Designers of straight winged aircraft usually express the Centre of Gravity (CG) as a distance relative to a datum or reference point. Those that design swept wing aircraft find it convenient for aerodynamic reasons to relate the longitudinal CG location with the Mean Aerodynamic Chord (MAC) of a wing. The CG is expressed as a percentage of its position along the MAC (%MAC) from the leading edge, also called LEMAC.

The Mean Aerodynamic Chord is the chord at the aerodynamic centre of the wing, not exactly half way to the tip, but close to it. Generally, most swept wing aircraft will have acceptable handling characteristics if the CG is between 15% and 25% of the way back along the MAC, described as 15% to 25% MAC.

When the CG of an aeroplane is at 25% MAC, this means that the CG is situated at 25% of the length of the MAC in relation to the LEMAC and is given from the formula: %MAC=((CG - LEMAC)/MAC) X 100.