Abstract: Lattice QCD has reached to the point that extraction of the parameters of the standard model from first principle is feasible. Of important example is the CKM matrix element, namely V_us. To get V_us, one needs the relevant vector form factor at zero momentum transfer as a theoretical input. Recent Lattice data provides the form factor in finite volumes (the volume of the Lattice) followed by extrapolation to large volume. On the other hand, we apply chiral effective theory to obtain an independent result for the vector form factor at finite spatial space. Our result may be considered as a guideline for lattice practitioners to do large volume extrapolation and on top of that, we see that some extra information dismissed in lattice calculations surfaces.