Ease is the difference between body measurements and the measurements of a finished garment.
The amount of ease added to a pattern will affect the look, fit, style and comfort of a garment.
In clothing manufacture there is usually a small amount of ease added to patterns just to give the right amount of comfort and movement. For example a skirt may have 3cm ease added to the hip and 1 - 2 cm ease to the waist, this would provide a minimum amount of comfort and movement. Each manufacturer would have their own standard set of ease measurements depending on who their client base is. For example if they are manufacturing garments for the older person they would most likely add more ease around the waist and hips to provide a certain look with greater comfort and movement.
Then there are the knitted fabrics that would not require any ease at all as these garments would stretch to fit the shape and size of the person. However, there are certain knits that may require some ease as they don't have much stretch and need to be treated similar to woven fabrics.
When buying a commercial pattern you don't have to consider ease when cutting out the fabric as the pattern company would have already calculated and included the ease to each pattern. If you like the silhouette and style of the pattern and think that enough ease hasn't been added then you can always add the extra ease that your require yourself. Commercial Pattern Companies usually have a standard set of ease measurements that they adhere to just like clothing manufacturers.
ADDING EASE TO PATTERNS
When adding ease to patterns first consider the type of garment your wish to make. You need to consider the function and style of the garment and the fabric that you intend to use. Also how is this garment going to be worn, do you require a zipper opening, in that case it may require less ease, etc.
Will you be making a loose fitting garment or a tight fitting garment? In both cases you will require a different amount of ease. What type of fabric will you be using?? Soft fabrics may suit a more loose fitting garment, for example an A-line skirt. A heavier and thicker type fabric may require a tailored type of garment, for example a jacket.
Adding ease is not difficult and it all comes down to experience and trial and error. As you become more familiar and experienced adding ease will become second nature.
Be confident and start experimenting and soon you will figure out the right amount of ease to add to each pattern.!!