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Stock Average Calculator

Find your average cost per share after averaging down on a position.

How it is calculated

The average cost per share is a weighted average: average = (shares₁ × price₁ + shares₂ × price₂) / (shares₁ + shares₂). The numerator is your total cost, the denominator your total number of shares. The second lot is optional; if left empty it counts as zero shares. Share counts and prices cannot be negative, and the total number of shares must be above zero. Fractional shares are allowed. Three results are shown: the average price, the total shares and the total cost. Commissions and fees are not included, so add them to the prices if you want a true break-even. For a third purchase, enter the previous average and share total as the first lot.

Worked example

You bought 100 shares at 50 and later 50 shares at 40. Cost of lot one: 100 × 50 = 5,000. Cost of lot two: 50 × 40 = 2,000. Total cost = $7,000.00 for 150.00 shares. Average price = 7,000 / 150 = $46.67 per share, not the simple mean of 45.

Try this example →

Common mistakes

  • Averaging the two prices (45) without weighting by the number of shares.
  • Leaving out commissions and fees, which makes your real break-even price slightly higher.
  • Entering prices in different currencies or before and after a stock split without adjusting.
  • Treating the average cost as the current value; it is only what you paid on average.
  • Trying to add a third lot directly instead of using the previous average and total shares as the first lot.

About this calculator

This calculator finds your blended average share price and total cost basis after buying the same stock at two different prices — the weighted average of both purchases based on how many shares you bought at each price.

Investors use it when averaging down (buying more shares after a price drop) or simply adding to a position over time, to know their true break-even price and total investment before deciding whether to buy more, hold, or sell.

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