Declaración Renta

Understanding the Wash Sale Rule

The tax rule that blocks deduction of losses on quick repurchases.

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Pol García Financial Advisor and Co-founder of Finturify • Published on October 12, 2026

1. Introduction to the Concept and Fundamentals

The wash sale rule is a regulation that prevents taxpayers from claiming a tax deduction for a security sold at a loss if they buy a "substantially identical" security within 30 days before or after the sale.

The rule prevents investors from selling a stock to artificially harvest a tax loss and then immediately buying it back to maintain their market position. If you violate the wash sale rule, you cannot deduct the loss this year; instead, the loss is added to the cost basis of the new security, deferring the tax benefit.

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2. Detailed Analysis and Market Data

To apply this concept with complete safety, it is essential to analyze the historical performance and data of the different options available. A detailed comparison is summarized below:

Transaction SequenceTimeframeSubstantially Identical?Tax Status of Loss
Sell fund at a loss, buy same fund back15 days laterYesWash sale; loss is disallowed and added to basis
Sell S&P 500 ETF, buy Total Stock Market ETFNext dayNoLoss is fully deductible; maintains market exposure
Sell stock at a loss, do not repurchaseOver 30 days laterNoLoss is fully deductible against capital gains

⚠️ Professional Warning

The IRS does not provide a definitive list of "substantially identical" securities. Swapping one S&P 500 ETF for another (e.g., SPY for VOO) is widely considered by tax professionals to be a wash sale, so it is safer to swap to a different index class.

3. Practical Application and Financial Context

In the US, IRS Publication 550 defines the wash sale rule. The rule also applies if you sell a security in a taxable account and buy it back in an IRA within the 61-day window.

The key steps you should follow to implement this strategy efficiently in your personal planning are listed below:

  • Step: Sell a fund or stock at a loss to harvest the tax deduction.
  • Step: Do not buy the same security within the 30-day window after the sale.
  • Step: Do not buy the same security within the 30-day window before the sale.
  • Step: To maintain market exposure, purchase a similar but not substantially identical fund (e.g., swapping an S&P 500 ETF for a Total Market ETF).

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Frequently Asked Questions (FAQ)

What happens to the disallowed loss?

The loss is added to the cost basis of the newly purchased shares. When you eventually sell those new shares, the higher basis will reduce your future taxable gain.

Does the wash sale rule apply to profitable sales?

No. The wash sale rule only applies to capital losses. Profitable sales always trigger immediate taxes, regardless of when you buy the asset back.

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Pol García Co-founder

Pol García is an independent financial advisor and co-founder of Finturify. Specialized in budgeting, family savings, and mortgage analysis. He helps families and young professionals build their finances and design efficient plans to acquire real estate wealth intelligently.