Stock average-down calculatorStock average cost calculator: combine purchase lots
Combine up to three purchase lots to find a share-weighted average cost. Buying 100 shares at $50 and 50 at $35 gives $6,750 invested in 150 shares, or $45 per share. At a market price of $40, their value is $6,000 and the unrealized loss is $750. A lower average price does not guarantee recovery and requires more capital at risk. The model excludes commissions, spreads, taxes, corporate actions, and tax-lot accounting. It is arithmetic for the lots you enter, not an investment recommendation.
Purchase lot 1
Purchase lot 2
Purchase lot 3
New average cost per share
$45
150 shares · $6,750 invested across 2 lots
Break-even price
$45
Total invested
$6,750
How to use
- 1
Add each purchase lot with its share count and price per share.
- 2
Optionally enter the current market price to see unrealized P/L.
- 3
Read your new average cost per share and break-even price.
- 4
Add a hypothetical new lot to preview its effect before you buy.
- 5
Remember: averaging down lowers break-even, it doesn't guarantee recovery.
Benefits
Break-even, precisely
Know the exact price your stock must reach for you to be whole.
Preview before you buy
Model the new lot first — see the new average before committing cash.
Multi-lot clarity
Three buys at three prices collapse into one honest average.
P/L at a glance
With a market price entered, unrealized gain/loss is instant.
How it works
- Σ shares × price
- Total invested — every lot’s shares multiplied by its buy price, added together
- total shares
- Shares from all lots combined
- Break-even
- The price the stock must reach for your position to be worth exactly what you paid
The average cost is a share-weighted mean: each lot contributes its total cost (shares × price), and the sum is divided by total shares. Lots with zero shares are ignored entirely. The break-even price equals the average cost because selling all shares at that price returns exactly the total invested — it is a property of the arithmetic, not a prediction. Unrealized P/L is marked to the market price you enter: current value minus total invested, with the percentage measured against total invested. This calculator ignores brokerage commissions, bid-ask spreads, and taxes, all of which move real-world break-even slightly above the figure shown.
Worked example
You buy 100 shares at $50 ($5,000), then the stock drops and you buy 50 more shares at $35 ($1,750). You now hold 150 shares with $6,750 invested, so your average cost falls from $50.00 to $45.00 — and the stock only needs to recover to $45.00 for you to break even.
Understanding your results
Average cost is total entered purchase cost divided by total shares. The market value and unrealized gain or loss use the market price you enter. Fees, taxes, lot accounting, and future price movement are excluded; a lower average does not remove investment risk.
Sources and methodology
These resources explain related concepts. They do not certify or endorse this calculator. See the assumptions and worked example above.
- Publication 550: Investment Income and Expenses — IRS ↗
Explains that cost-basis methods depend on the securities and account; this calculator's weighted average is not a tax-lot determination.
Frequently asked questions
Averaging down means buying additional shares below your current average cost, which lowers the share-weighted average if the added shares are included. It also increases the amount invested in that position. The calculation does not assess whether the purchase is suitable.
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Open calculatorFigures are estimates for education only — not financial advice. Results depend on the assumptions you enter. Read the full disclaimer.