# 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, NextSeq 1000/NextSeq 2000, NovaSeq 6000, and NovaSeq X systems utilize patterned flow cells, which result in a fixed cluster density, even if all nanowells are not occupied. In these systems, monitoring the Cluster PF (%) is a better measure of potential cluster occupancy.
See the following for further details:
* [Plotting % Occupied by % Pass Filter to optimize loading concentration for NovaSeq 6000 and iSeq 100 platform](https://support.illumina.com/bulletins/2020/03/plotting---occupied-by---pass-filter-to-optimize-loading-concent.html)
* [Calculating Percent Passing filter for Patterned and Nonpatterned Flow Cells](https://knowledge.illumina.com/instrumentation/general/instrumentation-general-reference_material-list/000006309)
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