Help with size/tension square

I’m busy knitting my first project, a throw, but decided to do a quick cushion in between. Size of cushion is 16 inches. The pattern is just plain stocking stitch, cast on 40 sts. Tension square is 8 sts x 12 rows = 4 inches, needle size 15. However I found that I needed 9 sts to make up 4 inches, so used next needle size up - size 17, which gave me the required 8 sts. Unfortunately though it now looks quite ‘holey’ and I’m not really happy with it. I was thinking of going back to size 15 needles but cast on maybe 5 extra stitches - would this be a solution to the problem?

At 8sts/4inches, casting on 40 sts gives you 20" across. Maybe the pattern is including some extra width over the 16" finished size? So anyway, if you’re getting 9sts/4inches, casting on 45sts will give you the same 20" across. As you suggest, 5 extra sts should do it.

Next time, cast on more sts and don’t include the edges when measuring for gauge. You can use more stitches and knit more rows - go by the measurements. 40 sts is supposed to be 20" according to the gauge (2 sts per inch) then you need however many sts will measure 20 in your gauge.

Thanks for your quick replies salmonmac and suzeeq. If I cast on 5 extra stitches so, I wonder how much more wool I will use? - By the way, the wool I’m using is absolutely amazing, don’t know if any of you have ever heard of Colinette yarns, they are a Welsh company and have fantastic yarns, the one I’m using is from the Firecracker range, it’s more like strings/ribbons than wool, not that easy to knit with but worth the extra effort!

I estimate the yardage by figuring out the percentage difference in the gauge. The 9 sts divided by the 8 sts would be 1.125 more so that’s about how much more you’d need.

salmonmac, if the pillow is 16" square and you start with a 20" flat piece (or 2) the extra width is for the thickness of it. Also, with a gauge of 2 sts per inch, the seams are going to take up a couple inches too.