Stock Split Calculator

A stock split is a corporate action in which a company increases or decreases the number of its outstanding shares by issuing new shares to existing shareholders in proportion to their current holdings. The total market capitalization — and therefore the underlying value of every shareholder's position — remains unchanged immediately after a split. What changes is the number of shares held and the price per share, which move in opposite directions to keep total value constant.

Companies most commonly announce forward splits when their share price has risen so high that smaller retail investors find it expensive or psychologically daunting to purchase even a single share. By splitting the stock, the company lowers the per-share price to a more accessible range while rewarding existing shareholders with a proportionally larger number of shares. Apple, Tesla, and Amazon have all executed high-profile forward splits for exactly this reason over the past decade.

Reverse splits, on the other hand, reduce the number of outstanding shares and raise the per-share price. These are frequently used by companies whose stock has fallen to very low prices — sometimes approaching penny-stock territory — to meet minimum price requirements for continued listing on major exchanges such as the NYSE or Nasdaq, or to improve institutional investor perception of the stock.

Understanding how splits work is essential for any investor. While the fundamental economics of your position do not change at the moment of the split, your cost basis per share, your average purchase price, and any limit orders you have placed must all be adjusted. Failing to account for these adjustments can distort your gain/loss calculations and lead to incorrect tax reporting.

This stock split calculator takes five inputs — your current share count, the current price per share, your original purchase price (cost basis), the split type, and the split ratio — and computes your adjusted share count, new price per share, updated cost basis, and both pre-split and post-split portfolio values.

The split ratio is expressed as New-for-Old. In a 4-for-1 forward split you receive four new shares for every one old share you held. In a 4-for-1 reverse split the formula inverts: for every four old shares you held you receive one new share. The calculator handles both directions automatically depending on which split type you select.

Once the split ratio is determined, all other results follow directly. The new share count equals your current shares multiplied by the effective split ratio. The new price equals the current price divided by the same ratio. Because shares and price scale in opposite directions by the same factor, the product — your total position value — is preserved exactly. The cost basis per share is adjusted in the same way as the price, ensuring that your total cost basis is also preserved.

The share change field shows you at a glance whether you gained or lost shares. For forward splits this number is positive; for reverse splits it is negative. The value change should always be zero (or negligibly close to zero due to floating-point arithmetic), confirming that the split itself creates no wealth — it simply repackages your existing holdings.

The stock split calculator uses the following formulas, which mirror the code exactly. The effective split ratio differs depending on whether you are computing a forward or reverse split, but the downstream formulas for shares, price, and cost basis are identical once the ratio is known.

Forward split — ratioFirst is the "New" number, ratioSecond is the "Old" number (e.g., 4-for-1 means ratioFirst = 4, ratioSecond = 1):

• splitRatio = ratioFirst ÷ ratioSecond
• newShares = currentShares × splitRatio
• newPrice = currentPrice ÷ splitRatio
• newCostBasis = purchasePrice ÷ splitRatio

Reverse split — the ratio formula inverts (the same ratio inputs now divide rather than multiply share count):

• splitRatio = ratioSecond ÷ ratioFirst
• newShares = currentShares × splitRatio
• newPrice = currentPrice ÷ splitRatio
• newCostBasis = purchasePrice ÷ splitRatio

Portfolio value is preserved in both cases: preValue = currentShares × currentPrice = newShares × newPrice = postValue.

One of the most important practical consequences of a stock split is the adjustment to your cost basis. Your cost basis is the original price you paid per share, and it is used to calculate your capital gain or loss when you eventually sell. After a stock split, the IRS and equivalent tax authorities in other countries require that you adjust your per-share cost basis proportionally so that the total cost basis of your position is unchanged.

For a 2-for-1 forward split, a share originally purchased at $200 becomes two shares with a cost basis of $100 each. Your total original investment was $200 and remains $200 — it is simply now spread over two shares rather than one. If you later sell both shares at $150 each ($300 total), your capital gain is $300 − $200 = $100.

For a reverse split, the adjustment runs in the opposite direction. If you owned 1,000 shares with a cost basis of $5 each (total cost: $5,000) and the company executes a 4-for-1 reverse split in this calculator's convention (ratioFirst=4, ratioSecond=1, reverse type), your new share count is 250 and your new cost basis per share is $20 — still totaling $5,000.

It is critical to update your brokerage records and any tax tracking software after a split. Most modern brokerages adjust cost basis automatically, but you should always verify, especially for shares held in accounts where the brokerage did not have the original purchase information. Errors in cost basis reporting can lead to overpaying capital gains tax.

Stock splits are generally not taxable events at the time they occur. You do not owe tax simply because your share count changed. Taxes are triggered only when you sell shares at a gain. However, the adjusted cost basis directly determines how large that gain will be, so getting it right matters.

While the mathematics of forward and reverse splits are mirror images of each other, they carry very different signals to the market and very different implications for investors.

Forward splits are almost universally associated with strong share price performance. A company typically splits its stock only after the price has risen substantially — often to hundreds or thousands of dollars per share. The split itself is neutral in terms of value, but the announcement is often interpreted as a bullish signal: management believes the current momentum will continue and wants to keep the stock accessible to a wide range of investors. Historically, stocks that announce forward splits have tended to outperform the market in the months following the announcement, though this effect is debated and should not be the sole reason to hold or buy a stock.

Reverse splits are more complex in their market signal. They are most often executed by companies in financial difficulty whose stock price has fallen below thresholds required by exchanges — typically $1.00 on the NYSE or Nasdaq. While the reverse split itself does not destroy value, the underlying reason for it often reflects business weakness. Statistically, reverse splits are associated with continued underperformance in many studies, though this is not universal: some companies have used reverse splits as part of a genuine restructuring and subsequently recovered.

As an investor, you should treat a reverse split announcement as a prompt to re-evaluate the investment thesis — not as a reason to panic-sell, but also not as something to ignore. Understanding whether the split is motivated by exchange compliance, institutional optics, or strategic restructuring is key to making an informed decision about whether to hold, add, or reduce your position.

Examining real corporate split history helps anchor the abstract mathematics in concrete experience. Some of the most discussed stock splits in recent memory include:

• Apple (AAPL) — 4-for-1 (August 2020): Apple's fifth split in its history brought the share price from roughly $500 down to around $125. Shareholders who held 100 shares before the split woke up with 400 shares, each worth one-quarter of the prior price. The cost basis per share was similarly divided by four.
• Tesla (TSLA) — 5-for-1 (August 2020): Executed the same month as Apple's split, Tesla brought its price from roughly $2,250 down to $450. Each 5-for-1 split multiplied share counts by five and divided per-share cost basis by five.
• Amazon (AMZN) — 20-for-1 (June 2022): One of the largest split ratios in recent history brought Amazon's share price from over $2,000 down to around $120, dramatically improving retail accessibility.
• Citigroup (C) — 1-for-10 reverse split (May 2011): After a severe decline during the financial crisis, Citigroup consolidated every 10 shares into 1, raising the per-share price from roughly $4.50 to $45 to improve institutional credibility and exchange compliance.

Each of these examples illustrates the same mathematical principle: the total market value of all outstanding shares was unchanged immediately after the split. What changed was how that value was packaged — into more, cheaper shares for forward splits and fewer, more expensive shares for reverse splits.