There is a standard formula for determining thread consumption. In the formula you will get multiplying factors according to machine type and stitching classes. To define thread consumption you just have to multiply seam length with factors. Thus, one can estimate total requirement of thread for making a garments. But actual thread consumption for a unit length of seam depends on many factors. Likes-
Stitch Classes
Stitches per inches (SPI)
Thickness of the seam (fabric thickness)
Thread tension
Thread count (thickness of sewing thread)
So I will advise you to calculate garment thread consumption by your own. You can develop multiplying factor according to your product categories and requirement using following simple steps.
Step 1: To calculate thread ratio, you need sewing machine, fabric and sewing thread that will be used for bulk production. For fabric and thread you can take similar thickness and same thread count in case actual is not available.
Step 2: Sew a seam of 12 cms long and take 10 cms seam out of it by trimming 1 cm from both edges.
Step 3: Unravel both needle thread and bobbin thread from the seam. Remove crimp from the unraveled thread and measure its length in cm. Generally it will be higher than the seam length. Now find the multiplying factor by dividing thread length with seam length. Assume unravelled needle thread length is 12.5 cm then needle thread multiplying factor is 12.5/10 or 1.25.
Using this method you can find out any types of machines’ thread consumption factors.
Step 4: Once you have consumption factors then it is easy to calculate total thread consumption. Measure seam length of all operations of the garment and get thread requirement by multiplying thread consumption factor. Add thread wastage 5% for the thread that trim out from each ends of the seam.
For quick reference you can follow the following thread consumption ratios. Source: Coats thread consumption guide.
