IRIS CSV bulk upload: 250 records per file, and how to split

The rules, from the IRS

• A CSV upload file can contain at most 250 records, not counting the header row^[3].
• One tax year and one form type per upload^[3].
• A file may contain more than one issuer only if your IRIS Taxpayer Portal TCC role is transmitter^[3].
• Only IRS-provided CSV templates can be used. Download them from the portal under Upload CSV with Form Data^[3].
• The portal lets you e-file up to 100 returns at a time^[2]. So after uploading a 250-record file, you submit it in batches.

How splitting works

A FIRE file can hold many issuers ("A" records) and many form types in one transmission^[9]. To turn it into uploads that follow the rules above, the converter:

1. Groups payee "B" records by form type and tax year, and by issuer TIN unless you choose the transmitter option.
2. Cuts each group into files of at most 250 returns. For example, 600 1099-NEC returns for one payer become files of 250, 250 and 100.
3. Names the files so you can see what is inside, for example IRIS_1099-NEC_TY2026_payer0011_part01of03.csv.
4. Checks the counts and totals of each group against the FIRE "C" record before anything is written.

Common reasons IRIS rejects a CSV

The IRS lists formatting rules for each form^[6]. The usual problems:

• TIN format. Business TINs must look like XX-XXXXXXX and individual TINs like XXX-XX-XXXX. Undetermined TINs are 9 digits with no dashes^[7].
• Characters. Addresses may contain letters, numbers, hyphen, slash and space only, up to 35 characters. City names may contain letters and spaces only^[6].
• Amounts. Digits with at most one decimal point and two decimals. Watch for scientific notation such as 1.045E+5^[6].
• Check boxes. Enter Y or N^[6].
• Excel. The IRS recommends formatting cells as Text and editing a saved CSV in a text editor, because reopening it in Excel removes formatting^[3]. Leading zeros in ZIP codes are the classic casualty.

When a CSV has an error, IRIS lists the row, the column title and the error message^[7].

Get the FIRE to IRIS Converter: $49, one-time
Single offline HTML file. Your file never leaves your computer.

Sources

1. [2] IRS newsroom: Information return e-file system transitioning to a new platform
2. [3] IRS Publication 5717 (Rev. 9-2026), IRIS Taxpayer Portal User Guide, section 3.5
3. [6] IRS IRIS CSV Template Formatting Guidelines, tax year 2025 (ZIP of PDFs)
4. [7] IRS IRIS Working Group Questions and Answers, January 14, 2026
5. [9] IRS Publication 1220, Specifications for Electronic Filing of Forms 1097, 1098, 1099, 3921, 3922, 5498 and W-2G

IRS pages checked 2026-11-14. Not affiliated with the IRS. Not tax advice.