Hello,
Am thinking of substituting Rowan Big yarn for Sirdar Bigga(I found it for 1/2 price on EBay). Both knit on 15mm needles. Would it work?
If you do, you’re going to end up with either having to change the pattern, or having a different drape to the fabric. Rowan Big Wool gets 8 sts per 4", and Bigga gets only 6 sts per 4". That’s a pretty significant difference in gauge. If you try to use the Rowan, you’ll get a much looser, drapier, wholeyer fabric.
It should be close enough, you’ll just have to knit a sample of the pattern to see what gauge [I]you[/I] get with it, and adjust your stitches or use a different size of the pattern.
Is this for the baby cardigan? And what pattern are you thinking of using, because we can help out more directly with better suggestions if you tell us that!
the patterns is actually for a bolero/ short cardigan for me and comes from a library book. I should have said that the patten is for Rowan and the Sirdar is on sale. The pattern is actually very open and uses a yo to creat big spaces.
Gauge is (over 10cm/4") 8sts and 10 rows using over crossed openwork and 12mm needles
openwork is created with:
P, YO2, k, slip stiches purlwise,back onto left needle and knit.
(yes I am practising)
In between the openwork is simple st st.
Irishmam
Oh-- okay, then the opposite of what I said is going to happen: The Sirdar is 25% thicker than the Rowan, so you’ll get a less wholey look. Just be careful of your gauge-- 25% is a lot over the course of an entire sweater/bolero.
Do a swatch before you start, definitely. 
Ok I took a deep breath and made that Ebay purchase. I ahve knitted a swatch and it is coming up a little bit bigger and thicker but that is no harm - it gets cold here and I am knitting to a size larger thatn I need.
Thank you for your help - I shall be back for help with this yarn over!
Irishmam
HOW little bit bigger is it? Remember that if it’s even just a tiny bit bigger over the gauge for an inch, you could end up with massive amounts of bigger! Especially if you’re already knitting a size up. Don’t be afraid to frog back and start again if you decide it’s too big. Really. I’m the queen of frogging and starting again, and it’s really better to end up with a garment that you actually want to wear that really does fit!
I always swatch larger than needed, cos that 1/4 stitch out that you didn’t think made much of a difference, does over a large garment. I’ve learned that the hard way! Maybe make the smaller size and it’ll be a big baggier anyway.