# Stock split

Stock split is a corporate action that increases the number of shares by a stated ratio while reducing the price per share proportionally, leaving an investor’s ownership percentage unchanged.

## What it means

A company can divide each existing share into multiple shares. In a 2-for-1 stock split, for example, a shareholder receives two shares for every one previously held. The shareholder owns more shares, but the split alone does not increase their ownership percentage or the total value of their holding.

The price per share adjusts in the opposite direction to the share count. Actual trading prices can then move for other reasons, including changes in demand and new information. A split is different from a reverse stock split, which combines shares into a smaller number.

The company announces a split ratio and an effective date. A broker typically updates eligible holdings to reflect the corporate action. Confirm the applicable dates and how your broker handles open orders rather than assuming every account screen updates at the same moment.

## How it is calculated

For an `A-for-B` split, where A and B are positive numbers:

- `New shares = Old shares × (A ÷ B)`
- `Theoretical post-split price per share = Pre-split price per share × (B ÷ A)`
- `Theoretical holding value = New shares × Theoretical post-split price per share = Old shares × Pre-split price per share`

**Old shares** is the number held before the split, measured in shares. **New shares** is the split-adjusted number, also measured in shares. **A** is the number of post-split shares corresponding to **B** pre-split shares; the ratio has no units. **Pre-split price per share** and **theoretical post-split price per share** are measured in US$ per share in this example. **Theoretical holding value** is measured in US$.

These formulas describe the mechanical adjustment, not a guaranteed market price or account value. The ownership percentage remains the same if all shareholders’ holdings adjust proportionally; rounding or cash-in-lieu treatment may matter when a split produces fractional shares.

## Worked example

Suppose, hypothetically, an investor owns 10 shares priced at US$100 each immediately before a 2-for-1 split.

1. **Calculate the original holding value:** 10 shares × US$100 per share = US$1,000.
2. **Adjust the share count:** 10 shares × (2 ÷ 1) = 20 shares.
3. **Calculate the theoretical adjusted price:** US$100 per share × (1 ÷ 2) = US$50 per share.
4. **Check the theoretical value:** 20 shares × US$50 per share = US$1,000.

The investor has 20 shares instead of 10, but the split itself creates no US$1,000-to-US$2,000 gain. If the shares trade at another price afterward, the holding’s market value changes accordingly.

## Why it matters before you trade

A split can make an old price, share quantity or order size misleading. For example, a plan to purchase 10 shares before a 2-for-1 split is not the same share-count instruction as a plan to purchase 10 shares afterward. If the aim is a particular dollar exposure, recalculate the quantity using the current quote, while accounting for execution costs and the possibility that the price moves.

Before placing an order near a split, verify that quotes and historical charts are displayed on a consistent split-adjusted basis. Check any open limit or stop orders with your broker: adjustment and cancellation practices can depend on the broker, market and order type. A split also does not, by itself, make the underlying company worth more or make execution costs disappear.

## How PreTrAIde uses it

The product facts do not state that PreTrAIde separately detects stock splits or adjusts split-affected orders. A pre-trade analysis in PreTrAIde starts with the basket the user builds, including each stock’s side, quantity and execution conditions. It returns a decision-support verdict and reasoning before the user places any order; PreTrAIde does not place orders. Around a split, the user should check that basket quantities and execution conditions reflect the intended share count and current market information.

## Common mistakes

- **Treating a lower split-adjusted price as a discount:** The company has more shares outstanding, so price per share alone is not a measure of value.
- **Comparing unmatched prices:** A pre-split price and a post-split price need a consistent share basis before calculating a return.
- **Assuming every order adjusts automatically:** Confirm the status and terms of open orders with the broker.
- **Ignoring fractional-share treatment:** Some ratios can produce fractions, which may be handled differently across brokers.

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Source: https://pretraide.com/glossary/stock-split