My knitting tension is too wide but the rows are fine - what do I do?

[Lionsea]

I've just started knitting a cardigan for a baby but I've realised it's too wide. Instead of 34 stitches in 10cm as per the tension advice i have 24.

I know the advice says go down a needle size but surely that will mean the numbers of rows as per the tension square will increase? The number of rows is fine and is not a problem.

The other problem is the pattern says for 0-9 months and has no diagrams as to what measurements I'm aiming for. I'm assuming that's cos babies come in all sorts of different sizes??

That is way off. Are you using the right yarn?

it’s often easier to use patterns that aren’t row-dependent (ie ones that say ‘knit for 5cm’ instead of ‘work Xrows’, but obviously you have to get the width right. I’m not sure what to advise with that amount of difference, but if you are using the right yarn you should start by going down a couple of needle sizes and see what happens.

I am using the wrong yarn - they didn't have the pattern wool in store so I changed to another DK wool.

I stead of King Cole Merino Blend DK; I'm using Sirdar replay a cotton/acrylic mix.

I suspect that the king Cole is much more squishy than the sirdar. According to the manufacturer specs they should knit up to the same st / row count on 4 mm needles.
If I were you, try a 3.5mm if you natural tension is quite tight. Just do a tension square and see what the difference is. I suspect you are going to have to make a compromise, as you are right to think a large step down in needle size will throw the rows out a lot. So it might a compromise and meet it somewhere in the middle eg a few stitches off/a few rows off. Or depending on the pattern and your skill set, adjust the pattern, so get the stitch count correct for the tension square, but add in extra rows as needed. But this will be tricky unless the overall pattern is quite plain eg no cables!
To make baby knits a little bit big isn’t really an issue as they (thankfully) grow! But it is important to keep the overall dimensions in line with each other.

That's good advice Alphabet1spaghetti2 that I may have to compromise and go with a smaller needle and adapt the pattern.

Bad new for me- it's not a plain pattern and has some bobbly bits at chest but if I cut down on stitches proportionally it should still work. I hope

I also knit wide and flat, no matter what yarn I use . . . Following for advice.

Luckily I had a short round child.