Licensing
Aircraft %MAC: How to Calculate Percent Mean Aerodynamic Chord
Percent MAC is just the CG position rewritten as a fraction of the wing's mean aerodynamic chord. The formula is one line; the inputs come from the aircraft's approved data.
Last reviewed: 2026-08-30
The short answer
%MAC = ((CG − LEMAC) ÷ MAC) × 100. CG is the loaded centre of gravity (a distance from the datum), LEMAC is the arm of the leading edge of the mean aerodynamic chord, and MAC is the length of the mean aerodynamic chord. The result is how far aft of the MAC leading edge the CG sits, as a percentage of the chord.
It is the same CG position you would get from an ordinary weight-and-balance calculation — just expressed against the wing instead of against the datum. See our guide on how the weight and balance calculation works for the CG step itself and the Canadian regulatory context.
MAC, LEMAC and MEAC
The mean aerodynamic chord (MAC) is a single reference chord that represents the whole wing for stability and balance purposes. Its length, and the position of its leading edge (LEMAC) and trailing edge (MEAC or TEMAC), are fixed geometric properties of a given aircraft's wing and are published in that aircraft's approved data.
LEMAC is expressed as an arm — a distance from the same datum used for the weight-and-balance calculation. That shared datum is what lets you subtract LEMAC from the CG directly.
The formula and its units
%MAC = ((CG − LEMAC) ÷ MAC) × 100.
CG, LEMAC and MAC must all be in the same length unit — all inches, or all millimetres, or all metres. The subtraction (CG − LEMAC) gives the distance of the CG aft of the MAC leading edge; dividing by MAC turns that distance into a fraction of the chord; multiplying by 100 makes it a percentage. A CG exactly at the leading edge of the MAC is 0% MAC; a CG one MAC-length aft of it would be 100% MAC.
If (CG − LEMAC) comes out negative, the CG is ahead of the MAC leading edge and %MAC is negative — which is valid arithmetic but, for a transport aircraft, almost always a sign of a data or entry error.
A worked example
The numbers here are illustrative — they are not any real aircraft's approved data.
Suppose a loaded CG of 52.0 in aft of datum, a LEMAC of 40.0 in, and a MAC of 60.0 in. Then %MAC = ((52.0 − 40.0) ÷ 60.0) × 100 = (12.0 ÷ 60.0) × 100 = 20.0% MAC.
If the same aircraft were loaded so the CG moved aft to 55.0 in, %MAC = ((55.0 − 40.0) ÷ 60.0) × 100 = 25.0% MAC. A 3-inch aft CG shift on a 60-inch MAC is a 5-percentage-point change in %MAC.
Reading %MAC against approved limits
Aircraft that use %MAC publish their forward and aft CG limits in %MAC too — for example a permitted range of roughly 15% to 35% MAC, widening or narrowing with weight. A loadsheet computes the %MAC for the takeoff, and often the zero-fuel and landing, conditions and checks each against the limits for that weight.
The specific limit values, and the LEMAC and MAC used to compute %MAC, are approved data for that exact aircraft type and configuration. There is no generic %MAC range. Any number used here as an example is only that.
Try it in the PASSCARs Weight & Balance tool
The PASSCARs Weight & Balance calculator shows a %MAC result alongside the CG whenever you supply LEMAC and MAC, and it can plot the CG envelope on a %MAC axis instead of an arm axis. As with every figure in the tool, the LEMAC, MAC and limits must come from the specific aircraft's current approved documentation.
Frequently Asked Questions
What is the %MAC formula?
%MAC = ((CG − LEMAC) ÷ MAC) × 100, with CG, LEMAC and MAC all in the same length unit. It expresses the centre of gravity as a percentage of the mean aerodynamic chord, measured aft of the chord's leading edge.
What are LEMAC and MAC?
MAC is a single reference chord representing the whole wing. LEMAC is the arm (distance from the datum) of the leading edge of that chord. Both are fixed geometric figures published in an aircraft's approved data.
Why do larger aircraft use %MAC instead of an arm?
Expressing CG as a fraction of the wing chord makes the balance limits comparable across weights and relates the CG directly to the aerodynamic reference the aircraft was designed around. The underlying CG position is identical to the arm value.
Can %MAC be negative or over 100%?
Arithmetically yes — a CG ahead of the MAC leading edge gives a negative %MAC, and a CG more than one chord aft gives over 100%. For a normally loaded transport aircraft, either result almost always means a data or entry error.
What is a normal %MAC range for an aircraft?
There is no generic range. Each aircraft type publishes its own forward and aft CG limits in %MAC, and they vary with weight. Values such as "15% to 35% MAC" are only illustrative — use the limits in the specific aircraft's approved data.
Are the %MAC numbers in this guide real?
No. Every CG, LEMAC, MAC and limit used here is an illustrative example. Real figures come from the aircraft's flight manual, type certificate data sheet, and approved supplements.
Weight & Balance Calculator
Shows a %MAC result alongside the CG, and can plot the CG envelope on a %MAC axis.
CARs Regulatory Assistant
Ask how the empty weight and balance report and Standard 571 Appendix C apply.
Official Transport Canada References
PassCARs is independent and not affiliated with or endorsed by Transport Canada. For authoritative, current requirements, always refer to the official sources below.
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