calculating from decimal to octal, hexadecimal or binary is easy... but the opposit way is harder, could someone tell me a way to calculate from octal to decimal?

like from decimal to octal:

lets say 11 decimal to octal

11/8 = 1 rest 0.375
1/8 = 0 rest 0.125

0.375 * 8 = 3
0.125 * 8 = 1

so result: 11 decimal is 13 in octal...