Stock split calculator
A stock split calculator works out how many shares you hold, what each is priced at, and what your cost basis per share becomes after a company multiplies or combines its shares — without changing what the position is worth.
Enter the split ratio and what you hold. You get your new share count, the adjusted share price, and the corrected cost basis for every lot — forward splits, reverse splits, and stocks that have split more than once.
The split
Enter the ratio as it was announced. A forward split gives you more shares; a reverse split gives you fewer.
ForwardEvery 1 share you own becomes 4 shares.
What you hold
One row per purchase. Cost per share is optional — leave it out if you only want the share count.
| Lot | Shares before | Shares after | Cost / share after | Total cost |
|---|---|---|---|---|
| Lot 1 | 10 | 40 | $500.00 $125.00 | $5,000.00 |
| Lot 2 | 15 | 60 | $700.00 $175.00 | $10,500.00 |
Runs in your browser. Nothing is saved or sent anywhere.
How this is calculated
A split ratio is one number in disguise. Written a:b, it means every b shares you own become a shares — so the whole adjustment turns on the factor a ÷ b. Above 1 it is a forward split; below 1 it is a reverse split. Everything on this page follows from four lines:
factor = a ÷ b shares after = shares before × factor price after = price before ÷ factor cost basis/share = cost basis/share before ÷ factor total cost basis = unchanged two splits = factor₁ × factor₂
Worked example
You bought 10 shares at $500, then 15 at $700. The stock does a 4:1 split.
- Factor:
4 ÷ 1 = 4 - First lot:
10 × 4 = 40 sharesat$500 ÷ 4 = $125 - Second lot:
15 × 4 = 60 sharesat$700 ÷ 4 = $175 - Total cost basis:
$5,000 + $10,500 = $15,500, before and after
You now hold 100 shares instead of 25. You did not make or lose anything — but every per-share number you had written down is now wrong.
Reverse stock split calculator
The calculator above handles reverse splits too — a reverse split just runs the arithmetic backwards, and the factor takes care of the direction. Enter the ratio the way it was announced, with the smaller number first.
Worked example
You hold 250 shares bought at $2.40, so your cost basis is $600. The company announces a 1-for-10 reverse split.
- Factor:
1 ÷ 10 = 0.1 - Shares:
250 × 0.1 = 25 shares - Cost basis per share:
$2.40 ÷ 0.1 = $24.00 - Total cost basis:
25 × $24.00 = $600— unchanged
If the stock was trading at $2.40 it now trades near $24.00. Ten times fewer shares, ten times the price, the same $600 of basis.
When the share count does not divide cleanly — at 1:10, 254 shares become 25.4 — the leftover is where reverse splits get awkward.
That decimal is the thing a forward split rarely raises. 25.4 shares is not a position anyone holds, so the fraction is normally paid out in cash instead. Tick cash out fractional shares and the calculator shows the whole shares you keep, roughly what the fraction is worth, and how much cost basis leaves the position along with it. Cash received in lieu of a fractional share is generally treated as a sale — what that means in your situation is a question for a tax professional.
Recent and upcoming stock splits
A few splits from this year, each taken from the company's own SEC filing. The date is the first day the shares traded, or will trade, on a split-adjusted basis. Use them to try the calculator with a real ratio, and check the filing before acting on any of them.
| Company | Ratio | Split-adjusted trading |
|---|---|---|
| Booking Holdings (BKNG) | 25-for-1 forward | April 6, 2026 |
| Alight (ALIT) | 1-for-20 reverse | July 1, 2026 |
| Cycurion (CYCU) | 1-for-8 reverse | August 28, 2026 |
| Tantech Holdings (TANH) | 1-for-50 reverse | September 4, 2026 |
| VisionWave Holdings (VWAV) | 1-for-20 reverse | September 22, 2026 (upcoming) |
Last checked: 2026-09-21. Source: each company's Form 8-K or 6-K on EDGAR. Not a complete list, and not a recommendation of any kind.
Stocks that have split more than once
Factors multiply. A 2:1 split followed by a 3:1 is a combined ×6 — an original share is now six shares, and the price you originally paid per share divides by six. Tick this stock split more than once to run both in one go.
This is where records tend to fall apart. A holding you have had for a decade may have been through two or three splits, each one silently invalidating the cost basis in your spreadsheet at the time. If the original numbers are long gone, the guide on finding the cost basis of old stock covers how to reconstruct them.
Why every lot is adjusted separately
Most split calculators ask for one share count and one price, which quietly assumes you bought everything at once. Almost nobody does. A split does not merge your purchases — it scales each of them, and they stay distinct afterwards, which matters the moment you sell part of a position and have to say which shares went. That is why the table above adjusts lot by lot rather than handing back a single blended number.
The blended figure is still useful, so the average cost per share is shown in the results too. If that is the number you are actually after, the cost basis calculator works out weighted average cost across any number of purchases, and cost basis methods explains when lot-by-lot and average-cost accounting actually differ.
The split is the easy part. Your records are the hard part.
The arithmetic on this page takes a second. What takes longer is everything that follows: the share count in your spreadsheet is wrong, the cost per share on every lot is wrong, and any formula referencing them is now quietly producing a number that looks plausible and isn't. If you hold the same stock in more than one account, that correction has to happen in more than one place, and no single broker app shows you all of them at once.
StoxDeck is where those corrected numbers can live. Enter each holding once — no brokerage login, no credentials — and it prices the whole deck live on every load, across every account, with gain and loss measured from what you actually paid. Holdings are yours to maintain, so a split is a number you update rather than a formula you repair.
Build your first deckOr try the demo deck first — it needs no signup. New to the term? What cost basis means explains it from the start.
More free tools
- Cost basis calculatorWeighted average cost per share across several purchases, plus unrealized gain.
- Stock profit calculatorProfit or loss, return on cost and break-even price on a sale, fees netted out.
- Portfolio rebalancing calculatorHow far each holding has drifted from its target weight, and what to buy or sell.
- Annualized return calculatorThe rate per year a holding has earned, from dated purchases and today's price.
- Portfolio tracker spreadsheetA free Google Sheets and Excel template with live price lookups wired in.
Common questions
How does a stock split affect your cost basis?
Your total cost basis does not change — only how it is spread across the shares. Divide your cost per share by the split factor and multiply your share count by the same number. 10 shares bought at $500 is $5,000 of basis; after a 4:1 split you hold 40 shares at $125 each, and it is still $5,000. A reverse split works the other way: after a 1:10, those 10 shares become 1 share at $5,000. The figure that has to be corrected in your records is the per-share cost, not the total.
What is the difference between a forward split and a reverse split?
A forward split gives you more shares at a proportionally lower price — 4:1 means every share becomes four. A reverse split does the opposite: 1:10 means every ten shares become one, at ten times the price. In both cases the value of the position is unchanged at the moment of the split. Enter the ratio the way it was announced and the calculator works out which direction it goes.
Does a stock split change how much your position is worth?
Not by itself. The share count and the price move by exactly offsetting amounts, so the position is worth the same immediately before and after. What happens to the price afterwards is a matter of ordinary market movement, and this calculator does not predict it — it does the arithmetic on the numbers you enter and nothing more.
What happens to a fractional share in a reverse stock split?
Reverse splits often leave a fraction of a share, and it is common for that fraction to be paid out in cash rather than held — a "cash in lieu" payment. Tick "cash out fractional shares" and the calculator shows the whole shares you keep, the approximate cash amount, and how much cost basis leaves the position with it. Cash in lieu is generally treated as a sale, which can have tax consequences depending on your circumstances — that one is worth checking with a tax professional.
How do you handle a stock that has split more than once?
Multiply the factors together. A 2:1 followed by a 3:1 is a combined 6:1, so an original share is now six and the original per-share cost is divided by six. Tick "this stock split more than once" to enter both and the calculator combines them. This is the case that catches people out on shares held for a decade or more, because each split quietly invalidated the cost basis they had written down.
Why does each lot need adjusting separately?
Because a split does not merge your purchases. If you bought at $500 and again at $700, after a 4:1 split you hold two lots at $125 and $175 — not one lot at an average. The lots stay distinct, which matters whenever you sell part of a position and have to say which shares went. If you would rather see the single blended figure, the average cost per share is shown in the results and the cost basis calculator works it out in more detail.
Does StoxDeck adjust my holdings for a split automatically?
No — StoxDeck holdings are entered and maintained by you, which is the deliberate trade-off for never asking for a brokerage login. After a split you update the share count and cost basis yourself, and this calculator gives you the numbers to enter. What StoxDeck does from there is keep the position priced live on every load, across every account, with gain and loss measured from the cost basis you entered.
Is anything I enter here private?
It never leaves your browser. The calculator does the arithmetic locally — no values are transmitted, stored, or logged, and there is no account to create before using it.
Disclaimer. This calculator is informational only and is not investment, tax, or financial advice. It does the arithmetic on figures you enter; it does not verify split ratios, and it makes no prediction about how a share price will behave after a split. StoxDeck is a portfolio-tracking tool, not a broker or an adviser. Cash received in lieu of a fractional share can have tax consequences that depend on your own circumstances — check them with a professional.