When drafting patterns for a knit fabric, negative ease must be subtracted from the circumference measurements to account for the fact that the garment will conform to fit the body when worn. Thus, the final garment will be cut smaller than the body measurements. To calculate total negative ease and negative ease percentage, you need to know the stretch ratio of your fabric. The stretch ratio is the amount by which a fabric can be stretched to its maximum length while maintaining wearer comfort (we do not want to max out a fabric on a wearer). This is calculated by dividing the amount of stretch of a fabric by its original length. For example, if a fabric is stretched to one and a half times its original length, then the stretch ratio would be 1.5 calculated (15"/10"). Once you have determined the stretch ratio of your fabric, you can then use the formula (1-1/Stretch Ratio)X100 to find the percentage of negative ease to be subtracted from each measurement. So, if you have a stretch ratio of 1.5, your percentage of negative ease would be 33.33% calculated (1-1/1.5)X100.
After calculating the negative ease percentage, you would use the negative ease percentage to determine the amount of negative ease to be subtracted from your measurements. To find the amount of negative ease that will be applied to your measurements, you would multiply your measurement by your negative ease percentage and then take that result and subtract it from your original measurement. For example, if you have a bust measurement of 36" and a negative ease percentage of 33.33%, you would multiply 33.33% X 36 (0.3333x36) to get 11.99" and subtract that amount from 36" to get a final measurement of 24.01". This means, that when the wearer puts on the garment, that 24" of fabric will stretch to 36" to fit and conform to the wearer's bust.
Stretch Ratio Formula: (Stretch Fabric Length / Original Fabric Length)Negative Ease Percentage: (1-1/Stretch Ratio) x 100