Stock Split & Reverse Split Calculator

Understanding the stock split calculator — concept and principle

A stock split changes the number of shares you own and the price per share without changing the total value of your holding. In a forward split such as 4-for-1, each share becomes four; in a reverse split such as 1-for-10, ten shares become one. This calculator shows your new share count, the new average cost per share, your unchanged total cost basis and any fractional share left over.

How it is calculated

For a split of a new shares for every b old shares:

• Ratio = a ÷ b.
• New shares = shares held × ratio (whole shares shown; any remainder is fractional).
• New average cost = old average cost ÷ ratio.
• Total cost basis = shares held × old average cost, which does not change.

The principle

A split is like cutting a pizza into more slices: there are more pieces, but the same amount of pizza. The company's market value, your ownership percentage and your total cost stay the same, so the share price adjusts by the inverse of the ratio. Companies split to make shares easier to trade at a lower price, and do reverse splits to raise a low share price, often to meet an exchange's minimum price rules. When a reverse split leaves you with a fraction of a share, it is usually paid out in cash.

Worked example

Default inputs: 100 shares at an average cost of $300, 4-for-1 split.

1. Ratio: 4 ÷ 1 = 4.
2. New shares: 100 × 4 = 400.
3. New average cost: 300 ÷ 4 = $75.00.
4. Total cost basis: 100 × 300 = $30,000.00 (unchanged).
5. Fractional shares: 0.

How to read the result

After the split, compare your account's share count and cost per share with the result; brokers update these automatically, but checking helps catch errors before you sell. A forward split is not a gain and a reverse split is not a loss. Any cash paid for fractional shares is a small sale that may have tax consequences.

Common mistakes

• Thinking a split makes the shares cheaper in value terms.
• Entering the ratio backwards for a reverse split (1-for-10 means a = 1, b = 10).
• Forgetting to adjust historical prices and cost basis when comparing charts.
• Overlooking cash paid for fractional shares.

Frequently asked questions

What happens to 100 shares at $300 in a 4-for-1 split?

You get 400 shares with a cost of $75 each; the total stays $30,000.

Is a stock split taxable?

In many tax systems a split itself is not a taxable event, but cash for fractional shares may be.

Is a reverse split a bad sign?

It is often used when a share price has fallen very low, but the split itself does not change the company's value.

Sources: U.S. Securities and Exchange Commission, Investor.gov glossary, "Stock Split" · IRS Publication 550, Investment Income and Expenses (stock splits and basis)