The current transfer length of multi-filamentary superconducting NbTi and Nb3Sn strands was measured and analyzed. The aim is to understand and quantify the current distribution process between matrix and superconducting filaments occurring at current injection joints or shunting localized interruptions like originated by transverse cracks or high strain, temperature, or magnetic field in filaments. The current transfer length was investigated on two different multi-filamentary Nb3Sn wires and one NbTi wire. Opposite to earlier clarifications, it was found that the current transfer length cannot be simply represented by a single parameter but depends on the ratio of transport current and critical current and the distance from the current injection point or local interruption of the superconducting path in the filamentary zone. With the aid of our numerical 3D multi-filamentary strand model, simulations were performed showing excellent agreement with the experimental data. For broader use, analytical formulae are proposed to determine the current transfer length for multi-filamentary superconductors with complex cross-sectional layout. The increasing current transfer length with the higher injected current and/or along with the distance away from the current injection point is explained by a progressive current penetration, which is caused by the high resistive matrix layers and complex layout. For the experimental results obtained here, the analytical and numerical simulation results show good agreement against the experimentally measured data from the potential-tips along the strand length.