Neutrinos, and in particular their tiny but non-vanishing masses, can be considered one of the doors towards physics beyond the Standard Model. Precision measurements of the kinematics of weak interactions, in particular of the ³H β-decay and the ¹⁶³Ho electron capture (EC), represent the only model independent approach to determine the absolute scale of neutrino masses. The electron capture in ¹⁶³Ho experiment, ECHo, is designed to reach sub-eV sensitivity on the electron neutrino mass by means of the analysis of the calorimetrically measured electron capture spectrum of the nuclide ¹⁶³Ho. The maximum energy available for this decay, about 2.8 keV, constrains the type of detectors that can be used. Arrays of low temperature metallic magnetic calorimeters (MMCs) are being developed to measure the ¹⁶³Ho EC spectrum with energy resolution below 3 eV FWHM and with a time resolution below 1 μs. To achieve the sub-eV sensitivity on the electron neutrino mass, together with the detector optimization, the availability of large ultra-pure ¹⁶³Ho samples, the identification and suppression of background sources as well as the precise parametrization of the ¹⁶³Ho EC spectrum are of utmost importance. The high-energy resolution ¹⁶³Ho spectra measured with the first MMC prototypes with ion-implanted ¹⁶³Ho set the basis for the ECHo experiment. We describe the conceptual design of ECHo and motivate the strategies we have adopted to carry on the present medium scale experiment, ECHo-1K. In this experiment, the use of 1 kBq ¹⁶³Ho will allow to reach a neutrino mass sensitivity below 10 eV/c². We then discuss how the results being achieved in ECHo-1k will guide the design of the next stage of the ECHo experiment, ECHo-1M, where a source of the order of 1 MBq ¹⁶³Ho embedded in large MMCs arrays will allow to reach sub-eV sensitivity on the electron neutrino mass.