The mean electrical axis (MEA) is is an important measurement in clinical assessment. However, currently there is no agreement on a standardized calculation of the MEA. In order to standardize the MEA calculation, the following issues need to be addressed: 1) The net direction or net potential in the measurement of MEA needs to be defined. 2) Which underlying heart model which describes the electrical activity of the heart to use. 3) Which electrocardiogram (ECG) leads to calculate MEA from. In present use for some applications, e.g. ECG-derived respiration, lead pairs I and III or I and aVF are used in axis calculation without further consideration of scaling and angle between the leads. This may cause an error up to 30° in the Einthoven model. The aim of this work is to derive correct systematic equations for determining the heart vector (dipole) and MEA from any pair of leads in the frontal plane. Since any pair of leads can define the frontal plane, it is more robust to calculate the MEA from each lead pair and take the mean of the results. From the results two different net potential definitions, $V_{area}$ and $V_{sum}$ gave approximately the same answer.