If you ask my FIL, the bigger the pie, the better. Lucky this isn't the case with the Birthday Pi. Last Thursday I managed to knit two swatches using different yarns. Below you'll find calculations done using those samples.
This is how they look before blocking:
Fingering weight yarn
Here goes the math (the "joys" I referred to on the title of this entry).
I know that 30 rows measures 15 cm (sorry folks, this is complicated enough without me trying to do it using imperial measurements) and I also know the the full shawl has 200 rounds. I multiplied the number of rounds by the height of my swatch and divided it by the number of rows I actually knit on the swatch, giving me an approximate measurement for the radius of the shawl = 100cm.
There are a number of variables I didn't include in my calculations as I didn't want to make it more complicated than my brains could handle.
- The gauge when knitting back and forth and when knitting in the round tends to be different,
- the way tension is applied to a rectangular piece and to a circular piece is different - on a rectangular piece all stitches will have a similar size, but on a circular item the inner stitches tend to be longer the the outer ones,
- on this particular shawl, the density of the fabric (the amount of holes vs. the amount of actual stitches) varies from one chart to the other...
Td;dr version: the shawl, done in fingering weight on size 4.5mm needles will be approximately 200cm in diameter (78.74 inches) - not counting the edging. The shawl will use approximately 1097 m(1200 yards) of yarn.
Dizzy yet? The final number is an approximation of the total length we'd need = 835m (916 yards). Due to some of the reasons I pointed above and to the fact that I didn't take the edging into the calculation, I'd feel more comfortable sticking with the 1200yards I recommended already - we can always find some use for extra yarn, but running short is no fun.
Lace weight yarn
For this swatch I used Jaggerspun Heather 2/20 and size 3.5mm needles. My swatch was 30 stitches by 30 rows and the measurements, after blocking were 16 cm x 11 cm.
Using the same calculations as the ones for the fingering yarn this is what I got: the shawl will be approximately 130 cm(51.18 inches) in diameter - without the edging and will probably require a similar amount of yarn.
Final considerations: If you want a smaller shawl and will be using fingering weight yarn, you can make the final chart smaller - I'll provide alternatives. If you want a bigger shawl and will be using lace weight, you can repeat one of the charts after knitting all of the charts for the body.