Same size swatch, different needle size: If I were to knit a 5"x5" square of stockingette using the exact same yarn.. once on size 8 and once on size 10.. the size 8 one would use more yarn. This is because the size 10 gives me a looser fabric.. more holes in it.. hence less yarn being used. The denser fabric is using more yarn to fill al those holes.
Same number of stitches, different needle size: If I were to cast on 20 stitches and knit 20 rows of stockingette in the same yarn.. once on size 8 and once on size 10 needles, the size 10 swatch would use more yarn - because both swatches have a total of 400 stitches (20x20) but the larger needles make bigger loops, hence more yardage needed.
Am I on the right track? Am I completly wrong? Has anyone ever actually experimented with this? I tossed this out on the knitty chat room where I am now, and none of us have any idea.. so I figured I'd ask here.
You've figured it right, but are you forgetting that even though you're making the same number of stitches on #10s in your second example, they measure larger than the same number on the 8s. So you'd need less stitches.
If you belong to yahoo's Clapotis knitalong - http://groups.yahoo.com/group/Clapotisknitalong2005
- there are databases where people have posted the yarns and needles they used, number of repeats of various sections, and finished sizes. That can be handy to figure out which to use. It was easy for me -- I don't like using anything smaller than 10.5s and that's the largest I could use without it looking too loose, so I did. With Caron's Simply Soft though. Am waiting for knitpicks Shimmer to make another.