Direct answer
Each octal digit is 0–7 and represents 3 bits. To convert: multiply each digit by 8^(position) and sum. 755 octal = 7×64 + 5×8 + 5 = 493.
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Octal input
Decimal output
About Octal → Decimal
When you see chmod 755 or chmod 644 in a tutorial, the number is octal. Converting to decimal lets you compare values, but in practice you almost always read octal directly because each digit corresponds to one Unix permission triplet.
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Frequently asked questions
Is 0644 different from 644?
In octal-aware contexts (C, Bash chmod), no — both are 644 octal = 420 decimal. But in JavaScript/Python ints, the leading zero is either an error or treated as decimal.