To calculate the voltage across capacitors in series, the formula V = Q/C is used, where V is voltage, Q is charge, and C is capacitance. For two capacitors with charges of 5E-6 C and 15E-6 C connected in series with a source voltage of 15V, the total voltage across both capacitors equals the source voltage. The charge on both capacitors remains the same in series, and the individual voltages can be calculated by determining the equivalent capacitance. The voltage across the first capacitor can be found by applying the formula with its specific capacitance. Understanding these principles is essential for analyzing circuits with capacitors in series.