In the numerical study of heat sinks, it is known that a sufficient amount of fluid domain should be added at each side of the heat sink. However, the question in this context is: what can be defined as sufficiently far away from the heat sink? Different authors use different sizes of the computational domain around the heat sink. In this work the impact of the size and location of the fluid domain on the calculated heat transfer coefficient is investigated. Three fin row types are studied: a rectangular, an interrupted rectangular, and an inverted triangular fin row. First, the influence of adding fluid domain to the sides of the heat sink is studied. A large decrease of the heat transfer coefficient on both sides and bottom is observed. Next, the influence of adding fluid domain on both the top and the sides is studied. For the rectangular fins, the impact on the lumped heat transfer coefficient is +12% compared to the case without any fluid domain added. For the inverted triangular fin shape, no net effect is observed on the lumped heat transfer coefficient. So the impact of adding fluid domain depends on fin shape that is investigated. For the sides only, a small amount of fluid needs to be added, while for the fluid domain on top of the heat sink, 130% of the equivalent fin height is found as a good option to simulate the fin in computational fluid dynamics.