A set of volume increments for ionic oxido‐borates is developed for trigonal‐planar and tetrahedral coordinated boron fragments in densely packed crystal structures. For this purpose, the cell volume of 277 alkaline‐ and earth alkaline borate structures was analyzed with respect to the volume of different anionic borate building units. The volume increments help to determine the number of formula equivalents in a cell which is especially useful for the structure solution process starting from powder diffraction data. This was demonstrated on a new methylammonium borate, [H3CNH3]B5O6(OH)4 ⋅ CH3NHCOH which could be solved from powder X‐ray diffraction data. The compound crystallizes in the monoclinic space group P 1 21/c 1 with the lattice parameters a=7.3040(1) Å, b=11.6377(1) Å, c=17.1065(3) Å, β=107.2167(9)° and Z=4. The structure model is supported as well by 1H, 11B, 13C and 15N solid‐state NMR and quantum chemical calculations.