How do you calculate lattice enthaply?

You can't directly measure lattice enthalpy of a solid crystal, so you have to look at a different method. Usually this would be experimentally or using Hess's Law to construct a Born-Haber cycle. The Born-Haber cycle uses molar enthalpies of formation, atomisation, ionisation and electron affinity of the various elements in the lattice to calculate the molar lattice enthalpy, which is enthalpy change when gaseous ions forms a lattice and is always negative.
For Example, the lattice enthalpy of Sodium Chloride can be calculated to be -786 kJ/mol with the given information below (in kJ/mol):Formation of NaCl: -411Atomisation of Na: +107Atomisation of 1/2 Cl2: +121Ionisation of Na: +496Electron affinity of Cl: -349