Diameter is one of the important parameters for quality measurement. Fruit classification, for example, takes diameter as one important indicator, apart from other indicators. Precision measurement has been observed and experimented for example it can be done using laser technology, however the instrument requires a lot of space, therefore mobility is almost impossible. When speed is necessity while doing measurement, a real-time measurement using camera and image processing can give a lot of contribution compared to manual measurement. This paper introduces a diameter measurement method using Simpson algorithm in real-time, using regular camera. Simpson algorithm gave high accuracy measurement with high computation speed. The design indicated good result with accuracy ranged from 93% to 99% depending on calibration.