Stock splits explained: share count, price, cost basis and fractional shares

A stock split changes the number of shares outstanding and the amount represented by each share. Investor.gov defines a split as an increase in share count without a change in shareholders’ equity; its example of a 2-for-1 split doubles the shares while halving the per-share price (Investor.gov: Stock Split). The same arithmetic works in reverse for a reverse split.

What happens to your shares in a 4-for-1 stock split?

You get four shares for each one you held, and the theoretical price of each falls to a quarter. Suppose you own 100 shares priced at $100 each. The position is worth $10,000 before a 4-for-1 forward split. The split arithmetic gives you 400 shares at a theoretical $25 each:

new shares = old shares × (new ÷ old)

new per-share price = old per-share price × (old ÷ new)

The position remains $10,000 in this calculation. The market price after trading begins can move for other reasons, so this arithmetic does not predict the stock’s return.

What happens to cost basis after a stock split?

The total stays the same and the per-share figure is divided across the new share count. If the 100 shares had a total basis of $5,000, the total remains $5,000 solely due to the split. The per-share basis changes from $50 to $12.50 because the same total is spread over four times as many shares. The IRS confirms that a split does not change overall basis, but does change per-share basis, and says the shares received in a split are not income until sold (IRS: Stocks (options, splits, traders) 7). IRS Publication 550 gives the same arithmetic for a split paid in identical stock: divide the adjusted basis of the old shares by the number of old and new shares together. When you later sell, the sale’s basis and gain also depend on the lots sold and your applicable tax rules. The cost-basis calculator handles lot-level sale calculations; the stock split calculator handles the share and per-share adjustments.

A reverse split works the same way in the opposite direction. In a 1-for-10 reverse split, 100 shares become 10; the theoretical per-share price and per-share basis each multiply by 10, while total position value and total basis are unchanged by the ratio math alone.

What happens to fractional shares in a split?

It depends on the corporate action and your broker. Some ratios can produce a fractional share. The treatment depends on the corporate action and broker: an account may retain a fractional share or receive cash in lieu. If cash is paid, check the broker’s corporate-action notice and tax statement to understand how it was recorded. Do not assume every broker handles fractions the same way.

Does a stock split make my investment more valuable?

Not by the split arithmetic alone. Share count rises or falls as the per-share price adjusts in the opposite direction. The market price can subsequently change, but that is a market move, not value created by the ratio itself.

Does a split change my total cost basis?

The split changes the per-share basis to reflect the adjusted share count; it does not by itself change the total amount originally paid. Keep the broker’s updated lot records and confirm any later sale reporting against your own tax situation.

For one-off numbers, use the stock split and reverse split calculator. This guide is educational and not tax or investment advice.