The wash sale rule: how to harvest losses without voiding them

Selling losers to offset gains — tax-loss harvesting — is one of the few free lunches in a taxable account. The wash sale rule is the string attached: sell a security at a loss and buy it (or something 'substantially identical') back within 30 days on either side of the sale, and the loss is disallowed for now. The rule doesn't fine you; it just cancels the deduction you were harvesting.

The mechanics: a 61-day window

The window is 30 days before the sale, the sale date, and 30 days after — 61 days total. Buy substantially identical shares anywhere in that window and the loss is disallowed. It's not gone forever: the disallowed loss gets added to the cost basis of the replacement shares, so you recover it whenever you eventually sell those. The real damage is deferral — plus bookkeeping — not destruction. Unless the repurchase happens inside an IRA, where the loss is destroyed permanently.

• The 30 days BEFORE the sale counts — buying more, then selling the old lot at a loss a week later, is a wash sale.
• It applies across ALL your accounts: your taxable account, your IRA, your spouse's accounts. Selling in taxable while your IRA auto-invests in the same fund is a classic accidental wash.
• Automatic dividend reinvestment is the most common trigger of all — a $40 reinvested dividend can wash part of a deliberate harvest.
• It applies per lot: partial repurchases wash a proportional part of the loss.
• It only applies to losses. Selling at a gain and rebuying instantly is fine (that's tax-gain harvesting).

'Substantially identical' — the gray zone

The IRS has never precisely defined it. Clearly identical: the same stock, the same fund, options on the stock you sold. Clearly different: stocks of two different companies, or a total-market fund vs. a sector fund. The practical gray zone is index funds: two S&P 500 funds from different companies track the same index, and many professionals treat swapping between them as risky. The standard playbook is to swap between similar-but-not-identical indexes — S&P 500 to a total-market or large-cap index fund — which keeps your allocation intact while being defensibly different.

How to harvest cleanly

1. Before selling, check for purchases of the same security in the last 30 days — including reinvested dividends — in every account you and your spouse own.
2. Turn off dividend reinvestment in taxable accounts (take dividends as cash and invest them manually) to kill the most common accidental wash.
3. Pick your replacement in advance: a similar-but-different fund (different index) so you're never out of the market.
4. Either hold the replacement permanently, or wait 31+ days before switching back to the original.
5. Check your 1099-B in January — brokers flag wash sales within one account, but only YOU can catch cross-account and spousal washes.

What a clean harvest is worth

Swap pairs that keep you invested

The practical heart of clean harvesting is having your replacement picked before you sell. The goal is a fund with near-identical behavior that tracks a DIFFERENT index: S&P 500 fund to a total-US-market fund (the two correlate at roughly 0.99), one total-international fund to another built on a different index, a Treasury fund to a similar-duration aggregate bond fund. What you should avoid is swapping a fund for another fund tracking the SAME index from a different provider — cheap identical exposure, but the least defensible position if 'substantially identical' is ever tested. And skip options-based cleverness entirely: buying calls on a stock you just sold at a loss is explicitly a wash sale under the rule's own text.

Worth internalizing once: the rule's cross-account reach includes retirement accounts you fund automatically. The classic self-inflicted wash is selling an index fund at a loss in taxable on the 10th while a 401(k) contribution buys the same fund on the 15th. Payroll contributions to a target-date fund are generally safe (a target-date fund isn't substantially identical to its components), which is one more quiet argument for target-date funds in workplace plans.

The bottom line

The wash sale rule says you can't claim a loss on something you never really stopped owning. Respect the 61-day window, remember it spans every account in the household, turn off automatic reinvestment in taxable accounts, and harvest into a similar-but-different fund the same day. Do that and tax-loss harvesting stays what it should be: a paperwork trick that turns market dips into real tax savings.