Calculation of frontal plane axis

If the QRS complex is positive in leads I and II, it falls between -30 and 90° and is normal, as indicated by the yellow area. If the QRS complex is negative in I and positive in aVF, there is right axis deviation. If the QRS complex is positive in I and negative in II, there is left axis deviation. If the QRS complex is negative in I and aVF, there is extreme axis deviation.