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The magnitude scale is logarithmic and inverted: five magnitudes is exactly a hundredfold brightness difference, so one magnitude is the fifth root of 100, about 2.512. A magnitude 1.25 star is 79.4 times brighter than a magnitude 6 star at the naked-eye limit. Smaller magnitudes mean brighter objects, a convention inherited from Hipparchus.
Brightness ratio
ratio = 10^((m2 - m1) / 2.5)
Per magnitude
one magnitude = 100^0.2 = 2.512 times
Because Hipparchus ranked the brightest stars as first class and the faintest visible as sixth, and the numbering stuck when the scale was formalised.
About 6 under dark skies, 4 in suburbs and barely 2-3 in bright city centres.