read doesn't represent a floating number voltage but a 10bit result .
The ACD result is 0 to 1023 where 0 represents 0 voltage and 1023 the Vref voltage.
To convert the result in voltage you have to use (ADCresult/1023) * Vref but in order to have an accurate result you need to store the result in a float.
An alternative is to multiply the result with 1000 so that the result becomes an integer, ((ADCresult*1000)/1023) * Vref , in that case the result will be in mV