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Investment Cost Basis: Purchases, Reinvestments and Adjustments

Cost basis is the tax starting value used with proceeds and adjustments to determine gain or loss. Purchase costs, reinvested distributions, nondividend distributions, wash sales, splits, gifts and inheritances can change or complicate it. Market value measures what an investment is worth now; it is not basis, and a…

Timeline

  1. 2026-10-12: Current IRS basis publications and securities-reporting instructions were rechecked.
  2. 2026-10-11: Primary and authoritative sources were reviewed for this guide.

Cost basis is the tax starting value assigned to property. For an ordinary investment purchase, it is generally the purchase price plus acquisition costs that the tax rules include in basis. When the investment is sold, the owner compares amount realized with adjusted basis to compute gain or loss. Current market value answers a different question: what someone may pay now. A holding can have a high market value and low basis, or the reverse, because basis records history rather than present price. [1][2][3]

Each purchase normally creates a tax lot with its own date, quantity and per-share basis. Automatic investments and dividend reinvestment plans create additional lots, including fractional shares. A cash distribution used to buy more shares is not “free” basis: IRS Publication 550 says the amount reinvested is the original basis of those new shares, even for certain exempt-interest distributions. Losing small reinvestment records can overstate gain when a long-held position is later sold. [2]

Adjusted basis changes after acquisition. A stock split generally reallocates the existing total basis across a different number of shares rather than creating new economic cost. A nondividend distribution can reduce basis; once applicable basis reaches zero, later distributions may have different tax treatment. A wash-sale loss ordinarily increases replacement-property basis, postponing the loss. Reorganizations, return of capital, fees and undistributed fund gains can also require adjustments documented by issuer notices and tax forms. [1][2][3]

Gifted and inherited property do not automatically take the recipient’s market value on the transfer date. Gift basis can depend on the donor’s adjusted basis, gift tax and whether the recipient later has a gain or loss. Inherited-property basis commonly uses date-of-death fair market value but has exceptions and special elections. Corporate actions and transfers between spouses can add more rules. Obtain the prior owner’s records and professional help instead of inventing a value from an old price chart. [1]

When identical shares were acquired at different times, the disposal method decides which basis enters the sale. Adequate specific identification requires telling the broker which shares to sell and receiving confirmation. If shares cannot be identified, first-in, first-out rules may apply. Eligible mutual-fund or dividend-reinvestment shares may use average basis under detailed conditions. Changing an average-basis election can be restricted after dispositions, so the method should be understood before the first sale. [2][3]

Broker reporting is helpful but not conclusive. Form 1099-B may report proceeds, basis, holding period and wash-sale adjustments for covered securities. Older or noncovered shares may lack reported basis, and a broker may not see purchases at another firm, spouse activity or pre-transfer adjustments. Account transfers can carry incomplete data. The taxpayer remains responsible for the correct return and may need Form 8949 codes and adjustments when the statement’s basis does not match substantiated records. [3][4]

A good basis file preserves trade confirmations, reinvestment statements, transfer records, Forms 1099-B and 8937, issuer action notices and prior-return schedules. Reconcile share counts after splits, mergers and transfers. Track basis separately from unrealized performance and label whether a figure is original, adjusted, reported to the IRS or estimated. If records are missing, request archives from the broker, fund and issuer before selling; a plausible estimate without support may not survive examination. [1][2][3][4]

Before a sale, list every available lot with acquisition date, adjusted basis, unrealized result and holding-period character. Confirm the intended identification with the broker and retain its response, then review the final confirmation and tax form. Do not choose a lot solely for the smallest current tax bill without considering wash sales, portfolio risk and future rates. Accurate basis prevents both overpaying tax by understating cost and underreporting gain by claiming adjustments that cannot be supported. [1][2][3][4]

Sources

  1. Internal Revenue Service — Publication 551: Basis of Assets
  2. Internal Revenue Service — Publication 550: Investment Income and Expenses
  3. Internal Revenue Service — Instructions for Form 8949
  4. Internal Revenue Service — Instructions for Form 1099-B (2026)

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