A humidifier includes a modular performance indicator that is removably attached to a grill of the humidifier air outlet. The performance indicator generally includes at least one humidity sensing means and a display. The humidity sensing means is positioned within the airflow for measuring the exit-relative humidity of the airflow and the display is coupled to the humidity sensing means for indicating when the humidifier wick requires replacement and/or the humidifier water supply requires refilling based upon the measured exit-relative humidity reaching a predetermined value.