What is a 123 file?
The .123 file extension is most commonly a Lotus 1-2-3 Spreadsheet. Created by IBM, this legacy format stores spreadsheet data, charts, and mathematical formulas used by the Lotus 1-2-3 software suite.
However, the .123 extension is also used by other applications. It can represent a printed circuit board (PCB) design file created with Autodesk 123D Circuits or Sunstone Circuits PCB123. It may also serve as a PKCS#12 security certificate managed by tools like Gpg4win. In some cases, users rename standard .zip, .rar, or .7z compressed archives to .123 to bypass file filters. Rarely, it appears as a ransomware-encrypted executable file.
How to open 123 files?
Because .123 has multiple possible meanings, viewer.online/123 analyzes .123 files to identify their exact format and creator software, shows which programs can open the file, and usually previews it.
If you know the file is a spreadsheet, you can open it with legacy versions of IBM Lotus SmartSuite or older versions of Microsoft Excel. If you suspect the file is a renamed archive, extraction utilities like WinZip, WinRAR, or 7-Zip can open it.
Software and tools
If you need to edit legacy .123 spreadsheets on modern systems, conversion is the best approach. We recommend using convert.guru to transform your .123 files into standard .xlsx or .csv target formats. For encrypted certificates, you can inspect the data using standard security tools like Gpg4win.
Best practices and troubleshooting
Always verify the source of a .123 file. Because malicious actors occasionally use the .123 extension for ransomware or disguised executables, scan unknown files with a trusted antivirus program before opening them. If your spreadsheet software fails to open the file, it is likely an archive, a PCB design project, or a security certificate instead.
Summary
viewer.online/123 directly opens and previews .123 files in the browser, eliminating the need to install software or troubleshoot compatibility issues.