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.