Beginner A to D Converter
The correct formula is \$V_{IN} = \$\$D \cdot V_{REF}\over 2^{NBITS}\$ where D is the digital value. NBITS is 10, so the denominator is 1024. That's assuming Vref(-) is 0V. 

If the reference is 3.3V/0V you'd expect a 1V input to produce a digital word of 310 base 10, so something is seriously wrong. I suggest you carefully read the reference manual chapter on the 10-bit ADC as linked here. The data sheet is generally sketchy in the operation of many things. 

As @Ignatio Vasquez-Abrams points out, adequate \$T_{AD}\$ is very important.