Tax-Loss Harvesting: Turning a Loser Into a Tax Break

Tier 5 · Advanced Instruments · advanced · ~6 min read · taxesinvestingtax-loss-harvestingcapital-gainsindex-funds

A loss on paper can pay a real dividend

Every long-term investor ends up holding something that’s down. The instinct is to look away and wait for it to recover. But a loss you’re sitting on is doing nothing for you — while a loss you realize (by selling) becomes a deduction the tax code genuinely values.

Tax-loss harvesting is the move:

Sell the losing investment to lock in the loss, use that loss to lower your tax bill, then immediately buy a similar but not identical investment so you stay in the market.

You barely change your portfolio — same rough exposure, a slightly different fund — but you walk away with a deductible loss you can put to work this April. The catch is subtle enough that plenty of people get it wrong, so let’s make it tangible.

What a harvested loss is worth

A realized capital loss is applied in a fixed order:

1. It cancels your capital gains first, dollar-for-dollar. Sold something else at a $10,000 gain? A $10,000 harvested loss erases the tax on it entirely.
2. Then up to $3,000 of ordinary income per year — your salary, interest, the stuff taxed at higher ordinary rates. This slice is especially valuable because ordinary income is taxed more heavily than long-term gains.
3. Anything left is carried forward to future years, indefinitely, to do the same job again.

Start with the default: an $8,000 loss with $10,000 of gains to offset. The whole loss lands on those gains, and at a 24% rate that’s about $1,920 off your tax bill this year. Now drag Gains to offset down below the loss and watch the stat cards: the overflow spills into the $3,000 ordinary-income offset, and the rest shows up as carried forward — waiting for next year.

The catch: it’s usually a deferral, not a gift

Here’s the part the headlines skip. When you sell and rebuy, your cost basis — the number your future gain is measured from — resets down to today’s lower price. So when you eventually sell the replacement investment, your gain is larger by exactly the loss you harvested, and you owe more tax then.

The tax you save now is, in large part, tax you’ll pay later. Harvesting moves the bill into the future more than it erases it.

That’s the amber region on the chart: the future tax cost of the basis reset. So why bother?

Why deferral still wins: the IRS’s interest-free loan

Because a dollar saved today is worth more than a dollar paid years from now. The tax you don’t pay this year stays invested and compounding, while the offsetting cost sits frozen until you sell.

Set your tax rate now and the rate at the future sale to the same number and look at the chart: the net benefit line still ends in the black. You saved, say, $1,200 now and will owe $1,200 later — but that $1,200 grew for years in the meantime. The break-even reads “Right away”: you’re never behind. It’s as close to free money as the tax code offers — an interest-free loan from the government, repaid in nominal dollars after they’ve lost a race against your portfolio.

Crank Investment growth up and the gap widens. The longer your horizon and the higher your return, the more that deferred dollar is worth.

When it’s a real win — and when it backfires

Deferral is the floor. The ceiling comes from rate arbitrage:

• Harvest high, pay low. Offset income taxed at a high rate today — ordinary income, or short-term gains — and pay the lower long-term rate when you finally sell. Set rate now above rate later and you’re ahead from day one, before any compounding. Push it far enough (a held position whose basis steps up at death, erasing the gain entirely) and the future bill never arrives at all.
• Harvest low, pay high — careful. Flip it: a low rate now and a higher rate later. Now the basis reset costs you more than you saved, and only years of compounding can dig you out. Set growth to 0% with a higher future rate and the break-even reads “Never” — harvesting actually cost you money. This is the genuine downside, and the simulator makes it visible.

The verdict isn’t “always harvest.” It’s: harvest when the rate you save at now is at least the rate you’ll pay later, or when your horizon is long enough for the deferral to pay for itself.

The one rule that ruins it: the wash sale

There’s a tripwire. To stop people from selling purely for the tax break and instantly buying right back, the IRS has the wash-sale rule:

If you buy the same — or a “substantially identical” — security within 30 days before or after the sale, the loss is disallowed.

Flip the When you rebuy slider to “Rebuy now (wash sale)” and the entire benefit vanishes: tax saved drops to zero, the chart flatlines. The loss isn’t destroyed forever — it’s added to the basis of the shares you rebought — but the harvest you were counting on this year is gone.

The professional workaround is simple: harvest the loss, then immediately buy a similar but not identical fund — sell one broad S&P 500 index fund and buy a different provider’s total-market fund, for instance. Your market exposure barely changes, but the two aren’t “substantially identical,” so the loss counts. After 31 days you can swap back if you like.

What the simulator leaves out

It’s a teaching model, so it keeps the edges clean:

• No state taxes or surtaxes. High earners may also owe a net-investment-income tax; many states tax gains too. Both make a harvest worth more.
• It assumes you sell at the horizon. Hold forever — or pass the shares on at death, when the basis steps up — and the deferred cost may shrink or disappear, making harvesting strictly better than shown.
• The carried-forward loss is reported but not compounded into the headline. Its future value is real; the chart’s net-benefit line is deliberately conservative.
• “Substantially identical” is a judgment call. The rule is fuzzy at the edges; the simulator models only the clean cases (rebuy the same thing = wash; rebuy a different fund = fine).

Key terms

• Cost basis — what you paid for an investment; your gain or loss is measured from it.
• Realized vs. unrealized — a paper (unrealized) loss does nothing for taxes until you sell and realize it.
• Capital loss / gain — the loss or profit from selling an investment; losses offset gains first.
• $3,000 ordinary-income offset — the most net capital loss you can deduct against ordinary income in one year; the rest carries forward.
• Carry-forward — unused losses applied to future tax years, with no expiration.
• Wash sale — buying the same or a substantially identical security within 30 days of the loss sale, which disallows the loss.
• Step-up in basis — at death, an heir’s cost basis resets to the market value, often erasing the deferred gain entirely.

Tax-loss harvesting is a clean lesson in a deep idea: taxes deferred are taxes discounted, because money has a time value. The same arithmetic drives the employer-match and Roth-vs-traditional math and the case for low-turnover index funds that rarely force a taxable gain in the first place. Harvest when the rates line up, swap into something similar to dodge the wash-sale rule, and let the saved tax compound — but never mistake a deferral for a gift.