  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.
