It is defined as the amount of heat liberated when one unit mass of fuel is burnt and the products of combustion are allowed to escape.
LCV = HCV - latent heat of water vapour formed
Since, 1 part by mass of hydrogen gives 9 parties by mass of water
LCV = HCV - (Mass of H2 in fuel * 9* latent heat of steam)
LCV = HCV - (H * 9 * 587 kcal/kg.)
or LCV = HCV - (0.09 H * 587) KCAL/KG.
where, H is percentage of hydrogen in fuel.