Tax-Loss Harvesting, Explained Without the Jargon

Selling a loser to lower your tax bill sounds like a trick, but it's an ordinary, IRS-recognized practice. What harvesting actually is, why the wash-sale rule exists, and why the math is all about lots.

The first loss we ever tried to harvest, we nearly fumbled by buying the position back a week later, which would have quietly voided the whole thing. So let's start with what it is. Every portfolio has losers. Tax-loss harvesting is the practice of putting them to work: selling a position that's down to "realize" the loss, which can then offset capital gains elsewhere, and in the US up to $3,000 of ordinary income per year, with the rest carrying forward.

That's the whole idea. No loophole, no trick, just the tax code's acknowledgment that if you're taxed when investments gain, losses should count too. The complexity is entirely in the details, and the details are where people get burned.

The wash-sale rule, in one paragraph

The IRS won't let you sell a stock on Monday, buy it back on Tuesday, and claim the loss. That's a "wash sale." The rule: if you buy the same (or a substantially identical) security within 30 days before or after the sale, the loss is disallowed for now. The window runs both directions, and it's the single most common way a harvest quietly fails.

It's about lots, not tickers

Here's the part most tools gloss over: you don't harvest a ticker, you harvest a lot, a specific batch of shares bought on a specific date at a specific price. If you bought shares of the same stock three times, you hold three lots, each with its own gain or loss and its own long-term or short-term clock. A position that's up overall can still contain an individual lot at a loss.

That "Clear" status is doing the heavy lifting. Before acting on any loss, you want to know the lot's holding period and whether recent purchases put it inside the 30-day window. This is bookkeeping: exactly the kind professionals run desks for and retail investors are left to do in spreadsheets.

Timing matters more than people think

• Losses first offset gains of the same type (short-term vs. long-term), and short-term gains are taxed at higher rates, so the same dollar of loss can be worth more against the right gain.
• December is the traditional harvest season, but losses exist year-round; a mid-year drawdown can be a better opportunity than a year-end scramble.
• Harvesting resets your cost basis lower, which can mean a bigger taxable gain later. It's usually a deferral, not a free lunch.