What Tax-Loss Harvesting Actually Does
Sell an investment at a loss and the tax code lets that loss cancel out gains dollar for dollar — then up to $3,000 per year of leftover losses can offset ordinary income (salary, interest), with anything beyond that carried forward indefinitely. Harvesting is simply doing this on purpose: realizing losses you already have, banking the tax benefit, and (usually) immediately buying a similar — but not identical — investment so your portfolio barely changes.
The savings are real but bounded: a $10,000 short-term loss saves $3,200 at a 32% bracket, $2,400 at 24%. Nobody harvests their way to wealth — they harvest to keep more of the wealth their gains already created. Think of it as a discount on taxes you were going to pay anyway.
Short-Term vs Long-Term: The Ordering Rules
Not all losses are equal. Short-term losses (assets held one year or less) first offset short-term gains, which are taxed at high ordinary-income rates — making short-term losses the most valuable kind. Long-term losses first offset long-term gains, taxed at the gentler 0%/15%/20% rates. Only after netting within each category can leftovers cross over.
Practical consequence: a short-term loss is worth more per dollar than a long-term loss for most earners, because it kills income that would have been taxed at 24–37% instead of 15–20%. The worksheet above values each bucket at its own rate — which is why entering the holding term per position matters.
The Wash-Sale Rule: The 61-Day Window
The IRS will not let you sell at a loss on Monday and buy it back on Tuesday. If you purchase a "substantially identical" security within 30 days before or after the loss sale — a 61-day window — the loss is disallowed. It is not destroyed (it gets added to the replacement shares' cost basis), but the current-year tax benefit evaporates, which defeats the entire exercise.
Traps that catch smart people: dividend reinvestment buying shares inside the window (turn off DRIP in taxable accounts before harvesting), purchases in your IRA or 401(k) (the IRS counts those too, and the disallowed loss is gone forever — no basis adjustment in a retirement account), and your spouse's accounts buying the same security. The guardrail above checks your dates; keep 31+ days of separation or swap to a similar-but-different fund.
What Counts as "Substantially Identical"?
The same stock or ETF is obviously identical. Beyond that, the IRS has never published a bright-line test, which creates the practical playbook: selling an S&P 500 ETF from one issuer and buying another issuer's S&P 500 ETF is probably fine (different fund, different structure) but not certainly fine — conservative harvesters swap to a similar but clearly different exposure instead, like S&P 500 ↔ large-cap Russell 1000. For individual stocks, swapping to a competitor in the same sector is the standard move.
Mutual fund share classes of the same fund are identical. Options on the same stock can trigger it. When in doubt, choose the clearly-different alternative — the tiny tracking difference costs far less than a disallowed loss.
The $3,000 Limit and Carryforward
After losses offset all your gains, only $3,000 per year ($1,500 married filing separately) can offset ordinary income. The rest carries forward indefinitely — it never expires, and it keeps its short/long-term character. Big harvest years (a $50,000 loss against no gains) create a carryforward that shelters income for a decade-plus: $3,000 a year, every year, until it is gone.
The tracker above shows the waterfall: gains first, then the $3,000 ordinary offset, then carryforward. One planning note: carryforwards die with you unused (heirs get a stepped-up basis anyway), so very large carryforwards argue for realizing gains strategically — rebalancing, Roth conversions in disguise — to soak them up.
When Harvesting Makes Sense (and When It Doesn't)
Harvest when you have gains to offset (a rebalancing year, a property sale, RSUs vesting), when you are in a high bracket (the savings scale with your rate), and when you can stay invested via a replacement security. Year-end is the natural moment, but volatile markets hand out opportunities all year — the best harvests happen in March panics, not December calm.
Do not harvest when the transaction costs exceed the benefit (a $200 loss at 12% saves $24 — not worth the paperwork), when you would have to go to cash and risk missing a rebound, or in retirement accounts (losses there are not deductible at all — harvesting inside an IRA is pure waste). And never let the tax tail wag the investment dog: if the replacement security is meaningfully worse, skip it.
Direct Indexing: Harvesting on Autopilot
Wealth managers now sell direct indexing — owning the individual stocks of an index instead of the ETF — largely as a harvesting machine: with 500 positions, something is always down, so the algorithm harvests year-round while tracking the index. It works, and the harvested losses are genuinely valuable. Whether the typical 0.2–40% fee earns its keep depends on your bracket and account size — at $500k+ taxable and a high bracket, the math often clears; below that, manual year-end harvesting captures most of the benefit free.
Recordkeeping: Boring but Mandatory
Every harvested loss needs a paper trail: trade confirmations with dates and amounts, the cost basis method used, and proof of the 61-day window (no repurchases). Brokers track basis for covered shares, but wash sales across different accounts or brokers are invisible to them — that reconciliation is on you. Keep a simple log: what you sold, what you bought instead, and the dates. An audit without records turns legitimate harvesting into disallowed losses plus penalties.
Tax-Gain Harvesting: The Mirror Image
Losses are not the only thing worth harvesting. In years when your income is unusually low — a sabbatical, early retirement before Social Security, a gap year — you may sit in the 0% long-term capital gains bracket (up to $48,350 of taxable income for single filers in 2025, roughly double for joint). Selling winners and immediately rebuying them resets your cost basis higher at zero tax, shrinking all future tax bills. Unlike losses, gains have no wash-sale rule — you can rebuy the identical security the same minute.
The two strategies pair beautifully across a lifetime: harvest losses in high-income years to bank deductions, harvest gains in low-income years to step up basis free. Retirees in the pre-RMD gap years (60s, delaying Social Security to 70) get the best of both — filling low brackets with Roth conversions and tax-gain harvesting. Run the numbers each December: the brackets reset January 1, and unused 0% space does not carry forward.
Putting It All Together
List your losers, value each bucket at its true rate, check every sale against the 61-day window (mind the IRA and DRIP traps), and track the carryforward so future gains meet it prepared. Harvesting will not make you rich — but done cleanly, year after year, it is one of the few reliable ways to keep more of what your portfolio already earned.
When NOT to Tax-Loss Harvest
Harvesting isn't always smart. Don't harvest in tax-advantaged accounts — losses inside an IRA or 401(k) are meaningless for taxes, and worse, buying a "substantially identical" security in your IRA within the window can trigger a wash sale that permanently disallows the taxable-account loss. Don't harvest tiny losses if transaction costs or your time exceed the benefit; a $200 loss saving $48 at a 24% bracket isn't worth an hour of paperwork and a tracking spreadsheet you'll neglect.
Don't harvest if you'll need the cash soon and the replacement security is volatile — the wash-sale rule forces you to stay exposed to similar market risk for 31 days, which can backfire. And don't let the tax tail wag the investment dog: selling a great long-term holding just to book a loss, then watching it rebound while you're sidelined, is a classic blunder. Harvest methodically from positions you'd rebalance anyway, keep the replacement genuinely comparable, and remember the goal is keeping more of your returns — not manufacturing losses for their own sake.