This html page with embeded javascript calculates damage reduction for unit armor in game Warcraft3

If armor is positive formula is: (armor*multiplier)/(1+armor*multiplier)
If armor is negative the formula is: -1+0.94^min(20, -armor)

^ represents power function. Values are in percents. %
Note that multiplier does not affect negative armor formula, it is hardcoded to 0.94
Also note that because of that min function unit cannot have more than -71% negative reduction
Negative reduction of -71% means that unit will take 171% damage.