In layman-friendly terms, the scale goes from G1 to G5.
The one in 1859 was classified as G5 (extreme), and the one the other day was the very first time G4 warning has been issued. Not as in "it'll definitely be G4", but more along the lines of "it'll be G3, possibly increasing to G4 for a brief period of time".
There's also a more "scientific" scale called the K-index (https://en.wikipedia.org/wiki/K-index), but I don't understand it well enough to be able to explain it.
May 2024 Event: Dst -250 from multiple up to X3.98 flares (and CMEs)
Dst is the disturbance to Earth's magnetic field, Xs are the W/m^2 of X-rays emitted from the solar explosion (flare). It's also important to consider the size and direction of the CME (if any) associated with the flare.
The one in 1859 was classified as G5 (extreme), and the one the other day was the very first time G4 warning has been issued. Not as in "it'll definitely be G4", but more along the lines of "it'll be G3, possibly increasing to G4 for a brief period of time".
There's also a more "scientific" scale called the K-index (https://en.wikipedia.org/wiki/K-index), but I don't understand it well enough to be able to explain it.