Does my data get uploaded anywhere?
No. Everything runs locally in your browser and nothing is sent to a server.
Is this free to use?
Yes. It is completely free, with no account, no signup, and no usage limits.
Does it work on a phone?
Yes. The page runs in any modern mobile browser, on iPhone and Android alike, with no app to install.
What is chmod 755 in decimal?
493. Each octal digit is a three-bit permission triad, so 755 is 111 101 101, which as a plain integer is 493. That is the number you will see if a script prints a file mode without formatting it as octal.
Why is 8 not a valid octal digit?
Because octal is base 8, so its digits run 0 to 7 only, exactly as decimal digits run 0 to 9. The value eight is written 10 in octal. The tool errors on an 8 or a 9 rather than guessing, which usually means the input was actually decimal.
What are the place values in octal?
From the right: 1, 8, 64, 512 and so on, each 8 times the last. So 644 is 6 x 64 plus 4 x 8 plus 4, which is 384 + 32 + 4, giving 420.
Can I paste a value with a leading zero?
Yes. A leading zero has no value, and it is the older convention for writing an octal literal in C and in Python 2, so 0755 converts exactly like 755. A modern 0o prefix is also stripped before parsing.
Where is octal still actually used?
Mostly in Unix file permissions and umask values, and in octal escape sequences inside string literals. Everywhere else hex has taken over, because 4 bits per digit fits bytes cleanly and 3 bits per digit does not.