1099-DA basis reconciliation.

How the reconciliation works

For each line the tool subtracts a basis from the proceeds. It uses your own basis when you give one, otherwise the broker’s box 1g. It then decides short-term or long-term from the two dates (more than one year is long-term) and adds up each side. A line is flagged when your basis and the 1099-DA basis differ by even a cent, because that is where a return and the broker’s record diverge .

It does not decide which of your lots the broker treated as sold. The 1099-DA instructions say that if you acquired the same asset on different dates or at different prices and sold part of the position, the broker reports according to your adequate and timely identification, and box 8 is checked if the broker relied on acquisition information that you supplied. When you see a gap, that is the place to look first.

Why your basis and box 1g may differ

• Transferred-in or pre-2026 coins. These are noncovered, so box 1g may be blank or voluntarily filled by the broker. If you enter your own basis, check box K or H on Form 8949 for those lines.
• Transaction costs. Box 1f is already reduced by fees, commissions and transfer taxes. Counting the same fee again in basis double counts it. Basis also includes acquisition fees, which the broker may not know about.
• Different lot chosen. A different identification method changes which purchase is matched to the sale, so basis and the holding period can both change.
• Units received in a transfer. The form has boxes for units transferred in and the transfer-in date (12a and 12b), which a broker may use for assets that arrived from outside.

Example

You paste two lines. The first, bought March 2, 2025 and sold May 9, 2026 for $12,400 with a $9,000 basis on the 1099-DA and the same figure in your records, is a $3,400 long-term gain with no mismatch. The second, bought June 10, 2024 and sold February 1, 2026 for $5,200 with no basis on the form and $4,300 in your records, is a $900 long-term gain, and the tool counts it because you supplied the basis. Totals show $4,300 net long-term, with zero short-term.

What to do with a mismatch

Form 8949 lets you enter the broker’s numbers and adjust them with a code and an adjustment amount when the form is wrong. The Form 8949 instructions for box A or G say to use that box when basis was reported, and the adjustments live in columns (f) and (g). Keep exchange statements, wallet transaction history and any purchase receipts. The IRS says to keep records that document your purchase, receipt, sale, exchange or other disposition of the digital assets, and the dollar value of anything you received.

Sources

• Instructions for Form 1099-DA (2026) — boxes 1f, 1g, 2, 8, 9, 12a and 12b
• Instructions for Form 8949 — which box matches each 1099-DA
• IRS: Digital assets — records to keep; short-term and long-term test

Figures and rules above were retrieved November 2026 from the IRS pages listed. Agencies revise forms and instructions, so confirm anything that affects your return against the current IRS page.

Frequently asked questions

How should I format the lines?

One sale per line with commas or tabs between fields: date acquired, date sold, box 1f proceeds, box 1g basis from the 1099-DA (blank if none), then your own basis (blank to use the broker figure). Dates can be YYYY-MM-DD or MM/DD/YYYY.

What counts as a mismatch?

Any line where your basis differs from the box 1g basis on the form, even by a cent.

Does this tool replace Form 8949?

No. It adds up gain or loss and compares basis. You still enter each sale on Form 8949 with the right box, and adjust the broker figures there if they are wrong.

Is my data sent anywhere?

No. The calculation runs in your browser and nothing is stored.