Investment Cost Basis on Your US Tax Return: 1099-B, Form 8949 and Schedule D
Reconcile investment sales to Form 1099-B before filing: covered and noncovered basis, reinvested distributions, bond adjustments, wash sales and Form 8949 codes.
US investors report gains and losses using adjusted cost basis, not simply the amount shown as profit in a brokerage account. Before filing Form 1040, reconcile your investment sales to Form 1099-B, correct missing or inaccurate basis and carry the results through Form 8949 and Schedule D.
This guide is for individuals reporting sales of stocks, bonds and mutual funds on a US federal income tax return, principally from taxable investment accounts. It focuses on the records and IRS reporting steps that affect the return, rather than every rule governing how securities acquire their basis.
Start with Form 1099-B and your transaction history
A US broker generally reports sales proceeds on Form 1099-B. For covered securities, it also reports cost or other basis to the IRS. For noncovered holdings, such as certain older investments, the basis may be blank or shown without being reported to the IRS. A missing figure is not evidence that the investment cost nothing.
Match every disposal to its purchase lot, acquisition date, sale date and any subsequent adjustment. Check whether the broker received complete basis records when securities were moved from another account or broker. Even when a broker reports basis, you remain responsible for the amounts on your return.
Correct stock basis before calculating a gain or loss
Purchased shares generally begin with what you paid, including qualifying acquisition costs such as purchase commissions. A stock split usually changes the number of shares and the per-share basis, not the aggregate basis. Nontaxable distributions classified as a return of capital generally reduce basis; amounts beyond zero basis may produce capital gain.
Do not assume a brokerage statement captures every corporate action. Transfers, mergers, spin-offs, gifts and inherited holdings can require supporting documents. Employer-provided shares can also need special care: where share value was already included in taxable compensation, the basis calculation should reflect the applicable compensation treatment rather than treating the shares as acquired for nothing.
Include reinvested mutual fund distributions
When a dividend or capital gain distribution buys additional fund shares in a taxable account, the reinvestment generally creates another purchase lot with its own basis and acquisition date. Failing to include those shares can overstate the gain when the holding is eventually sold. Check Form 1099-DIV against the fund's reinvestment history; the distribution and the later sale are separate tax events.
For identifying shares sold, specific identification and first-in, first-out (FIFO) are common methods. An average-basis method may be available for eligible mutual fund or dividend-reinvestment-plan shares, subject to the relevant requirements and elections. It is not a general option for ordinary shares. Confirm the method applied by the fund before calculating the amount to enter on your return.
Reconcile bond basis with interest reporting
A bond's reported sale proceeds do not always translate entirely into a capital gain. Original issue discount (OID) included in income generally increases adjusted basis. Bond premium amortised under the applicable rules generally reduces basis, while accrued market discount may need ordinary-income treatment on sale or redemption. An election to include market discount currently can affect the basis calculation.
Compare the broker's bond trade history with Forms 1099-INT, 1099-OID and 1099-B, as applicable. If you bought a bond between interest dates, separately identify accrued interest paid to the seller. These checks help prevent interest income being included again as a capital gain or an adjustment being counted twice.
Check wash sales beyond one brokerage account
Selling shares at a loss and acquiring substantially identical securities within 30 days before or after the sale can trigger the US wash-sale rule. For an ordinary taxable replacement purchase, the disallowed loss is generally added to the replacement holding's basis and its holding period is adjusted. Form 1099-B may show a wash-sale adjustment, but the broker may not identify purchases in other accounts or purchases made by your spouse.
An important exception applies where the replacement securities are acquired for an IRA or Roth IRA: the disallowed loss is not added to the IRA holding's basis. Do not describe every wash-sale loss as merely deferred. Where reporting on Form 8949 is required, the disallowed loss is generally shown using adjustment code W.
How to correct an incorrect Form 1099-B basis
Suppose your Form 1099-B shows sale proceeds of $7,400 and a basis of $5,800, but purchase and transfer records substantiate a basis of $6,300. Your correct gain is $1,100, not $1,600. The reporting treatment depends on whether the broker reported its basis figure to the IRS.
If the incorrect $5,800 basis was reported to the IRS, the Form 8949 instructions generally require that figure in column (e), code B in column (f), and a negative $500 adjustment in column (g). This produces the correct $1,100 gain in column (h). If the basis was not reported to the IRS, enter the correct $6,300 in column (e), following the applicable noncovered-security instructions. Retain the evidence supporting the correction.
Wrong basis means you overpay tax you never owed.
Get your basis reviewedCarry the corrected amounts to Form 8949 and Schedule D
Form 8949 reconciles relevant sales to broker information and provides fields for adjustments. Its totals flow to Schedule D, which combines short-term and long-term capital gains and losses. In general, a holding period of one year or less produces a short-term result; more than one year is long term. Certain sales with basis reported to the IRS and no adjustments can be reported directly on Schedule D under the IRS instructions.
Capital gain distributions reported on Form 1099-DIV are separate from gains on sales. Keep them distinct from the proceeds of fund shares sold, particularly where distributions were reinvested. Also check whether your state return requires corresponding investment-income adjustments.
Documents to have ready before you file
Gather current Forms 1099-B, 1099-DIV, 1099-INT and 1099-OID; purchase confirmations and earlier brokerage statements; transfer records; reinvestment schedules; and notices of stock splits, mergers or returns of capital. For gifted or inherited securities, obtain the donor's records or relevant estate valuation rather than assuming the broker's displayed basis is correct.
If a holding has several acquisition lots or a long history, prepare a lot-by-lot reconciliation showing how each reported basis amount was calculated. This gives you a defensible audit trail and makes it easier to identify discrepancies before submitting the return.
Get the US return and investment reporting checked
Taxule's US individual tax service covers Form 1040 and applicable state returns, including investment disposals and basis discrepancies.
See Form 1040 and state return servicesIf your investments also create reporting or tax obligations in the UK, those separate cross-border questions are addressed through Taxule's US-UK individual services. This article deals with the US return, not the interaction of two countries' capital gains rules.
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