To calculate the lattice constant from a diffraction pattern, first analyze the intensity versus 2θ graph obtained from the diffraction data. Utilize Bragg's Law, which relates the angle of diffraction to the spacing between crystal planes, to convert the measured θ into the desired interplanar spacing. This spacing can then be used to determine the lattice constant based on the crystal structure type. Accurate measurements and proper interpretation of the diffraction pattern are crucial for reliable results. Following these steps will enable effective calculation of the lattice constant from diffraction data.