formula to calculate dew point from temperature and humidity

Can anyone help me to calculate dew point from the temperature in both (°C or °F) with the relative humidity. I have searched lot on Google but not found any appropriate formula to calculate this. I am using the below formula but it gives wrong values in Fahrenheit unit.

float Temp = [value floatValue];
float Humi = [[data humidity] floatValue];
float Td = Temp - ((100 - Humi)/3.6);
return Td;

I have the temperature (°C or °F) and humidity and need help in formula to calculate dew point in objective C.