Mainsail Aspect Ratio;

Mainsails should have as Aspect Ratio around 3, they tend
to make the best sails.

The aspect ratio is determined by dividing the luff length
by the foot length.
For instance a sail with a luff of 13m and a foot of 4.1m
would have an aspect ratio of  13 / 4.1 = 3.25

Ratios higher than a 3 represent a tall skinny mainsail.

Ratios less than 3 represent a short fat mainsail.

The type of mainsail your vessel uses is based on its sailplan.

Checking the aspect ratio of your sail is a good way to double
check your measurments. For instance, if you had an aspect ratio
of 4, you might want to recheck your measurements.

We would think 3.5 on the tall side and 2.5 on the fat side would
be about the limits of normal.