Stock Cost Basis Calculator
Average cost basis across multiple purchases.
Your Purchases
Average cost / share
How to Use This Calculator
2. Add as many lots as you need with “+ Add lot”.
Your average cost per share and total cost basis update instantly.
Calculation Method
Total cost = Σ (shares × price + commission) for every lot
This uses the average-cost method, common for mutual funds and a simple baseline for stocks.
Source: Average-cost basis method.
Two lots
100 @ $50 + 50 @ $60
$53.33 / share
Dollar-cost
3 monthly buys
Blended average
With fees
Commission added per lot
Raises basis slightly
Frequently Asked Questions
What is cost basis?
The total amount you paid for an investment, including commissions. It is used to compute capital gains when you sell.
Which cost-basis method does this use?
The average-cost method. Brokerages may also offer FIFO or specific-identification, which can change your taxable gain.
Do commissions affect cost basis?
Yes — purchase commissions increase your cost basis and reduce your taxable gain.
How do stock splits affect cost basis?
A split changes your share count and per-share basis but not your total basis. Adjust the shares accordingly.
Is this tax advice?
No. This is an educational estimate — confirm your basis with your broker and a tax professional.
Related Calculators
Related Guides
Last updated: May 24, 2026