Stock Average Calculator
Enter every buy to get your average cost per share, then see exactly how many shares it would take to average down to the price you want.
Your purchases
- Average cost per share
- $0.00
- Total shares
- Total cost
- $0.00
Unrealized gain / loss
n/a
Average down (or up) to a target
How average cost is calculated
Your average cost is a weighted average: bigger purchases count for more. Buying 100 shares at $50 and 100 more at $40 gives an average of $45, but buying 100 at $50 and 300 at $40 gives $42.50.
Average cost = (shares₁ × price₁ + shares₂ × price₂ + …) ÷ total shares
The average-down formula
To find how many shares (x) you need to buy at price P to reach a target average T, given S shares already held at average A:
x = S × (A − T) ÷ (T − P)
The calculator solves this for you and shows the cash it would take. If the number looks large, that's the point: averaging down a big loss gets expensive fast.
Frequently asked questions
How do you calculate the average cost of a stock? +
Add up what you paid for every purchase (shares × price for each buy), then divide by the total number of shares. That's your weighted average cost per share, the price the stock has to get back to for you to break even.
What does averaging down mean? +
Averaging down means buying more shares after the price falls, which lowers your average cost. It reduces the rebound you need to break even, but it also increases how much money you have in a position that's moving against you.
Why can't I reach my target average? +
Your target average has to sit between the new purchase price and your current average. If you're buying at $40, no number of shares will pull your average below $40.
Should I include commissions? +
If you paid any, yes. Add them to the price of that buy (fee ÷ shares) to get a true cost basis. Most US brokers no longer charge stock commissions, so many people can leave this out.
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Zentrix MCP connects Claude, GPT, and Gemini to live market data so your agent can watch the positions you're averaging into.