Retail investors who sell a losing stock and buy it back within 30 days lose the right to claim that loss on their federal tax return. The rule, codified in Title 26 of the U.S. Code, creates a 61-day restricted window around every loss sale. With the 2026 filing season now open, any trader who triggered a wash sale in 2025 will see the consequences on their brokerage statements and tax forms, often as a surprise adjustment that raises their future taxable gain instead of lowering this year’s bill.
How the 61-Day Window Blocks a Tax Deduction
The wash-sale rule bars a taxpayer from deducting a loss when that person buys substantially identical stock or securities inside a period that starts 30 days before and ends 30 days after the sale. That 61-day span is spelled out in Section 1091 of the Internal Revenue Code, the statute that has governed these transactions for decades. The same prohibition applies when the taxpayer enters into a contract or option to acquire the same security, not just an outright purchase.
The Treasury regulation implementing the statute, found at Treasury Regulation 1.1091-1, repeats the rule in operational terms: a loss is nondeductible if the taxpayer acquires, or contracts to acquire, substantially identical shares anywhere inside that window. The disallowed loss does not vanish permanently. Instead, it gets added to the cost basis of the replacement shares, which defers the tax benefit until those new shares are eventually sold.
For the investor, the practical effect is straightforward. A $2,000 loss on a stock sold in October 2025, followed by a repurchase of the same stock three weeks later, cannot reduce 2025 taxable income. The $2,000 instead increases the basis of the replacement position, meaning the investor will owe less tax only when that replacement position is finally closed, potentially years later or never, if the stock is held indefinitely.
The 61-day window also applies if the investor buys first and sells later. A purchase of substantially identical shares within 30 days before a loss sale can taint the later sale and turn what appears to be a deductible loss into a wash sale. Because the rule looks both backward and forward from the date of sale, frequent traders can easily trigger multiple overlapping windows without realizing it.
Broker Reporting and Form 8949 During the 2026 Filing Season
Brokers are required to flag wash-sale disallowances directly on the year-end tax documents they send to clients. The disallowed loss amount appears in Box 1g of Form 1099-B, according to IRS guidance. That number flows through to the investor’s return, where it must be reported on Form 8949 using adjustment code “W.” The nondeductible loss is entered as a positive number in the adjustment column, effectively canceling the loss that would otherwise reduce taxable gains.
The instructions for Form 8949 explain that taxpayers must list each affected transaction, identify it with code W, and adjust the gain or loss to reflect any wash-sale amount already reported by the broker. When software imports Form 1099-B electronically, these adjustments are often carried into the return automatically, but filers still need to verify that the totals match their records.
Investors who trade frequently across multiple brokerage accounts face an added complication: each broker tracks wash sales only within its own account. A sale at one firm and a repurchase at another can still trigger the rule, but neither broker may catch it. The taxpayer bears the responsibility to reconcile those transactions and apply the correct adjustment on their own return. Failure to do so can lead to overstated losses and potential correspondence from the IRS if the agency later matches trades across accounts.
Substantially Identical Securities and Modern Products
Revenue Ruling 2008-5, published in Internal Revenue Bulletin 2008-3, provides additional IRS guidance on what qualifies as a substantially identical security. That ruling has shaped how preparers and software handle edge cases involving mutual funds and index products, particularly when an investor sells one fund tracking a specific index and quickly buys another that follows the same benchmark.
The IRS has not issued a newer ruling that directly addresses the rapid growth of exchange-traded funds, leveraged products, and narrowly focused sector funds. As a result, taxpayers and advisers often rely on the principles in the ruling and the statutory language to evaluate whether two positions are substantially identical. When the economic exposure and underlying holdings are nearly the same, the risk that the wash-sale rule applies increases, even if the ticker symbols and fund sponsors differ.
For 2025 transactions now appearing on 2026 tax forms, this uncertainty means investors who harvested losses in one index fund and moved immediately into a closely aligned alternative should review their trades carefully. If those positions are deemed substantially identical, the expected loss deduction may be postponed rather than realized in the current year. Understanding how the 61-day window, broker reporting, and the substantially identical standard interact can help traders avoid unwelcome surprises at tax time and plan more deliberate loss-harvesting strategies in future years.
David M. Keller is a finance writer based in Columbus, Ohio, covering personal finance and consumer-focused economic topics. He earned his degree in journalism from Ohio University and began his career reporting on local business and economic trends for a regional media outlet. Since then, he has contributed to a variety of online publications, focusing on clear, practical coverage of topics such as cost of living, debt, and everyday financial decision-making.
