To calculate the voltage for a grill igniter project, using a voltmeter is recommended for accurate measurement. The igniter generates a spark in air, and understanding the breakdown voltage of air can provide an estimate of the voltage produced. The breakdown voltage of air is approximately 3 million volts per meter, which can help in calculations over short distances. For practical purposes, measuring directly with a voltmeter will yield the most reliable results. Accurate voltage measurement is crucial for the success of igniter projects.