The Wash-Sale Rule in Plain English

Tax-loss harvesting is one of the few places in a financial plan where a down market actually works in your favor, but one rule can quietly erase the benefit before you realize what happened. The wash-sale rule is that rule, and most of the people who get caught by it thought they understood it.

What the Wash-Sale Rule Actually Says

The IRS disallows a loss on a security sale if you buy a "substantially identical" security within 30 days before or after the sale. That is a 61-day window total: 30 days prior, the day of the sale, and 30 days after. The loss is not gone forever. It gets added to the cost basis of the replacement security. But the timing advantage, which is often the whole point of harvesting the loss this year, disappears.

The rule comes from IRC Section 1091. It was designed to stop investors from selling a position, claiming the loss, and immediately buying right back in as if nothing changed. In practice, it catches people who move faster than they think and who forget they own the same fund in multiple accounts.

The Rules, Traps, and the Index-Fund Swap Most People Miss

"Substantially identical" is narrower than "similar." Two funds that track different indexes are generally not substantially identical, even if they hold overlapping stocks. Selling Vanguard's S&P 500 ETF (VOO) and buying iShares' S&P 500 ETF (IVV) is almost certainly a wash sale because both track the same index. But selling VOO and buying a total market fund or a large-cap fund tracking the Russell 1000 is a different story. The underlying indexes are distinct, the holdings differ at the margin, and the IRS has not historically treated that swap as substantially identical.

This is the move that retail investors most often miss. You do not have to sit in cash for 31 days. You can stay invested, maintain very similar market exposure, and still book the loss, as long as the replacement security clears the substantially identical test.

The spousal account trap. The wash-sale rule applies across accounts you control and accounts your spouse controls. If you sell a position in your taxable brokerage account and your spouse buys the same security in their IRA on the same day, the wash sale is triggered. One household, one rule.

The IRA trap is worse. If a wash sale is triggered because you repurchased inside an IRA, the deferred loss does not get added back to your basis anywhere. It is lost entirely. The basis-transfer mechanic that softens a wash sale in taxable accounts does not apply when the replacement purchase happens inside a tax-advantaged account. This is the version of the mistake that has no recovery.

The basis-transfer mechanic. When a wash sale is triggered inside taxable accounts, the disallowed loss is added to the cost basis of the replacement shares. That higher basis reduces the gain (or increases the loss) when you eventually sell. You have not lost the tax benefit permanently. You have postponed it. Whether postponement matters depends on your expected bracket in the year you finally sell and how long that takes.

Worked Example: Harvesting $50,000 Without Tripping the Rule

Say it is October and you hold $200,000 in a large-cap growth ETF in your taxable account. The position is sitting at a $50,000 unrealized loss from a rough year. You want to harvest that loss before year-end without losing market exposure.

Here is a clean path. On day one, you sell the ETF and immediately purchase a large-cap ETF tracking a different index, one with meaningfully different constituents. You stay fully invested. You have realized $50,000 in losses with no gap in market participation.

You then wait 31 days. At that point, you have two options: hold the replacement fund if you prefer it, or sell it and buy back the original. Either way, the wash-sale window has passed and the $50,000 loss is intact.

At the 37% federal bracket, that $50,000 loss shelters capital gains at a rate of up to 20% plus the 3.8% net investment income tax, which is real money. Paired with a Roth conversion in the same tax year, the harvested losses can offset ordinary income that would otherwise push the conversion into a higher bracket. I cover that combination in more detail in Tax-Loss Harvesting: How to Turn a Down Market Into Real Tax Savings.

One thing I walk through with clients before pulling this trigger: do not repurchase anything automatically. A dividend reinvestment on the old fund, scheduled before the sale, can create a wash sale on a small lot and contaminate the entire harvest. Turn off auto-reinvestment before you execute.

How This Connects to the Soil Layer of Your Plan

This is squarely a Soil-layer decision. The Sporos Doctrine uses the Soil stage to describe the tax architecture of a plan: which accounts hold which assets, how gains and losses are managed across the household, and how current-year moves interact with future bracket exposure. The wash-sale rule is not a one-transaction problem. It is a household-level coordination problem, which is exactly where structure matters.

Frequently Asked Questions

Does the wash-sale rule apply to cryptocurrency?

As of now, no. Crypto is treated as property, not a security, so the wash-sale rule under IRC 1091 does not apply. You can sell bitcoin at a loss and buy it back the next day. Congress has proposed closing this gap in prior sessions, and it is worth watching, but the rule does not currently reach crypto.

What counts as "substantially identical" for mutual funds versus ETFs?

The IRS has not issued exhaustive guidance, so practitioners apply a facts-and-circumstances test. Two funds tracking the same index are high-risk. Two funds tracking different indexes with similar but not identical holdings are generally considered safe. When in doubt, look at the index name, not just the asset class.

Can I harvest losses in a tax-deferred account?

No. Losses inside an IRA or 401(k) have no tax effect because gains are not taxed annually in those accounts either. Tax-loss harvesting is a taxable-account strategy only.

What happens to the disallowed loss if I never sell the replacement shares?

The disallowed loss sits in the basis of the replacement shares indefinitely. If you donate those shares to charity or pass them at death with a step-up in basis, the deferred loss is lost permanently.

Is there a dollar threshold below which the wash-sale rule does not apply?

No. The rule applies to any amount. Whether it is worth the administrative attention on small lots is a practical judgment, but there is no de minimis exemption.

How do I document a tax-loss harvest for my accountant?

Keep a record of the sale date, the purchase date of the replacement security, the fund tickers, and the specific lot(s) sold. Your brokerage will issue a 1099-B, but it will not flag a wash sale across accounts at different custodians. That coordination falls on you.

What to Do Next

1. Identify unrealized losses in your taxable accounts before November to leave adequate time before year-end settlement.
2. Screen replacement securities against your current holdings for the substantially identical test, and disable automatic dividend reinvestment on any fund you plan to sell.
3. Coordinate across every account in your household, including your spouse's accounts at any custodian, before executing any sale.
4. Talk to a tax advisor about pairing harvested losses with a Roth conversion if your current-year bracket creates an opportunity.

The information provided is for educational purposes only and does not constitute investment, legal, or tax advice. Tax law changes frequently — verify current rules before acting. Consult with qualified professionals for guidance specific to your situation.