Form 1099-B field reference
Form 1099-B Box 12: Basis reported to the IRS
Form 8949 destination
Helps select box A/D versus B/E
Reconcile this field with the rest of the transaction before calculating gain or loss. A Form 1099-B label does not override the official basis, holding-period, or adjustment rules.
Before filing
What to check
- Read the broker statement's basis-reporting indicator.
- Do not confuse basis reported to you with basis reported to the IRS.
- Use Form 8949 adjustments when the reported basis must be corrected.
How it works
Box 12 mechanics
Box 12, combined with the holding period from Box 2, is what actually determines which of the six Form 8949 categories a transaction belongs in: A for short-term with basis reported, B for short-term without basis reported, D for long-term with basis reported, and E for long-term without basis reported. Category F, covering transactions not reported on any 1099-B at all, is rare for a typical retail brokerage account. A checked Box 12 confirms the broker sent that basis figure to the IRS — it does not guarantee the figure is correct. If the reported basis needs correcting (a missed wash-sale adjustment is the most common cause), the transaction still belongs in category A or D because the IRS did receive a basis figure; the taxpayer adds a Code B adjustment in column (f) and the corrected amount in column (g) rather than moving the transaction to category B or E, which are reserved for securities where no basis was reported at all.
Worked example
A covered security shows Box 12 checked (basis reported to the IRS) with long-term holding period, so the sale generally belongs in Form 8949 category D. If the broker's reported basis turns out to be wrong — say it missed a wash-sale basis adjustment — the taxpayer still uses category D but adds a Code B adjustment in column (f) and the corrected figure in column (g), rather than switching to category E.
Box 12 FAQs
Does a checked Box 12 guarantee the broker's basis figure is correct?
No. It only means the broker sent that basis figure to the IRS. You can still need to correct it using the Form 8949 adjustment columns if your records show a different basis.
What Form 8949 category applies to a noncovered security?
Generally category B for short-term or E for long-term, since noncovered securities pair with Box 5's flag rather than a basis-reported indicator in Box 12.