Split ratios also known as seeding densities are used to maintain optimum growth and viability of cell cultures for future applications. It is important to note that cell lines require specific seeding densities to ensure proper growth (i.e. check manufactures guidelines). When a cell density exceeds the capacity of its growth medium it must be sub-cultured or passaged to maintain optimal density for continued proliferation. As a general guideline cell cultures should be split when cells are approximately 80% confluent (i.e. 80% of the surface area/volume of the substrate is covered).