1 = 1, 2 = 2, 3 = 3, 4 = 4, 5 = 5, 6 = 6, 7 = 7, 8 = 10, 9 = 11, 10 = 12, 11 = 13, 13 = 15, 14 = 16, 15 = 17, 16 = 20, 17 = 21, 18 = 22 and so on...
So the actual formula for in our mind is every 8 numbers, it add 2 instead of 1 and all the others are adding one. The exel formula for this is =DEC2OCT(A1), just put the number you want converted into base 8 in A1 and put the formula in A2