A stock split doesn’t change your total dollar cost basis — it only spreads that same basis across more (or fewer, for a reverse split) shares.
How it works
The original share count times the split ratio gives the new number of shares owned. Dividing the unchanged total cost basis by that new share count gives the new cost basis per share.
What this does not include
This handles a standard forward split — a reverse split (a split ratio below 1) uses the identical math but produces fewer shares at a higher basis per share, worth noting since it’s a much less common scenario for most investors.
How to use this calculator
- Enter your original shares owned, total original cost basis, and the split ratio.
Frequently asked questions
Does a stock split trigger a taxable event?
No — a stock split itself is not a taxable event; the total cost basis and holding period carry over unchanged, just redistributed across a different number of shares.
What if I bought shares at different times before a split?
Each purchase lot’s cost basis is adjusted separately by the same split ratio, since each lot may have a different original price per share and holding period.
How does a reverse split affect cost basis?
The same formula applies with a split ratio below 1 — fewer shares result, each carrying a proportionally higher cost basis, with the total dollar basis still unchanged.
