Giovanni Mana trained as a theoretical physicist. After transitioning to metrology, he developed a broad range of interests in theoretical/mathematical and experimental/technical fields. They include metrology, precision measurements, mathematical physics, modelling, data analysis, Bayesian inference, neutron, X-ray, optical interferometry and diffractometry, Fourier optics, linear system, and control theory. He developed a split-crystal X-ray interferometer that operates over centimetre scans, which allowed precision measurements of the silicon lattice parameter and Avogadro constant. This way, he demonstrated that it is possible to count atoms and obtain the mass unit from the Planck constant, thus paving the way for the 2019 redefinition of the international system of units. The Italian Physical Society awarded him the Enrico Fermi Prize for these results.