What it does
The IRIS Taxpayer Portal accepts a CSV upload built on the IRS template for each form, and Publication 5717 (Rev. 9-2026) sets the size: “The CSV file can include up to 250 forms per file, header row not included.” A payer with more forms has to split the file. This tool does that without opening the file in Excel, which is the step that usually breaks it: Excel drops leading zeros from TINs and ZIP codes, turns long numbers into scientific notation and can change the encoding.
- The header row is copied into every part, unchanged.
- Records are copied byte for byte. Quoted values, commas inside quotes, line endings and a byte order mark are kept as they were; nothing is reformatted.
- Blank rows are dropped, such as the empty lines a spreadsheet leaves at the end of a file.
- Form types and tax years are kept apart when the file mixes them, since a file is for one form type and one tax year.
- Files are named after the original, with the part number:
payees-part-1-of-3.csv. When files are grouped, the form type, tax year and the last four digits of the payer TIN are added. Download each part, or all of them in one ZIP.
After splitting
Check each part with the IRIS CSV upload checker before you upload it. In the portal, upload each file separately; uploaded forms land on the Unsubmitted Forms page, and you can submit at most 100 forms at a time from there. The portal walkthrough shows each screen.
Source: IRS Publication 5717 (Rev. 9-2026), IRIS Taxpayer Portal User Guide. This tool splits the file; it does not check its contents.