Stock Average Cost Basis Calculator

Buying additional shares after a price drop pulls your blended average cost basis lower than your original purchase price, which lowers the exact price the stock needs to reach for your overall position to turn a profit. It doesn't change whether the stock's decline was justified — it only changes your personal breakeven math on the total position.

Not automatically — it works well when the drop is temporary noise unrelated to the company's actual prospects, but it can compound losses if the stock is falling because the underlying business is genuinely deteriorating, since you're then committing more money to a weakening investment rather than a merely cheaper one.

No, it only blends the raw share counts and prices you enter, so any per-trade fees or commissions charged on either purchase would need to be added into your effective price per share manually before entering it, or your true breakeven point will sit slightly above the number this tool reports.

Treat your first two purchases as one combined lot using this calculator's output as the average price and total shares figure, then run a second calculation pairing that combined lot against the details of your third purchase — the resulting blended average correctly folds in all three transactions regardless of how many separate buys you're combining.