Being able to accurately determine weir discharges is of key importance in urban water management. The most common method is performing a level measurement and calculating the discharge using the standard weir equation. Since this equation is only valid in certain conditions, this can lead to large deviations from the actual discharge. In this paper, lab measurements and CFD calculations are applied to determine a new method for the derivation of accurate Q(h) relationships. Detailed scale model experiments have been performed for the internal weir of a SST. The scale model is precisely reproduced in a CFD model, with which corresponding calculations have been performed. It is shown that the water levels from the scale model can be reproduced by the CFD model without parameter tuning. Q(h) relationships have been derived from both the scale model experiments and the CFD calculations. They have been mutually compared with the relationship from the standard equation as well. It is shown that the derived relationships are very similar indicating the applicability of CFD for this purpose. All derived relationships form a much better description of the actual weir discharge than the standard equation.