People with special nutritional needs must monitor their food intake as accurately as possible. One of the problems with food-intake monitoring is a correct estimation of portion sizes. This paper presents an open-source solution to the problem of portion-size estimation consisting of a pocket-sized kitchen scale and a demo mobile application. It is implemented as a soft real-time, low-power consuming embedded system with a Bluetooth connection to the smartphone. Releasing both the hardware and software of the system as open source paves the way towards accurate nutrition tracking and personalized nutrition. This can lead to important advances in the field of nutrition research, as well as practice. This is demonstrated with a proof-of-concept integration of our solution with a commercial mobile application that helps users to keep track of their health and lifestyle goals.