As an European, I use Metric system. 1m = 100cm, 1cm = 10mm, 1,5cm = 1cm and 5mm and so on. Everything divides nicely, don't you think? It’s the metric power of ten!
Now in Imperial system how ever 1 inch divides simply into fractions: binary, quarter, eighth, and so on.
It is almost impossible to convert inches into centimeters precisely. The most common seam allowance on commercial patterns for home sewers produced in the U.S is ⅝ inches. When converted into millimeters, it’s 15,8750. In centimeters it’s 1,5875cm. And rounded to even it’s 1,6cm, not 1,5cm! but 1,6cm. By rounding it to even I’ve added 1,25 mm to the seam allowance. See what I mean?
My finished garment will be 5mm (thats slightly more than 3/16 of an inch) smaller in width. Or when not to be so anal and precise about decimal places, I could simply use a 1,5cm seam allowance and end up with a bigger finished garment.
I also think using a 1,5cm seam allowance almost everywhere is a waste, because you almost always trim the seam allowances down when the seam is sewn. Maybe it’s the years spent working in RTW industry speaking, where 1cm is like the “holy grail” of seam allowances, but there are seams where you just don't need that much allowance.
I usually spend hours grading the ⅝ inch seam allowance on patterns to 1cm on all the waistbands, collars, neck seams etc. Basically all of the somewhat horizontal seam allowances get trimmed to 1cm.
I have never ever tried to set in a sleeve or attach a collar (not to mention assembling the collar) with a 1,5cm seam allowance.
How do you sew a curve precisely with the ⅝ inch seam allowance people???!!!
However, I have learned to almost love ⅝ inch seam allowance on side seams, center back or front seams, any vertical seams. There is something great in knowing that even though I do not muslin most of the garments; I will still have room to let out seams, if the clothing item will somehow end up being wee bit too small for me.
Now most of you lovely internet people do a muslin before cutting into your fashion fabric. You fit it properly, mark all the changes to the paper pattern etc.
I challenge you to try and set in a sleeve on a jacket with a 1cm seam allowance, or attach a collar!
Sew something with a 1cm seam allowance if you already havent!
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