Poncho knitted from two rectangles

When I decided to make a poncho that is constructed of two rectangular panels, I thought about it really hard, because I read somewhere that when knitting or crocheting a poncho one should double the width. So if, for example, I want to make a poncho that is wide 20 inches (which would be a length from neck to the wrist of a person) then my length of each panel should be 40 inches before I start sewing it together.  Now, if one has to sew the short side to the long side - that person would be shortening the length by half,  leaving us with remaining 20 inches. Then we would do the same with other panel and therefore we would have 20 more inches left - which these 20 inch measurements combined would give us 40 inches and that would be our opening for head - a little too big, IMHO, since a regular head circumference is somewhere between 21-25 inches for adults. I can clearly imagine that I would be pushing such poncho up my shoulder all the time. Very uncomfortable.
The same thing would happen to a small baby. If the width would be 10" and length would be 20", then by calculation the opening for the head would be 20". I can just see a baby running around playground and poncho slipping of the shoulders to her/his hips or knees.

So, I arrived at the conclusion that the "best" (hopefully) measurements are perhaps combination of the length from the neck to the wrist (or up to knuckles on a hand) plus the circumference of a head + a little bit more just to be on the safe side. That makes sense right? I think that first the poncho has to cover the arms, and second has to go over the head, BUT this opening for a person's head cannot be too big (because a poncho would be slipping of one's shoulders) nor too small so the person can pull the poncho comfortably over the head.

I think I will experiment with this formula I thought of so hard:
(Note: I didn't put it into a use yet, so be careful when using my "formula" - it is not completely checked out by me yet)

Width of a poncho = measurement from a neck area to a wrist (arm length)
Length of a poncho = measurement from a neck area to a wrist + 1/2 of a head circumference + 1 or 2 extra inches (depending on a person's upper body size around the neck and shoulder area)

Diagram Of "How To Assemble A Poncho"

In the meantime see my free written pattern (below) for a poncho for 10" width & 16" head circumference - most likely to fit a year old baby.

Also, I made videos:

1. how to knit this beautiful, simple and easy (woven) stitch - RIGHTIES CLICK HERE, and LEFTIES CLICK HERE
2. how to assemble the poncho and how to crochet around the edges  - RIGHTIES CLICK HERE, and LEFTIES CLICK HERE

Baby poncho from variegated yarn

Materials:
Yarn - worsted #4 thickness, 220 yds/200m, 100g/3.5oz, (I used Cascade Yarns, Casablanca - wool/silk/mohair) - buy more yarn if fringe or tassels are desired
Knitting needles - US size #8 (5.0 mm)
Hook - size F (3.75mm) and G (4.0 mm)
Scissors

Gauge:
4"x4" = 18 sts x 24 rows (stockinette stitch with 4.5mm needles)

Size:
about 12-18 months old

Abbreviations:
CO - cast on
st(s) - stitch, (stitches)
K - knit
P - purl
sl st - slip st
SSP - slip stitch purlwise from left needle onto right needle without knitting
sc - single crochet
BO - bind off


Step-by-Step:

Make 2 identical rectangles

CO any number of odd stitches that you need per your measurements (Note: for my project I casted on 41 sts = 10 inches)

Row 1               *[K1, SSP]* - repeat from * to *, K last st
Row 2               P all sts;  (Note: I usually knit first st)
Row 3               K2, *[SSP, K1]* - repeat from * to *, K last st
Row 4               P all sts; (Note: I usually knit first st)

Repeat Row 1 & Row 2 until desired length (Note: for my project I knitted 16" long panel)

BO all stitches and after you assemble your poncho (see image above). Crochet or knit around the edges and/or add some fringe or tassels. Weave in all ends.

If you find any errors, please, let me know. Thank you.


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