  We calculate the complete tree and one-loop matching of the dimension 6
Standard Model Effective Field Theory (SMEFT) with unbroken $U(3)^5$ flavour
symmetry to the operators of the Weak Effective Theory (WET) which are
responsible for flavour changing neutral current effects among down-type
quarks. We also explicitly calculate the effects of SMEFT corrections to input
observables on the WET Wilson coefficients, a necessary step on the way to a
well-defined, complete prediction. These results will enable high-precision
flavour data to be incorporated into global fits of the SMEFT at high energies,
where the flavour symmetry assumption is widespread.
