Split-Adjusted Cost Basis Calculator
Enter your original purchase and any stock splits since then to compute your adjusted per-share cost basis.
Ratio: 2 = 2-for-1 split (doubles shares), 0.1 = 1-for-10 reverse split (shares become 1/10th).
For informational purposes only — not tax advice. Does not account for wash sales, dividend reinvestment, or partial-lot purchases. Consult a tax professional or your broker's cost basis records before filing.
Frequently Asked Questions
Why does cost basis need to be adjusted for stock splits?
A stock split changes your share count but not the total value of your position — a 2-for-1 split doubles your shares and halves the price per share. Your original per-share cost basis needs to be divided by the same ratio so that basis × shares still equals what you actually paid. Skipping this understates your basis and overstates your taxable gain when you eventually sell.
How do I enter a reverse split?
Use a ratio below 1. A 1-for-10 reverse split (10 old shares become 1 new share) is entered as a ratio of 0.1 — it reduces your share count and increases your per-share cost basis proportionally.
Does a split change my total cost basis?
No. Splits only redistribute the same total dollar amount across a different number of shares — your total cost basis (shares × price) stays the same immediately before and after a split; only the per-share figure changes.