Stock Average Cost Calculator
The cost-weighted average price across multiple purchases, plus unrealized gain or loss.
Purchase 1
Purchase 2
Purchase 3
Every calculation runs locally in your browser. This computes a cost basis from what you enter — it doesn't account for wash sales, dividends reinvested, or brokerage fees. Not financial advice.
Average cost per share
$117
across 15 shares, $1,750 total cost
Unrealized gain
$350
20% at current value $2,100
For informational and educational purposes only — not financial, investment, or tax advice, and not a substitute for consulting a qualified professional about your specific situation. TrueMeasureKit is not liable for decisions made based on these results. See our Terms of Service.
Why the average is weighted, not just averaged
Averaging the per-share prices directly would treat a 100-share purchase the same as a 10-share purchase — this instead weights each lot by how many shares it contributed, dividing total dollars spent by total shares owned. That's the same cost-basis math brokerages use for tax reporting, and it's why "buying the dip" (adding shares at a lower price) pulls the average down more than a same-size purchase at a higher price would pull it up.
Frequently asked questions
Why does average cost matter for taxes?
Your average (or specific-lot) cost basis determines the capital gain or loss you report when you sell. Getting it right matters for accurate tax reporting, though brokers typically track official cost basis separately.
Is 'averaging down' on a losing position a good strategy?
It lowers your average cost basis, which reduces the gain needed to break even, but it also increases your total exposure to a stock that's currently declining — this calculator shows the math, not whether it's a good decision for your situation.