Buy the same stock at several different prices over time, and “what did I actually pay per share” stops being any single one of those prices — it’s a share-weighted average across every purchase.
How it works
Each lot’s shares are multiplied by its price and summed, then divided by the total shares across all lots. A bigger lot counts for more in the average, exactly as it does in the real cost basis — a plain average of the prices paid would treat a 5-share lot the same as a 500-share one, which is not what actually happened.
The break-even price
The average cost is also the break-even price: sell every share at exactly that price and the position is a wash, before fees or tax. It’s the number “am I up or down overall” actually has to be measured against, not the most recent price paid.
How to use this calculator
- List each purchase as shares@price — for example, “10@50, 15@45”.
- Optionally enter the current share price to see unrealised gain or loss.
Frequently asked questions
Is this the same as what my broker calls “cost basis”?
Conceptually yes for the average-cost method, though your broker’s tax reporting may use a different method (like specific-lot identification) that produces a different taxable gain even on the identical shares.
Does “averaging down” actually lower my risk?
It lowers your average cost and therefore your break-even price, but it also means more money is committed to a position that has already fallen — a decision worth making deliberately, not automatically.
Does this account for dividends or fees?
No — it’s the raw share-weighted price paid. Dividends and trading fees would adjust the true economic cost basis further.
What if I sold some shares along the way?
This calculator assumes only purchases, no sales — a sale would need its own accounting method (average cost, FIFO, or specific lot) to determine which shares’ cost basis it reduces.
