How to use Stock Average Calculator
A stock average calculator works out your weighted average price per share — your cost basis — across several purchases of the same stock. Each time you buy at a different price, your blended cost changes, and tracking it by hand is error-prone. Enter each purchase with its price and number of shares, and the tool returns your total shares held, the total amount invested, and the average cost per share. This is essential for knowing your break-even price, deciding whether to average down, and reporting cost basis when you eventually sell.
- Enter the price and share count for your first purchase.
- Add a row for each additional purchase.
- Use the add button to include as many buys as needed.
- Read the average cost per share and total invested.
- Adjust any purchase to see how the average changes.
Your data never leaves your device — 100% private processing.
How the average price is calculated
The average is weighted by the number of shares in each purchase, not a simple average of the prices. To find it, the calculator multiplies each buy price by its share count to get the money spent on that lot, sums those amounts to get the total invested, and divides by the total number of shares. A large purchase therefore pulls the average toward its price more strongly than a small one. This weighting is what makes cost basis meaningful: if you bought 10 shares at 100 and 90 shares at 50, your average is far closer to 50 than to 75, because most of your shares were bought cheaply. The result is your break-even price — the level at which selling everything would return exactly what you paid, before fees and taxes.
| Purchase | Cost | Running average |
|---|---|---|
| 10 @ 100 | 1,000 | 100.00 |
| 15 @ 80 | 1,200 | 88.00 |
| Total: 25 shares | 2,200 | 88.00 |
Averaging down: a tool, not a strategy
Averaging down means buying more shares after the price falls, which lowers your average cost and your break-even price. It can be powerful: a smaller recovery in the share price is then enough to return to profit. But a lower average is only worthwhile if your reason for owning the stock still holds — adding to a position simply because it dropped can concentrate risk in a falling asset. The calculator deliberately stays neutral on that decision; it shows you precisely what your new average and total exposure would be so you can judge whether the extra investment is justified. It also excludes brokerage commissions and taxes, which slightly raise your real cost basis, so add those in when precision matters for tax reporting.
Worked examples
Two buys
Inputs: 10 @ 100 · 15 @ 80
Result: Average 88.00 · 25 shares · 2,200 invested
Averaging down
Inputs: 10 @ 100 · 40 @ 50
Result: Average 60.00 · 50 shares
Glossary
- Cost basis
- The total amount invested in a position, used to work out gains when you sell.
- Weighted average
- An average where each value counts in proportion to its share quantity.
- Averaging down
- Buying more shares at a lower price to reduce the average cost.
- Break-even price
- The price at which selling returns exactly what you paid, before fees.
- Lot
- A single purchase of shares at one price.
Related reading
Frequently Asked Questions
Why use Stock Average Calculator?
- Get your weighted average cost across many purchases
- See total shares held and total amount invested
- Know your true break-even price per share
- Model the effect of averaging down before you buy
- Runs in your browser — your holdings stay private
Common use cases
- Track cost basis as you build a position over time
- See your new average after buying the dip
- Decide whether averaging down meaningfully lowers your cost
- Confirm your break-even price before setting a sell target
- Keep a simple record of a multi-buy position
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