Low-cost sensors have been studied extensively in recent years due to their price advantage, compact size, and moderate accuracy. Different manufacturers use different calibration methodologies and report a factor for the user. This study compared nine types of low-cost PM monitors (AirVisual, Alphasense, APT, Awair, Dylos, Foobot, PurpleAir, Wynd and Xiaomi) in a chamber with a well-defined aerosol. Additionally, two reference instruments (GRIMM and SidePak) were also used. These nine types of monitors were divided into two groups for comparison, according to their working principle and data reporting format. A linear correlation factor based on PM2.5 mass concentration was reported for all monitors. Apart from linear correlation, the differences of the PM2.5 mass concentrations reported by the various monitors and reference instruments were plotted against their average to demonstrate the degree of improvement that was possible after calibration. A bin-wise calibration was also conducted for monitors reporting size distributions to illustrate any coincidence error that could bias the results. For monitors designed for residential use, an important parameter often reported is the air quality index and is illustrated with a simplified index and color. The color display scheme of various monitors was compared with the US EPA regulation to demonstrate whether they could convey overall air quality levels accurately and promptly. The residential monitors indicate the air quality moderately well, but their different color display schemes make the comparison difficult and possibly misleading. Compared to only the PM sensor block, the tested monitors are convenient to use and ready for deployment without additional fabrication. However, user-defined calibration is still recommended for the target PM source to improve their accuracy.