Base32 allows to encode each 5
bits ( as 32 = 2^5
) using single character.
It means that you need output buffer size for encoding:
dst_size = src_size * 8 / 5 (1.6 times larger)
But as base32 string length must be multiple of 40 bits:
dst_size = (src_size * 8 + 4) / 5
Thus, for decoding (base32->binary) required buffer size is accordingly
dst_size = ceil( src_size / 1.6 )