An image forming apparatus includes an image carrier, a toner-image
forming unit, a photosensor, an information obtaining unit, and a
toner-amount calculating unit. The toner-image forming unit forms a toner
image on the image carrier with toner. The photosensor detects a toner
area where the toner is sticking to the image carrier. The information
obtaining unit obtains particle-size-change information about how
particle size distribution of the toner changes over time. The
toner-amount calculating unit calculates a toner amount of the toner
image based on the toner area and the particle-size-change information.