September is a good time to start looking for investment losses you might use to offset gains before year-end. But there’s a catch: selling a losing investment doesn’t necessarily mean you get to claim the loss on your taxes.
You might buy back the same stock too soon. You might have bought it before selling. Or you might trigger a wash sale in another brokerage account without realizing it.
The result can be frustrating: a trade intended to reduce your tax bill produces no immediately deductible loss, even though you genuinely lost money.
To understand why, you need to know two things: the wash-sale rule looks both backward and forward in time, and a disallowed loss doesn’t always disappear. Sometimes it moves somewhere else.
The 30-day rule is actually a 61-day window
Most investors who have heard of wash sales know the familiar warning: don’t sell a stock at a loss and buy it back within 30 days.
That’s only half the rule.
A wash sale generally occurs when you sell stock or securities at a loss and acquire substantially identical stock or securities within 30 days before or after the sale. Certain options and other contracts can also trigger the rule.
That creates two common traps.
Scenario 1: You sell first, then buy back too soon.
Suppose you bought 100 shares for $5,000. On September 15, you sell them for $4,000, realizing a $1,000 loss.
On September 25, you buy 100 shares of the same stock again.
You may have thought you harvested a $1,000 tax loss. Instead, you’ve triggered a wash sale. The $1,000 loss is disallowed for the September 15 sale.
Scenario 2: You buy first, then sell at a loss.
Now reverse the order.
On September 5, you buy 100 additional shares of a stock you already own. On September 15, you sell 100 older shares at a $1,000 loss.
The earlier purchase can trigger the wash-sale rule just as the later repurchase did in the first example.
Waiting 30 days after the sale won’t undo a wash sale caused by a purchase that already occurred.
The relevant period runs from 30 days before the loss sale through 30 days after it: 61 calendar days, counting the sale date itself.
That is why simply setting a reminder to avoid buying back a stock for 30 days after selling it is not enough. You also need to know what you bought during the preceding 30 days.
What does “disallowed loss” actually mean?
The word disallowed sounds ominous. But in an ordinary wash sale involving a replacement purchase in a taxable account, the loss is generally deferred rather than permanently lost.
Let’s return to our first example.
You originally paid $5,000 for 100 shares and sold them for $4,000, realizing a $1,000 loss. Ten days later, you repurchased 100 shares for $4,200.
Here’s what happens:
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Original investment: $5,000
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Sale proceeds: $4,000
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Realized loss: $1,000
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Cost of replacement shares: $4,200
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Disallowed loss added to the replacement shares’ tax basis: $1,000
Your replacement shares now have an adjusted tax basis of $5,200.
Tax basis is essentially the amount used to calculate your gain or loss when you eventually dispose of an investment.
Suppose you later sell those replacement shares for $4,700, in a transaction that doesn’t trigger another wash sale.
Without the basis adjustment, you would appear to have made a $500 gain: $4,700 minus $4,200.
With the adjustment, you instead have a $500 loss: $4,700 minus $5,200.
The original loss has effectively carried forward into the replacement investment. The holding period of the original shares also generally carries over to the replacement shares.
This is the key distinction: a wash sale generally changes when you can recognize a loss, not whether you economically suffered one.
The timing can matter enormously. A loss deferred beyond December 31 may no longer be available to offset gains in the tax year you intended.
And if you repeatedly sell at a loss and repurchase substantially identical shares within the wash-sale window, the loss can continue moving from one set of shares to another.
When the loss really can disappear
There is an important exception to the usual basis-adjustment treatment.
Suppose you sell shares at a loss in your taxable brokerage account and then buy substantially identical shares in your IRA or Roth IRA within the wash-sale window.
The IRS treats that acquisition as a wash sale. But unlike an ordinary taxable-account replacement purchase, the disallowed loss does not increase the basis of the IRA shares.
That means the loss generally cannot be recovered through a later taxable sale of the replacement shares.
This is one reason wash-sale monitoring cannot stop at the boundary of a single taxable brokerage account. Transactions elsewhere can change the tax consequences of a trade.
Other situations—including partial repurchases, options and purchases involving a spouse—can make the analysis more complicated. When the number of replacement shares differs from the number sold at a loss, only part of the loss may be disallowed.
Your broker’s wash-sale report may not tell the whole story
Investors often assume their brokerage account will catch wash sales automatically.
For certain transactions involving the same security in the same account, brokers are required to report disallowed wash-sale losses and adjust the basis of replacement shares.
But those reporting requirements are narrower than the wash-sale rule itself.
A broker generally isn’t required to detect a wash sale involving a purchase in a different account—even if both accounts are held at the same brokerage firm. It also may not know about relevant purchases at another broker or in a spouse’s account.
Imagine that you sell a stock at a loss in Brokerage Account A. Two weeks later, an automatic investment plan purchases the same stock in Brokerage Account B.
Account A may show a realized loss. Account B may show an ordinary purchase. Neither account’s individual transaction history necessarily tells you that the two trades must be considered together.
And the absence of a wash-sale adjustment on your Form 1099-B does not mean the loss is deductible.
This creates a practical problem at tax time.
By the time you’re preparing your return, the trades have already happened. You may need to reconstruct purchases and sales across accounts, determine which shares were affected, calculate the disallowed amount and make the appropriate tax-basis adjustments.
Tax software and a qualified tax professional can help with the reporting. But neither can retroactively turn a wash sale into the timely tax-loss-harvesting opportunity you intended.
The better time to identify a potential wash sale is before placing the trade.
The tax lot you didn’t know was losing money
Wash-sale tracking becomes even more useful when combined with another piece of investment data: tax lots.
Every time you purchase shares, you create a separate group of shares with its own acquisition date and cost basis.
Consider an investor who bought 100 shares at $50 and another 100 shares at $100. The stock now trades at $80.
The position as a whole is profitable. The investor paid $15,000 for the 200 shares, which are now worth $16,000—an unrealized gain of $1,000.
But look at the individual purchases.
The 100 shares bought at $50 have gained $3,000.
The 100 shares bought at $100 have lost $2,000.
The investor may have a potential $2,000 loss-harvesting opportunity hidden inside a position that appears to be up $1,000.
Whether that loss can be recognized depends on the shares actually sold, applicable lot-identification rules and any wash-sale-triggering acquisitions. For example, a recent purchase of substantially identical shares could affect the intended loss sale.
This is why looking only at a portfolio’s overall gains and losses can miss important tax information.
A wish list for tax-aware investing technology
The mechanics of wash sales suggest a useful set of capabilities for investment software. Rather than merely displaying an annual realized-gains report, a tax-aware system could help investors make decisions while there’s still time to act.
1. Warn before a trade creates a wash sale.
Check relevant purchases and sales across connected accounts, including transactions during the preceding 30 days, and identify potential conflicts before an investor acts.
The system should also explain the limitations of any accounts it cannot see.
2. Find losing tax lots inside winning positions.
Analyze individual purchase lots rather than relying on a position’s average cost or overall return. Surface potential loss-harvesting opportunities that a portfolio summary might conceal.
3. Keep track of the dates.
Show which recent acquisitions may affect a proposed loss sale, when a replacement purchase could trigger a new wash sale, and when a planned transaction would fall outside the relevant window—assuming no other triggering transactions occur.
Investors shouldn’t have to maintain a spreadsheet of purchase dates and count calendar days every time they consider a trade.
4. Identify suitable replacement investments.
An investor harvesting a loss may want to maintain exposure to a sector or market while avoiding a substantially identical replacement security.
Software could compare alternatives by investment characteristics and flag potential wash-sale concerns.
But it should not promise that any replacement is guaranteed to satisfy the IRS. Whether securities are substantially identical depends on the facts and circumstances.
None of these capabilities eliminates the need for accurate account data, sound investment judgment or appropriate tax advice. They do, however, address a fundamental weakness in a tax-planning process that often begins only after the year’s trades are complete.
Don’t wait until tax season to find out
A wash sale isn’t necessarily a disaster. In an ordinary taxable-account transaction, the loss generally moves into the basis of replacement shares and may become recognizable later.
But the investor who intended to harvest a loss this year may discover that the deduction has been deferred. A purchase in an IRA can have more lasting consequences. And transactions across multiple accounts can create complications that no single broker’s tax report fully captures.
As the final quarter approaches, the practical lesson is to review individual tax lots, recent purchases and activity across accounts before making year-end tax trades.
The goal isn’t merely to find investments that have lost money. It’s to understand whether selling them will produce the tax result you expect.
This article is for educational purposes only and does not constitute individualized investment or tax advice. Actual wash-sale treatment depends on the transactions and applicable tax rules.