A portfolio's average price compresses several purchases into one number. US cost basis records describe the amount attached to particular shares, including applicable costs and adjustments. The two figures can begin with the same purchases and still answer different questions.
This explanation uses fictional US shares and dollars. It describes records rather than recommending a tax lot method or calculating a personal tax liability.
Calculate a purchase average
Suppose an account buys 10 shares at $40 and another 10 at $60. Excluding fees, the total purchase cost is $1,000 for 20 shares. The weighted average is $50 per share.
That average is a convenient summary of the remaining position before any sale. It does not preserve the separate $40 and $60 purchase prices or their acquisition dates. A report containing only 20 shares at $50 has less information than the original two trade confirmations.
The IRS explains that average basis is not generally an available method for ordinary stock sales, apart from specified exceptions such as certain mutual fund shares and dividend reinvestment plans. A tracking average therefore cannot be assumed to determine the US tax basis of shares sold.
Purchase costs change the recorded amount
Now add a fictional $5 purchase commission to each trade. The first purchase has a $405 cost and the second a $605 cost, giving $1,010 in total. A tracker using only share price times quantity would still show $1,000.
The $10 difference is explained by the omitted purchase charges. It is not a market loss. In a complete record, the share quantity, price, transaction cost and total consideration remain separate enough to show how the amount was built.
The example does not imply that every charge from a broker changes basis in the same way. An account service fee and a cost directly associated with an acquisition are different records. The relevant tax rules determine their treatment.
Reinvestment adds another purchase
Suppose a later $100 dividend buys two additional shares at $50, with no fee. There is a $100 dividend receipt and a separate $100 acquisition. The position now has 22 shares and $1,110 of purchase cost in the fee-inclusive example.
The new shares have their own purchase date and record. Dividing $1,110 by 22 gives a summary average of about $50.45, but that calculation does not erase the three acquisitions. The dividend's income classification is also separate from the purchase it funded.
Recording only the extra shares with no acquisition amount would leave the purchase history incomplete. Recording the $100 twice as income would create a different error. The dividend reference connects the receipt to its use without merging them into one event.
Adjustments need supporting documents
A stock split changes share quantity and the amount associated with each share. In a simple two-for-one split, a lot of 10 shares becomes 20 and its total basis is spread across those 20 shares. That arithmetic does not create an extra investment contribution.
Other events can also change adjusted basis. A nondividend distribution, for example, can have a different treatment from ordinary dividend income. Transfers between brokers can leave one account screen with incomplete acquisition information until the relevant records arrive.
These events explain why a number copied from an old holdings screen can differ from a later tax document. The transaction history provides the explanation that a single average cannot contain.
FinTarget's tracking calculation
FinTarget's Trades page updates a holding's average cost after buys and uses that average for its tracking profit on a sale. It does not provide a US tax lot engine, wash sale calculation or Form 1099-B preparation.
The original confirmations, fee details, acquisition dates and adjustment notices preserve the separate evidence behind a US basis figure. A portfolio average remains useful as a summary when its purpose is labelled accurately.