Average Down / Cost Basis Calculator

How it works

What this calculator does

When you buy more of a position you already hold at a lower price, your average cost per share changes. This tool blends every purchase — share counts, prices and fees — into a single weighted-average cost basis and the price at which the combined position breaks even.

The formula

It uses a units-in model: you enter the number of shares bought at each price.

avg cost = Σ(quantity × price + fee) / Σ quantity

The fee-inclusive break-even price is the same figure once commissions are spread across all shares held.

Worked example

You bought 100 shares at $50, then add 100 shares at $30, paying $5 commission on each order. Total cost is (100×50 + 5) + (100×30 + 5) = 5005 + 3005 = 8010 across 200 shares, so your average cost is 8010 / 200 = $40.05. The stock now only needs to reach $40.05, not the original $50, to break even.

What it deliberately does not do

It will not tell you whether averaging down is wise — adding to a losing position increases your exposure if the price keeps falling. It uses the prices you type, not live market data, and it is for education, not investment advice.