Stock Price Calculator — Average Cost Basis
Calculate your average stock purchase price across multiple buys, see your total cost basis, current gain or loss, and break-even price.
About the Stock Price Calculator — Average Cost Basis
When you buy a stock at different prices over time — a strategy called dollar-cost averaging — your true cost per share is the weighted average across all purchases. This stock price calculator handles any number of purchase lots and instantly shows your average cost basis, total invested, and current gain or loss.
Knowing your average cost basis is essential for tax reporting (Schedule D / Form 8949), evaluating whether to hold or sell, and understanding your actual return on investment. Add your current price to see real-time P&L against your position.
What is average cost basis and why does it matter?
Your cost basis is what you paid for your shares — the starting point for calculating taxable gain or loss when you sell. If you buy 10 shares at $100 and 10 more at $120, your average cost basis is $110/share and total basis is $2,200. If you sell at $130, your taxable gain is $20/share × 20 shares = $400, not $30 × 10 + $10 × 10. Using weighted average cost is one of the IRS-approved accounting methods (along with FIFO and specific identification).
Dollar-cost averaging: lower risk, smoothed entry
Dollar-cost averaging (DCA) means investing fixed amounts at regular intervals regardless of price. When prices are low, your fixed amount buys more shares; when high, fewer. Over time this lowers your average cost basis compared to a large lump-sum at the wrong time. The strategy works best for long-term investors who aren't trying to time the market. Our calculator helps you track your evolving average as you add to positions.
Break-even price: when your position turns profitable
Your break-even price equals your total cost basis divided by total shares — identical to the average cost basis in most cases. If your current market price is below break-even, you're in an unrealized loss. Above it, you're in profit. Some investors use break-even as a decision point for adding more shares or setting stop-loss orders.