The idea in plain language
Tax-loss harvesting means selling an investment that is worth less than your cost basis so you realize a capital loss on your tax return. That loss can offset capital gains from other sales in the same year. If losses exceed gains, U.S. federal rules generally let you deduct up to $3,000 of the net loss against ordinary income, with the remainder carried forward to future years.
Harvesting does not create free money. It can defer or reduce taxes if you stay invested in a similar economic exposure and if you respect wash-sale rules. It is most useful in taxable brokerage accounts. Traditional IRAs and 401(k)s do not generate annual capital-loss deductions in the same way, so harvesting inside those wrappers is usually irrelevant to the year-end tax filing.
When harvesting is worth the effort
Focus on positions with meaningful unrealized losses relative to trading costs and your time. A $200 paper loss on a thinly traded stock may not justify commissions, bid-ask friction, and paperwork. A multi-thousand-dollar loss on a broad ETF in a year when you also realized gains from rebalancing or selling concentrated stock can be highly relevant.
Also consider your expected tax rate. Harvesting is more valuable if you are realizing short-term gains taxed at ordinary rates, or if you expect to be in a higher bracket in the harvest year than in a future year when deferred gains appear. It is less urgent in a year with no gains and little ordinary income to absorb the $3,000 allowance—though carryforwards still have lasting value.
Wash-sale rules: the trap that undoes the benefit
Under U.S. wash-sale rules, if you sell a security at a loss and buy a “substantially identical” security within thirty days before or after the sale, the loss is disallowed for current deduction and instead adjusts the basis of the replacement. Buying the same ETF the next day in the same account is an obvious wash sale. Buying it in your IRA, or having your spouse buy it in a household account, can also create problems depending on facts—treat related accounts carefully and consult a tax professional for edge cases.
A common compliant approach is to sell Fund A at a loss and immediately buy Fund B that tracks a similar but not identical index—for example, an S&P 500 ETF replaced with a total U.S. market ETF, or one total international fund replaced with another from a different index family. The goal is to maintain market exposure while avoiding “substantially identical” status. Exact determinations can be fact-specific; when in doubt, increase differentiation or wait thirty-one days.
Mechanics of a clean harvest
- Identify lots: Use specific identification if your broker supports it; do not accidentally sell high-basis lots you wanted to keep.
- Estimate net loss: Subtract proceeds from adjusted basis; include commissions.
- Place the replacement trade: Buy the substitute ETF the same day if you want continuous exposure.
- Log the lot: Record sale date, proceeds, replacement ticker, and intended tax year.
- Avoid dividend-driven buys: Automatic dividend reinvestment in the sold ticker during the wash window can spoil the harvest—disable DRIP on that position first.
Gains, carryforwards, and year-end crowding
December is crowded with harvesters, which can widen spreads slightly in niche funds, but major ETFs usually remain liquid. Do not wait until the final trading day if you need settlement and confirmation buffer. Remember that harvesting resets basis downward on the replacement only when a wash sale applies; a successful harvest leaves you with a lower-cost replacement that may produce larger gains later. You are often deferring tax, not erasing the economic gain forever.
If you have carryforward losses from prior years, apply them intentionally when planning large gain realizations such as selling a business stake or concentrated stock. Coordinate with charitable donation strategies only with professional advice—these interact in ways that DIY spreadsheets miss.
What harvesting is not
It is not a reason to churn a portfolio. It is not advice to realize losses in a way that abandons your asset allocation. It does not replace emergency savings or debt payoff priorities. And software “auto-harvest” features from brokerages can help, but you still need to understand wash sales across multiple accounts you control.
Used selectively, tax-loss harvesting is a maintenance tool for taxable investors: realize losses when markets give them to you, stay invested, follow the thirty-day discipline, and keep records clean enough that April is boring instead of chaotic.