The Wash-Sale Rule

The wash-sale rule disallows a loss on the sale of a security if, within 30 days before or after that sale, you buy the same or a substantially identical security. The purpose is straightforward: without it, an investor could sell a holding purely to claim a paper loss for tax purposes and then instantly rebuy the identical position, capturing the deduction while never really changing their exposure. The rule closes that door by refusing the loss when you effectively stay in the same investment.

It matters most in the context of tax-loss harvesting, where the entire goal is to realize a loss while staying invested. The wash-sale rule is what forces harvesters to use a replacement that is similar but not substantially identical, rather than simply rebuying what they sold. Understanding it is therefore the prerequisite for harvesting correctly. This is general information, not tax advice.

The timing is the part people most often get wrong. The rule covers 30 days before the sale and 30 days after it. Counting the day of the sale itself, that is a 61-day window. Most investors remember the "after" half and carefully wait past 30 days to rebuy, but forget the "before" half: if you bought additional shares of the security shortly before selling other shares at a loss, that earlier purchase can trigger the rule just as easily.

A frequently missed trigger is automatic dividend reinvestment. If a fund reinvests a dividend into new shares inside the window, even a small amount, it can create a wash sale on part of your loss without you doing anything deliberately. The same goes for recurring automated buys. Because of the before-and-after span and these automatic purchases, careful harvesters avoid buying the same or a substantially identical security anywhere in the full 61-day window. Verify the mechanics with a tax professional.

A crucial and reassuring detail: a disallowed wash-sale loss is not forfeited. Instead, the amount of the disallowed loss is added to the cost basis of the replacement shares you bought. Your holding period from the original shares also carries over to the replacement. So the tax benefit is not erased; it is deferred until you eventually sell the replacement shares, when the higher basis reduces your gain or enlarges your loss.

A quick example: you buy a stock at 100 dollars, sell at 70 dollars for a 30 dollar loss, then rebuy within the window at 72 dollars. The 30 dollar loss is disallowed now, but it is added to the replacement's basis, making it 102 dollars rather than 72. When you later sell that replacement, the built-in 30 dollars flows through. The one important exception: if the replacement is bought inside an IRA, the basis benefit generally cannot be recovered, so that loss can be permanently lost. This is educational, not tax advice.

The wash-sale rule does not stop at a single account. The IRS looks at all of your accounts together, so selling a stock at a loss in your taxable brokerage account and buying the same security in your IRA within the window can trigger the rule. As noted above, a replacement purchased in an IRA is the worst case, because the loss cannot be restored through a basis adjustment there, so it is effectively gone.

The reach extends further: the IRS generally treats a purchase by your spouse, or by a corporation you control, as if you made it for wash-sale purposes. This means a household cannot sidestep the rule by having one spouse sell and the other buy. This wide scope is exactly why harvesting requires looking at your entire financial picture, not just the account where you placed the sale. Because these cross-account situations get intricate, confirm any specific case with a tax professional.

The phrase "substantially identical" is the heart of the rule and, frustratingly, the IRS has never defined it precisely, which makes it a genuine gray area. Some cases are clear: the exact same stock is substantially identical to itself, and the same mutual fund or ETF is substantially identical to itself. Options and contracts to acquire the same security can also count. Selling and immediately rebuying the identical ticker is the textbook wash sale.

The harder question is funds. Two broad-market ETFs from different providers that track different underlying indexes are generally treated as not substantially identical, which is why harvesters swap between them. But two funds tracking the same index are far riskier, and there is no bright-line safe harbor for funds, so this remains a judgment call. Individual stocks of two different companies, even close competitors, are not substantially identical. Because the answer depends on specifics and is genuinely a tax-advice question, confirm any particular swap with a tax professional before relying on it.

One clarifying point that removes a lot of confusion: the wash-sale rule applies only to losses. If you sell a security at a gain and rebuy it immediately, the rule is simply irrelevant, the gain is taxable as normal, and nothing is disallowed. The entire machinery exists to prevent claiming a tax loss while staying in the same position, so a profitable sale is outside its scope.

This is worth knowing because it explains why a related strategy, sometimes called tax-gain harvesting (realizing gains on purpose in a low-income year and rebuying), is not restricted by the wash-sale rule, since there is no loss to disallow. For how gains themselves are taxed and the levers around them, see how to reduce investment taxes. This is general information, not tax advice.

Put together, the rule is narrow in what it targets (a loss where you rebought the same or a substantially identical security in the 61-day window) but broad in reach (all your accounts, including IRAs and a spouse's) and forgiving in outcome (the loss is deferred into the replacement's basis rather than destroyed, except inside an IRA). It applies only to losses. The gray zone is what counts as substantially identical, especially for funds, so verify current guidance and treat this as a framework, not tax advice. Confirm any specific situation with the IRS or a licensed professional.

The wash-sale rule is hard to police by hand precisely because it spans every account you and your household hold: to spot a risk you have to see all your positions and recent buys in one place, not one brokerage at a time. That is a question about your real, cross-account holdings, which is where an AI assistant that reads across your accounts helps as context. Walnut, an AI investing app, lets you connect any major US broker, then chat in plain words through Claude, ChatGPT, or its built-in AI to see which holdings you own across accounts and which are below what you paid. It surfaces that context only: it does not track wash sales, calculate cost basis, execute trades on its own, or judge whether two funds are substantially identical. Walnut is not a registered investment adviser or a tax adviser, it does not calculate your taxes, and it does not tell you what to buy or sell.