# How to calculate lux value?

I tried calculating lux value for my LED by using the formula the (luminous flux / area), i obtained viewing angle and luminous flux value from LED datasheet, then with the angle i calculated the area the i calculated the lux value but for the same values i'm getting different outputs in online calculators, i gone through some article and found that few other factors need to be added like utilization factor and loss factors but still i can't get how the online calculators calculate, i'm posting the snaps below, can someone give directions to get clarified of this doubt ?