Abstract: There is disclosed a method and means for compacting (encoding) and decompacting (decoding) binary bit strings which avoids the blocking of string elements required by Huffman coding and the ever increasing memory as is the case in simple enumerative coding. The method and means arithmetically encodes successive terms or symbols in a symbol string s=a.sub.i a.sub.j . . . , in which each new term a.sub.k in a source alphabet of N symbols gives rise to a new encoded string C(sa.sub.k) and a new length indicator L(sa.sub.k). The method and means comprises the steps of forming L(sa.sub.k) from the recursion L(sa.sub.k)=L(s)+l(a.sub.k), where l(a.sub.k) is a rational approximation of log.sub.2 1/p(a.sub.k), p(a.sub.k) being the a'priori probability of occurrence of a.sub.k, and l(a.sub.k) being so constrained that the Kraft inequality is satisfied: ##EQU1## AND FORMING C(sa.sub.k) from the recursion C(s)+[P.sub.k-1 2.sup.L(sa.sbsp.k.sup.) ],where: ##EQU2## AND WHERE P.sub.