The wash-sale rule: the 61-day window and the partial-match math

You sell a stock at a loss, meaning to bank the loss for tax purposes, and a few weeks later you buy the same stock back because you still like it. If that purchase falls close enough to the sale, the loss can be disallowed under the wash-sale rule — and "close enough" is a specific, countable window, not a vague sense of "too soon."

The method

The window is 30 days before the sale date through 30 days after it, with both ends and the sale date itself included — 61 calendar days in total. Count the days by hand: the sale date is day 0, thirty days earlier is −30, thirty days later is +30. Any purchase of the same symbol landing anywhere in that −30-to-+30 range is a same-symbol repurchase inside the window. One day outside either edge and it doesn't count.

If the repurchase covers every share you sold, the whole loss is flagged. If it only covers some of them, the flag is proportional: flagged amount = loss × shares repurchased ÷ shares sold. The remaining loss, on the shares that weren't repurchased, is not flagged.

A worked example: full match

You sell 100 shares at a $1,800 loss on March 10. On March 28 — 18 days later, inside the −30/+30 window — you buy 100 shares of the same stock back.

1. Day distance: March 28 is 18 days after March 10, so +18 — inside the window (≤ 30).
2. Shares repurchased vs. shares sold: 100 ÷ 100 = 100% covered.
3. Flagged amount: $1,800 × 100 ÷ 100 = $1,800 — the entire loss is flagged for review.

A worked example: partial match

You sell 150 shares at a $2,400 loss on June 1. On June 20 — 19 days later, inside the window — you buy back only 60 shares of the same stock.

1. Day distance: +19 — inside the −30/+30 window.
2. Coverage: 60 ÷ 150 = 40% of the shares sold were repurchased.
3. Flagged amount: $2,400 × 60 ÷ 150 = $960.
4. Not flagged: $2,400 − $960 = $1,440 — the loss on the 90 shares that weren't replaced.

Same loss, same repurchase date — but because only 40% of the position was bought back, only 40% of the loss is worth reviewing as a possible wash sale. The other 60% of the loss stands on its own.

What a flag doesn't mean

A flag means "same symbol, inside the window, worth a second look" — nothing more. The actual rule turns on securities that are substantially identical, which is broader than an exact symbol match in some directions (an option on the same stock, or two funds tracking the same index, can count) and narrower in others. It also reaches across every account you or your spouse hold, including IRAs, none of which a symbol-matching check can see. No flag doesn't mean you're clear — it means nothing matching was found in the data reviewed. A reinvested dividend counts as a purchase too, and it's one of the easiest ones to forget, since it happens automatically.

What this doesn't include

This is date-window and proportional-share arithmetic only. It doesn't determine whether two securities are substantially identical, doesn't adjust cost basis or extend a holding period (the real consequence of a confirmed wash sale, which shifts the disallowed loss onto the replacement shares' basis), and computes no tax. Whether a flagged pair is actually a wash sale is a question for your broker's records and whoever prepares your return.

Run your own numbers

The free Wash-Sale Flagger tool takes a pasted transaction list and flags every loss sale with a same-symbol purchase in the 61-day window, with the day distance and the proportional-match arithmetic shown for each one — entirely on your device.