One step of magnitude is 10 times the ground motion and about 32 times the energy. Two steps is 100 times the motion and 1,000 times the energy, so it takes a thousand magnitude 5s to match one magnitude 7.
The scale is a logarithm: magnitude is the log of the amplitude on a seismogram, so each unit is a factor of 10 in motion. Energy goes as 10 to the power 1.5 per unit (log E = 1.5 M + 4.8 joules) because a bigger quake ruptures a longer fault and shakes for longer as well: 10 to the 1.5 is 31.6. That puts a magnitude 6 at about the Hiroshima bomb (15 kt), a 7 at about 30 of them, an 8 near a thousand and the 2011 Tōhoku magnitude 9.1 at about 45,000. The USGS catalogue agrees on the other side of the ledger: since 1973 the world has had about 125 quakes a year from 6.0 to 6.9, 13 from 7.0 to 7.9 and under one from 8 up, a drop of about ten for each step, which is why a magnitude 8 is rare and a magnitude 5 is routine.
Magnitude is not damage. Haiti (7.0, 2010) killed an estimated 100,000 or more and Anchorage (7.1, 2018) killed nobody, because depth, distance, soil and buildings decide what the energy does. The old Richter scale also stops climbing near 7, so a number quoted for a big quake is really moment magnitude, and the energy relation is an average that scatters by about a factor of three.
Estimate with the rule, then check it against the calculator that models it properly.
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