Cost Basis

What Is Cost Basis?

Cost basis is the amount an investor is treated as having paid for an asset. In most cases it starts with the purchase price and adds certain costs tied to the acquisition, such as commissions or transfer fees. It matters most at the time of sale, because the difference between the sale proceeds and the cost basis determines the capital gain or loss that must be reported for tax purposes.

How It Is Calculated

For a simple purchase of shares, the basis is often calculated as:

Cost basis = purchase price + acquisition costs

If an investor buys 100 shares at 50 per share and pays a 10 commission, the basis is 5,010. If those shares later sell for 6,000, the taxable gain is 990 (before any adjustments for holding period or wash sale rules).

Basis can change over time. Events such as stock splits, return of capital distributions, and certain corporate actions can require the basis to be adjusted up or down. Reinvested dividends also add to basis, since they are treated as new purchases.

Why It Matters to Investors

Accurate cost basis records help investors report gains and losses correctly and avoid paying more tax than necessary. Brokerages in many countries now track and report cost basis for covered securities, but investors who hold assets bought in multiple lots, received as gifts or inheritances, or acquired before reporting rules took effect may need to keep their own records.

Methods for choosing which shares are sold, such as first-in, first-out (FIFO), specific identification, or average cost, can produce different gains from the same trades. Understanding the method in use helps investors anticipate their tax results.

Cost basis rules vary by country and by the type of asset, so investors often consult official tax guidance or a qualified tax professional when questions arise.