The JMA Scale runs from 0 to 7, with 7 being the strongest. The Mercalli intensity scale is sometimes used together with the Shindo; however this is not usual practice in Japan. Real-time earthquake reports are calculated automatically from measurements of ground acceleration. The JMA reports the earthquake level based on the ground acceleration, measured automatically with seismic intensity meters. There is no simple and linear relationship between Shindo Number and peak ground acceleration as Shindo Number also depend on duration, so the values of the table below should only be regarded as an approximate orientation.
Poster explaining JMA Seismic Intensity Scale
Shindo Scale on Wikipedia