# Cluster density guidelines for Illumina sequencing platforms using non patterned flow cells
Cluster density is an important factor in optimizing data quality and yield. The following table lists the recommended raw cluster densities for balanced libraries (such as PhiX):
The iSeq 100, MiSeq i100 Series, NextSeq 1000/2000, NovaSeq 6000, and NovaSeq X Series systems utilize patterned flow cells. This results in a fixed cluster density, even if all nanowells are not occupied. In these systems, monitoring the Cluster Pass filter (%) is a better measure of potential cluster occupancy.
See the following for further details:
* [Plotting %Occupied by %PF to optimize loading for the NovaSeq 6000 and X, MiSeq i100, and iSeq 100](/instrumentation/general/instrumentation-general-reference_material-list/000002308.md)
* [Calculating Percent Passing Filter for Patterned and Nonpatterned Flow Cells](/instrumentation/general/instrumentation-general-reference_material-list/000006309.md)
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