Wash Sale Calculator

How it is calculated

The tool assumes you already know you repurchased within the 30-day window; it does not take dates, it only splits the loss by share count.

loss per share = cost basis − sale price
total loss = shares sold × loss per share
matching shares = min(shares sold, shares repurchased)
disallowed = matching shares × loss per share
allowed = total loss − disallowed
new basis = repurchase price + disallowed ÷ matching shares

All prices are dollars per share. The disallowed amount is not lost; it is added to the basis of the matching replacement shares only. If you bought back more than you sold, the extra shares keep their plain repurchase price, and the new basis shown is per matching share. If the sale was at a gain or breakeven (sale price not below cost basis) there is no loss and no result is shown. Zero shares sold, zero shares repurchased or a zero cost basis also give no result.

Common mistakes

• Entering the sale price at or above the cost basis. A gain is not a wash sale; the tool returns nothing instead of a disallowed amount of zero.
• Entering the total proceeds rather than the price per share. Both price fields are per share, and the share counts are separate fields.
• Treating the disallowed loss as gone for good. It is moved into the basis of the repurchased shares and is recovered when you finally sell them.
• Entering a repurchase of zero to see the loss as fully allowed. With zero shares repurchased the tool shows no result; if nothing was bought back, the whole loss is simply deductible and no wash-sale calculation is needed.
• Forgetting that the tool does not check dates or whether the securities are substantially identical. It only does the arithmetic; you decide whether the rule applies, and other countries have different rules.