An image forming apparatus includes a latent image carrier, a developing unit to develop a latent image on the image carrier with developer including toner and carrier, a developer bearing member, a first detector to detect a toner adhesion amount per unit area of a toner image developed by the developing unit, a second detector to detect humidity, and a controller to calculate an index indicating toner chargeability of the carrier based on detection results of the toner adhesion amount obtained by the first detector and perform a given control process based on calculation results thereof and including a data storage unit to store an algorithm to correct the index to a specific value according to a specific humidity, based on detection results of the humidity obtained by the second detector. The controller corrects the index obtained based on the amount of toner adhesion based on the algorithm.