I’m working on several projects that I want to include a voltmeter to monitor battery levels.  One of the easier methods is to use a resistor voltage divider network and a zener diode to scale the input voltage to a value safe for the Analog-to-Digital (ADC) pin.  I developed a voltage divider calculator in C# to help me determine the correct resistor values for the selected voltage range I would like to measure.  You can find it here.