Aspect ratio is the wing span divided by the mean wing chord. An aircraft with a rectangular wing of area 12 m² might have a wing span of 8 m and wing chord of 1.5 m. In this case the aspect ratio is 5.33. If the span was 12 m and the chord 1 m then the aspect ratio would be 12. However because wings may have varied plan forms it is usual to calculate aspect ratio as:

Aspect ratio = wing span² / wing area

For example, the Jabiru's aspect ratio (span 7.9 m, area 8.0 m²) = 7.9 × 7.9 / 8 = 7.8 whereas an aircraft like the Thruster would have an aspect ratio around 6 and consequently you would expect such an aircraft to induce much more drag at high angles of attack, and thus slow much more rapidly than the Jabiru.

And incidentally, the mean chord of a wing is span/aspect ratio. A high performance sailplane wing, designed for minimum induced drag over the C_L range, might have a wingspan of 22 m and an aspect ratio of 30 thus a mean chord of 0.7 m. Most ultralights would have an aspect ratio between 5.5 and 8 and light general aviation aircraft between 7 and 9, averaging around 7.5. There are a few ultralight aeroplanes, designed to have some soaring capability, which have aspect ratios around 16 or 17.

Higher aspect ratio also has the effect of a higher rate of lift increase, as aoa increases, than lower aspect ratio wings. Aspect ratio also affects the lift curve, a high aspect ratio wing will have a higher maximum C_L but a lower stalling aoa than a low aspect ratio wing utilising the same aerofoil