Actually Does
Selling an investment at a loss realizes that loss for tax purposes. It first offsets any realized capital gains elsewhere in your portfolio, dollar for dollar. If losses exceed gains, up to $3,000 of the remainder can offset ordinary income each year, and anything beyond that carries forward indefinitely. There is no expiration on unused losses.
The netting happens in a specific order: short-term losses offset short-term gains first, long-term losses offset long-term gains first, and only the leftover amounts cross over. That ordering changes how much a harvested loss is worth to you depending on what else has already happened in the account this year.
The Wash Sale Rule, Explained Plainly
The IRS disallows the loss if you buy the same security, or one that is “substantially identical,” within 30 days before or after the sale. That is a 61-day window in total. The rule applies across every account you own, including your spouse's and including IRAs, which is where it catches people most often.
“Substantially identical” does not mean “similar.” Selling one S&P 500 index fund and buying a different provider's S&P 500 fund is generally treated as the same exposure and can violate the rule. Selling a large-cap index fund and replacing it with a total-market fund keeps you invested in a genuinely different holding. The distinction is easy to get backwards.
Where the Deduction Actually Helps
A harvested loss is most valuable when it offsets a gain you were going to realize anyway — from a rebalance, a concentrated stock sale, or a fund's capital gain distribution. It is less valuable, though never worthless, when there is nothing to offset and it is simply carrying forward or trimming $3,000 of ordinary income.
The Three Mistakes We See Most
1. Harvesting inside a tax-deferred account like an IRA, where there is no tax benefit to capture in the first place.
2. Replacing a sold position with something close enough to trigger the wash sale rule without realizing it.
3. And selling the wrong tax lot — the specific shares matter, and “first in, first out” is not always the right choice once lots can be identified individually.
The Windward Approach
Tax-loss harvesting is not a standalone move here. It gets reviewed alongside your realized and unrealized gains for the year, any Roth conversion on the table, and any concentrated position you are working down — because our CPA roots mean we read the tax return and the portfolio as one document, not two.
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