To be honest that is quite a fat chunky at 8 stitches per 10cm. (And very nice!) I normally go by, if the tension listed in the pattern more or less matches the little tension square on the ball band, you're good to substitute.
If it's an unusual stitch pattern then I compare the ball band (plain stitch) tension of the specified yarn vs. the one I want to substitute.
Scarfs are also quite tolerant of slight variations in size so you will probably be OK, just make sure you buy enough yarn.
The suggested needle size for that yarn is 10-12mm so if you use a 9mm you're going to end up with tight stitches and a very firm fabric. I'd recommend using a 10 or 12mm needle with this yarn and adjusting the pattern accordingly - if it's a simple scarf, it should be easy enough to do.
Thanks for your responses but i dont really understand the technical points re tension etc. The pattern suggests super chunky wool, so maybe i should just stick with that. I have no idea how 2 adjust the pattern accordingly, where would i start? As i say its a plain garter stitch scarf.
All you really need to do is make it as wide as you want it to be. So knit a bit (more than a few rows as the first couple are always tighter than the rest) and see if you think it looks okay. If too narrow, you'll need to pull it back and cast on more stitches. If too wide, likewise but cast on fewer stitches. If you really want to, you can knit what's called a gauge swatch, which is a small piece about 15cm by 15cm, and then you count the number of stitches in the measurement specified in your pattern, in this case per inch. If you've got 5 stitches in 2 inches, you're spot on. If it's 6 or 4 - well, it'll probably be fine. If it's 10, you definitely need bigger needles!
I would go up a needle size or two, though - the Ravelry page for the yarn suggests 10 mm - 12.75 mm.
Tension is basically how tight or loose your knitting is. If your tension is tight, all the stitches will be smaller, and so your finished piece of knitting will be smaller, and if your tension is too loose, your fabric will be loose and a bit baggy looking, and bigger than it should be.
This doesn't really matter when you are knitting a scarf - if it is a bit wider or narrower, and a bit shorter or longer - that's usually not a big issue (though you don't want your knitting to look baggy and loose).
If you progress on to knitting garments, tension becomes more important, because you want the finished garment to be the right size - so most knitters I know recommend a tension square - you knit a test square, and measure to see if you get the right number of stitches and rows, in a 10cm square - too many stitches/rows means you are knitting too tightly, too few means your knitting is too loose.
The more knitting you do, the better you will get at creating a fabric that is neither too loose nor too tight. You can also use different sized needles - that's what I used to do as a teenager, when my knitting was very tight - I went up both a needle size and a size in whatever pattern I was knitting - but now I really don't need to do that, as my tension is pretty much OK.
that yarn is super chunky as others have said you might want to knit it on slightly bigger needles but it should work fine. You might need slightly more wool too so check how many yards your pattern calls for.
thanks choco and tweety, I joined Ravelry, but I'd really like to try the pattern I've got. I will also look at the Rowan big wool - that was my alternative choice. Thanks everyone, I will report back. Off to buy the wool and needles tomorrow, Im going to go with the 10mm needles to be on the safe side. Im very excited!
I have just finished my beautiful scarf tonight! I am so proud of myself! I only dropped one stitch (which my very helpful neighbour sorted out) and had a little help with attaching the second ball of wool and casting off. I am in heaven, its the most beautiful thing I've ever seen apart from my DD.
DD (4) is amazingly impressed and has put an order in for a pink, sparkly version with tassles...