If you’ve bought shares of the same stock or ETF at different prices over time, your average cost basis is what you actually paid per share on average. It’s the number you need for tracking gains, losses, and taxes. Add each purchase below to calculate it.
How It’s Calculated
Average Cost Basis = Total Amount Invested ÷ Total Shares Owned. For example, if you bought 10 shares at $45 and later 5 shares at $52.50, you’ve spent $712.50 for 15 shares — an average cost basis of $47.50 per share.
Why Average Cost Basis Matters
Your cost basis is what you compare a stock’s current price against to see your actual gain or loss, and it’s also what tax reporting is based on when you sell. This calculator gives a simple average — note that if you sell only part of a position, your broker may use a different accounting method (like FIFO or specific-lot identification), so check your brokerage statements for the exact figures used at tax time.