The modern guitar has 6 strings of approximately 65 cm in length tuned to: `E_2 (82 Hz), A_2 (110 Hz), D_3 (147 Hz),G_3 (196 Hz),B_3 (247 Hz), E_4 (330 Hz)`
This equation returns the frequency for the chosen string.
This equation, Modern Guitar Frequency, references 6 pages