Before hexadecimal took over the programming world, octal — base 8 — was the compact way to write binary, and it still survives in Unix file permissions, where 755 and 644 are octal numbers every developer has typed. Each octal digit is worth a power of 8, so a two-digit number like 17 in octal means one eight plus seven ones: 15 in decimal. This converter takes the two digits separately — the Tens octal digit (0-7) and the Octal digits ones position (0-7) — and reports the Decimal value, the Hex value, and how many bits the number needs. Enter 1 and 7 and you get decimal 15, hex F, and 4 bits. Push both digits to 7 and 77 octal becomes 63 decimal, hex 3F, needing 6 bits. The digits-only-0-to-7 rule is the classic beginner trap: 8 and 9 do not exist in octal, so 18 is not a number here at all. File-permission math is the payoff — once you see that 7 is read+write+execute (4+2+1), the cryptic chmod numbers finally read like plain English.

With the pre-filled defaults — Octal digits (ones position value, 0-7) 7, Tens octal digit (0-7) 1 — the Octal to Decimal returns Decimal value 15, Hex value (0-F range) 15, Needs N bits 4. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.