How is average cost per share calculated?

Average cost equals your total investment (each purchase's price times shares, summed) divided by your total shares owned. Buying more shares at a different price changes the weighted average, not a simple average of the prices.

What is break-even price?

Your break-even price is the same as your average cost per share — the price at which selling all your shares would return exactly what you invested, with no profit or loss before fees and taxes.

How is unrealized gain or loss calculated?

Unrealized gain or loss is (current price − average cost) × total shares. It only appears once you enter a current market price, and it is 'unrealized' because you have not sold the shares yet.

Does averaging down lower my risk?

Averaging down (buying more shares after a price drop) lowers your average cost and break-even price, but it also increases your total exposure to that stock. It reduces the price needed to break even — it does not reduce the underlying investment risk.

Does this calculator account for fees or taxes?

No. This calculator uses only your entered purchase prices and share counts. Brokerage fees, commissions, and capital gains taxes are not included and would change your real-world cost basis and returns.

Stock Average Cost Calculator — Free Online

About this calculator

A stock average cost calculator computes cost basis per share across multiple purchases (weighted average).

Who should use it: Investors tracking positions built over time.

When to use it: Use it after each buy to refresh average cost before measuring gains.

Weighted average cost per share

Average cost = Σ(price × shares) ÷ Σ shares

Each purchase lot contributes its own cost basis (price × shares). Average cost is the total of all cost bases divided by total shares — a weighted average, not a simple average of the entered prices.

Step-by-step

Add every purchase lot
Enter the price you paid and the number of shares for each separate buy.
Let the calculator total your investment
Each lot's cost basis (price × shares) is summed into your total investment.
Divide by total shares
Total investment ÷ total shares gives your weighted average cost per share.
Add a current price (optional)
Enter today's market price to see unrealized gain or loss versus your average cost.

Worked example: Two purchases of the same stock

You buy 10 shares at $100, then later buy 15 more shares at $80, with the stock now at $95.

Lot 1 cost basis: 10 × $100 = $1,000
Lot 2 cost basis: 15 × $80 = $1,200
Total investment: $1,000 + $1,200 = $2,200 across 25 shares
Average cost: $2,200 ÷ 25 = $88.00 per share
Unrealized gain: ($95 − $88) × 25 = $175

Your average cost is $88.00 per share, with a $175 unrealized gain at a $95 current price.

How to interpret the result

Unrealized gain compares market price to average cost × shares — lots may still matter for taxes.

Key definitions

Average cost (cost basis): The weighted average price paid per share across all purchase lots.
Break-even price: The price at which selling all shares returns exactly your total investment — equal to average cost.
Unrealized gain/loss: The paper profit or loss on shares you still hold, based on the current market price.
Averaging down: Buying additional shares after a price drop to lower your average cost per share.

Common use cases

Tracking cost basis across multiple buys of the same stock
Deciding whether averaging down makes sense at a new price
Estimating unrealized profit or loss before selling
Preparing cost-basis figures for tax record-keeping

Tips

Add every lot, including reinvested dividend purchases, for an accurate average.
A lower average cost means a lower break-even price, but does not guarantee the stock will recover.
Keep your own purchase records — brokers may compute cost basis differently (e.g. FIFO or specific lot).

Common mistakes

Averaging the purchase prices instead of weighting by shares

Fix: Always weight by shares: a large lot at a low price affects the average more than a small lot at a high price.

Forgetting to include a lot, which skews the average

Fix: List every purchase of the position, even small or older ones, to get an accurate cost basis.

Treating average cost as guaranteed break-even after fees

Fix: Average cost ignores commissions and taxes — your real break-even price is usually slightly higher.

Limitations

Tax lot methods (FIFO, specific ID) can differ from average cost.
Not tax advice.