Average Cost Is Weighted by the Number of Shares
The new average cost is total dollars invested across the entered lots divided by total shares acquired. A larger purchase therefore has more influence on the average than a small purchase at the same price difference.
Lowering average cost does not reduce the market risk of the position or establish that buying more is appropriate. The calculator only measures cost basis from the values entered and does not forecast the share price.
Averaging down changes your cost basis; it does not improve the quality of the investment. Buying more shares at a lower price reduces the weighted average price per share, but it also increases the amount of money exposed to whatever caused the stock to fall. Use the calculator to understand the new break-even price and total capital at risk. The decision to buy more still depends on the investment thesis, not on the fact that the average price would look lower.
Frequently Asked Questions
Why is this not a simple average of purchase prices?
Each purchase can contain a different number of shares, so the correct average is weighted by the shares in each lot.
Does a lower average cost mean the position is safer?
No. Cost basis and investment risk are different concepts; future market value can still rise or fall.
Does a lower average cost mean the investment is safer?
No. It lowers the price needed to recover the position, but it also commits more capital. If the stock continues falling, the larger position can produce a larger dollar loss.
Methodology & Related Tools
Review how RatioCalc builds, tests and updates calculator models before using an estimate for a material decision.
Accuracy review
Investing & Savings family audit: .
Site-wide consistency review: September 4, 2026.