The $Lambda$-parameter in 3-flavour QCD and $alpha_s(m_Z)$ by the ALPHA collaboration

We present results by the ALPHA collaboration for the $Lambda$-parameter in 3-flavour QCD and the strong coupling constant at the electroweak scale, $alpha_s(m_Z)$, in terms of hadronic quantities computed on the CLS gauge configurations. The first part of this proceedings contribution contains a review of published material cite{Brida:2016flw,DallaBrida:2016kgh} and yields the $Lambda$-parameter in units of a low energy scale, $1/L_{rm had}$. We then discuss how to determine this scale in physical units from experimental data for the pion and kaon decay constants. We obtain $Lambda_{overline{rm MS}}^{(3)} = 332(14)$ MeV which translates to $alpha_s(M_Z)=0.1179(10)(2)$ using perturbation theory to match between 3-, 4- and 5-flavour QCD.