The number of ADC steps is constant, it has a 10-bit ADC so there are 1024 possible readings from 0 to 1023.
The steps are equally spaced between Vref- and Vref+ so if you know what those are, you can work out the voltage represented by each step in measurement.
Multiply the ADC value by the step size and it will give you the actual voltage on the input pin but bear in mind if you used a Vref- other than ground, your reading is relative to that and not 0V.
Vref+ should never be less than the voltage you want to measure or higher than VDD. Vref- should never be higher than the voltage you want to measure or less than 0V (not be negative). In other words the voltage should be kept between Vref- and Vref+.
Example, if Vref- is ground and Vref+ was 2.5V, each step would be (2.5 - 0)/1023 = 0.00244V and and ADC reading of 123 would mean the actual voltage at the pin was (123 * 0.00244) = 0.3V.
Brian.