RSC is expressed in meq/L units. RSC should not be higher than 1 and preferably less than +0.5 for considering the water use for irrigation. The formula for calculating RSC index is:
* RSC index = [HCO + CO] − [Ca + Mg]
* RSC index = HCO/61 + CO/30 – Ca/20 – Mg/12 (in case the ionic concentrations are measured in mg/L or ppm as salts)
While calculating RSC index, the water quality present at the root zone of the crop should be considered which would take into account the leaching factor in the field. Calcium present in dissolved form is also influenced by the partial pressure of dissolved  at the plants root zone in the field water.