Tax Loss Harvesting: What It Saves, and What It Only Postpones

A down market hands you exactly one consolation prize: positions trading below what you paid can be converted into tax savings without changing what you own. That conversion is tax-loss harvesting, and it is both more useful and less magical than its reputation. The honest one-line summary: it mostly defers tax rather than deleting it, and deferral done at the right moments is still worth real money.

The mechanic in three moves

1. Sell a position below its basis. The paper loss becomes a realized loss the IRS recognizes.
2. Immediately buy something similar but not substantially identical. Your market exposure barely blinks; only the tax position changed.
3. Spend the loss on your return. Realized losses first cancel realized capital gains dollar for dollar. If losses exceed gains, a capped slice offsets ordinary income each year, and anything left carries forward indefinitely to future returns.

Nothing about your portfolio strategy has to change. You held a broad stock fund before; you hold a very similar one after. The harvest is purely a tax maneuver layered on top.

A worked example

Say you bought a fund for $20,000 and a rough year has it sitting at $14,000. Selling realizes a $6,000 loss. Earlier this year you also rebalanced and realized $4,500 of long-term gains. The harvest applies in a fixed order: the loss first wipes out the $4,500 gain entirely. At a 15% long-term rate, that alone saves $675 in federal tax. The remaining $1,500 of loss then offsets ordinary income, up to the annual cap, and those are the most expensive dollars on your return because they are taxed at your top marginal rate, not the capital gains rate. Whatever the cap leaves unused rolls forward and waits for next year's gains.

Notice the asymmetry hiding in there: losses spent against ordinary income save at your highest rate, while the gains you will eventually repay are often taxed at lower long-term rates. That spread between rates, not the deferral alone, is where harvesting earns its keep. The capital gains tax calculator puts current-year numbers on both sides of that trade for your income.

The wash-sale coordination problem

The one rule that can void the whole exercise: rebuy the same or a substantially identical security within 30 days, before or after the sale, and the loss is disallowed. The window reaches every account you control, including IRAs and a spouse's accounts, and the quiet triggers are automated: dividend reinvestment, recurring monthly buys, a retirement contribution that lands mid-window. Harvesting well is mostly logistics: pause automatic purchases of the harvested security everywhere, pick the replacement fund in advance (similar exposure, different index), and hold the plan for the full 31 days.

Why high-income years are the sweet spot

Harvesting is most valuable exactly when your marginal rate is highest. Three reasons stack. Losses that offset ordinary income save at that high rate. Losses that offset short-term gains, which are taxed like wages, do the same. And if you expect a lower rate later (retirement, a planned career downshift), you are deferring tax out of an expensive year and repaying it in a cheap one, which converts a timing trick into a permanent saving. The reverse also holds: in a genuinely low-income year the 0% long-term rate can make harvesting gains, not losses, the smarter move, since you can reset basis upward and pay nothing federally to do it.

The honest limits

• The basis resets lower. Harvest the $20,000 fund at $14,000 and your new cost basis is $14,000. If the market recovers and you sell at $20,000, there is now a $6,000 taxable gain that would never have existed without the harvest. You saved tax today and scheduled tax for later; the net win is the rate spread plus years of compounding on the deferred dollars.
• Some exits make deferral permanent, most do not. Appreciated shares donated to charity, and the basis treatment heirs receive under current law, can mean the deferred gain is never taxed. Planning on that is estate planning, not portfolio management; for the ordinary case, assume the bill eventually arrives.
• It only works in taxable accounts. Losses inside a 401(k) or IRA cannot be harvested at all, so a household whose investments live mostly in retirement accounts has little to harvest.
• The tail should not wag the dog. A harvest that moves you into a fund you like less, or that you would not otherwise hold, spends investment quality to buy a tax discount. The replacement has to stand on its own.

A December habit, not a December scramble

Most people think about harvesting in the last week of the year, which is when the wash-sale traps are thickest and the choices most rushed. The calmer version is a quick review a few times a year: which lots sit below basis, what gains are already realized, and whether the spread is worth the trade. That review takes minutes when every account and every lot already sits in one complete picture of what you own.