Stock Split Calculator

A stock split calculator tells you how many shares you own and what your new per-share cost basis is after a company splits its stock — forward (more shares, lower price) or reverse (fewer shares, higher price). Per the IRS, a stock split never changes your total cost basis — only how it's divided across your (new number of) shares. Get the split ratio wrong on your own records, though, and you'll misreport your capital gain when you eventually sell: an unadjusted basis after a forward split overstates your taxable gain, and an unadjusted basis after a reverse split understates it.

Most brokers automatically adjust "covered" securities for you, so for a recent split on a normal brokerage account you often don't need to do this by hand. This calculator is for checking your broker's math, older or uncovered positions, DRIP shares purchased outside a standard brokerage, or simply understanding what happened to your position without digging through a corporate-actions notice.

Every split — forward or reverse — follows the same two-line formula:

Forward split example (per the IRS's own worked example): you own 100 shares with a $15 per-share basis, for a total basis of $1,500. In a 2-for-1 split, you now own 200 shares — but your total basis is still $1,500, reallocated to $7.50 per share.

Reverse split example: you own 1,000 shares with a $2 per-share basis ($2,000 total). In a 1-for-10 reverse split, you now own 100 shares at $20 per share — again, $2,000 total, unchanged.

Fractional shares (common in reverse splits): if the ratio doesn't divide evenly, most companies pay "cash in lieu" (CIL) for the leftover fraction instead of issuing a partial share. The IRS treats this as if you received the fractional share and immediately sold it — a small, separately taxable capital gain or loss. Example: you own 15 shares at a $10 basis ($150 total). A 1-for-4 reverse split entitles you to 3.75 shares — you get 3 whole shares plus a cash payment for the 0.75 fraction. Your new basis is $150 ÷ 3.75 = $40/share. If the company pays cash-in-lieu at $42/share, your CIL payment is 0.75 × $42 = $31.50, and your taxable gain on that fraction is $31.50 − (0.75 × $40) = $1.50 — reported on Form 8949/Schedule D even though it's a tiny amount.

1. Enter your shares and cost basis before the split. Use your original purchase confirmation or brokerage cost-basis report — the per-share price you actually paid, not today's price.
2. Enter the split ratio. Check the company's press release or your broker's corporate-actions notice for the exact ratio (e.g., "2-for-1," "1-for-10"). Enter the new-shares number and old-shares number separately — the calculator computes the ratio for you.
3. Optionally enter the current share price to see your position's current value and unrealized gain or loss at the new, split-adjusted basis.
4. Check the result against your 1099-B or broker statement if you're verifying a recent split — for covered securities, brokers are required to track and report adjusted basis themselves, so this is mainly a cross-check.

A forward split (2-for-1, 3-for-1, etc.) increases your share count and proportionally lowers the price — companies typically do this to bring a high share price back into a more "affordable" trading range, with no change to the company's actual value or your stake in it. A reverse split (1-for-10, 1-for-20) does the opposite — fewer shares at a proportionally higher price, often done by companies trying to meet a stock exchange's minimum price listing requirement. Neither event, by itself, changes what your position is worth or triggers a taxable event — only a subsequent sale, or a cash-in-lieu payment for a fractional share, does that.

• Keep every corporate-actions notice. If a stock splits multiple times over years you hold it, you need every ratio in order — a single missed split will throw off your basis on eventual sale.
• Don't confuse a split with a stock dividend. A stock dividend (e.g., "5% stock dividend") uses similar reallocation math but is a distinct corporate action with its own IRS treatment — check your 1099-DIV, not just a splits calendar.
• Report cash-in-lieu even when it's small. The IRS receives a 1099-B for it; a blank line on your return next to a broker-reported amount can trigger an automated mismatch notice.
• Reverse splits are sometimes a warning sign, not just a listing-requirement fix. They're frequently used by companies in financial distress to avoid delisting — the split itself doesn't cause the trouble, but it's worth understanding why the company needed one.