Cost basis after a stock split

A stock split gives an investor more shares, but it does not create extra investment cost. Same total basis. The number of shares changes; the total basis generally does not. The basis per share falls in proportion to the increase in shares, which is why a split alone normally does not create taxable gain.

The per-share formula

For identical shares, divide the adjusted basis of the original lot by the number of original and new shares after the split. If a company declares a 2-for-1 split, each old share becomes two shares. The total basis assigned to that lot remains the same, so each resulting share carries half of the old per-share basis. In a 3-for-1 split, the per-share basis is one-third of the prior amount.

Worked example with one lot

An investor owns 100 shares with a total basis of $1,000, or $10 per share. In a 2-for-1 split, the investor now owns 200 shares. The total lot basis remains $1,000, and the basis per share becomes $5. If the investor later sells 40 shares for $8 each, the proceeds are $320 and the basis of those shares is $200. Before commissions and other sale adjustments, the gain is $120. The split itself was not the taxable sale; the later disposition is where gain or loss is calculated.

Keep different purchase lots separate

Investors often buy the same stock on different dates and at different prices. Keep lots. Suppose one lot is 100 shares with a $1,000 basis and a second lot is 100 shares with a $1,200 basis. After a 2-for-1 split, each lot becomes 200 shares. The first lot retains $1,000 total basis, or $5 per share; the second retains $1,200, or $6 per share. Combining them too early can obscure holding periods and which basis applies to a later sale.

When shares are sold, specific identification may determine which lot was disposed of if the taxpayer and broker adequately identify the shares. If shares cannot be identified, applicable tax rules and broker records determine the default method. A planner should reconcile the client's own records with the brokerage's adjusted basis reporting.

Fractional shares and corporate actions

A split can leave a fractional share when the exchange ratio does not produce a whole number of shares. A company may pay cash instead of issuing the fraction. The cash-in-lieu transaction is separate from merely adjusting the basis per share: determine the basis allocable to the fractional interest and compare it with the cash proceeds under the applicable reporting rules.

Do not assume every distribution of additional shares is a simple split. A nontaxable stock dividend of identical shares generally spreads the existing basis across old and new shares. A distribution that is taxable, involves nonidentical stock, or forms part of a reorganization can require a different basis treatment. Use the corporation's and broker's corporate-action information and the relevant tax guidance.

What changes and what does not

Exam calculation sequence

1. Identify the split ratio and the number of post-split shares in each original lot.
2. Carry forward the adjusted total basis for each lot unless the facts describe a different corporate action.
3. Divide each lot's basis by its post-split share count to find per-share basis.
4. For shares sold, compare proceeds with the basis of the identified shares and account for the holding period.
5. Treat cash received for a fractional share as a separate disposition question.

Fewer per share. The clean check is total basis: immediately after a basic split, adding the adjusted basis of all resulting shares should equal the original lot's adjusted basis. If your calculation changes the investor's total basis without another taxable or basis-adjusting event, recheck the split ratio and lot arithmetic.

Allocate the same total basis across more shares

A conventional stock split changes share count and per-share basis but generally does not by itself create taxable gain or loss. Multiply the pre-split shares by the split ratio to determine the new share count, then divide the lot’s adjusted total basis by that new count. If 100 shares with $4,000 basis split two-for-one, the investor now has 200 shares with a $20 basis per share; total basis remains $4,000.

For a reverse split, fewer shares represent the same aggregate basis before any cash paid for a fractional share. If a reverse split produces a fractional entitlement that the broker cashes out, allocate basis to the fractional share and calculate any resulting gain or loss. Do not simply round shares and keep the old per-share basis.

Keep each lot and tax date intact

When shares were acquired in multiple lots, apply the split ratio to each lot separately. The aggregate total basis may be the same, but acquisition dates and per-share basis differ. A sale of only some post-split shares still requires lot identification. A split ordinarily does not restart the holding period; the holding period of the old shares carries to the replacement shares under the applicable rules.

Fractional shares, spin-offs, mergers, return-of-capital distributions, and stock dividends can have different tax treatment from a simple split. A “stock dividend” may be nontaxable or taxable depending on its structure and cash/property alternatives. Read the issuer notice and broker corporate-action statement. Correct Form 1099-B basis when broker records do not reflect the proper corporate-action adjustment.

A recordkeeping checklist

Keep the purchase confirmation, lot-level basis, split ratio, effective date, share count before and after, and broker statement showing the adjustment. If shares move between custodians, verify that the receiving firm carries the correct split-adjusted basis. A basis error can surface years later when the investor sells or transfers shares as a gift.

For exam problems, calculate total basis first, adjust share count by the ratio, divide total basis by new shares, and preserve the acquisition date. If the prompt includes cash for a fractional share, isolate that disposition. The common mistakes are doubling total basis after a two-for-one split, failing to halve per-share basis, and resetting the holding period.