How it works
How to use the Add Line Numbers
Paste or open your input
Type, paste or drop a file into the left editor — or press Sample to try it.
Adjust the options
Use the options in the toolbar; the result updates as you type.
Copy or download the result
Copy the output or download it as a file.
About the Add Line Numbers
Prefix each line with its number — choose the starting number, the separator and zero-padding.
Built for developers
Why developers use it
Private by design
Runs 100% in your browser. Nothing you paste is uploaded, logged or stored on a server.
Instant results
Output updates as you type, with precise error locations when something is wrong.
Works offline
Once loaded, the tool keeps working without a connection. Install it as an app if you like.
Free, no sign-up
No accounts, no limits, no watermarks.
FAQ
Frequently asked questions
Is my data uploaded anywhere?
No. Everything runs locally in your browser — nothing you type, paste or open is sent to a server, logged or stored. The tool keeps working offline once the page has loaded.
Keep going
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