When for example the rain probability value shows something like 75% it is not possible to reach 100%, as the increment is fixed to 10%. (and the final increment from 95 would go to 105) You surely do limit the value to not be over 100, but you should allow the value to reach 100 in any situations. just like Code: // on increment: rainProbability += 10; if(rainProbability > 100) { rainProbability = 100; } Or you simply could let a slider and not a spinbox handle that.