The measurement of the $ZZ$ final state in the mass range above the 2$m_Z$ threshold provides a unique opportunity to measure the off-shell coupling strength of the observed Higgs boson. In this note a determination of the off-shell Higgs boson signal strength in the $ZZ \rightarrow 4\ell$ and $ZZ\rightarrow 2\ell 2\nu$ final states is presented. The result is based on the data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb$^{-1}$ at a collision energy of $\sqrt{s}$ = 8 TeV. The 95% confidence level upper CLs limit on the off-shell signal strength $\mu_{\rm off-shell}$ is in the range 5.6–9.0 when varying the unknown $gg \rightarrow ZZ$ background K-factor from higher-order QCD corrections between half and twice the known signal K-factor, with an expected range of 6.6–10.7. Assuming no energy-scale dependence of the relevant Higgs boson couplings, a combination with the on-shell measurement of $\mu_{\rm on-shell}$ in the $H \rightarrow ZZ \rightarrow 4\ell$ channel yields an observed (expected) 95% confidence level upper limit on $\Gamma_{H}/\Gamma_H^{SM}$ in the range 4.8–7.7 (7.0–12.0) under the same variations of the background K-factor.

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