I am knitting sweater and I need a little help with tension. The instruction say that casting on 18sts and knitting 24 rows I sould have a 10cm pice using 5mm needles. I use 4mm needles and tension measures 7cm. How will I know how many sts to cast on

Help with tension please

First of all please don’t write your question in the subject line. Just put basic info about your question there and the rest in the text box.

It sounds like you may need to go up a needle size. If you use a smaller needle your knitting will be tighter and smaller.

Oh, sorry .My mistake.
Yes but the yarn I am using is for needles size 4mm. Should I go up one or two sizes?

Not using the yarn suggested for a pattern, especially a garment, will affect the gauge which is what you need to meet to make it fit. Sometimes just changing the size you’re knitting will work. For instance if you’re using a lighter weight yarn and smaller needles you might need to make a size bigger. That’s not a fool proof method, but it can work. But try using a bigger needle size and see if it works and if you like the fabric.

Assuming that you like the hand and drape of the swatch that you got on the 4mm needles, you obtained 70% of the gauge needed. You need to add 30% more stitches to get the same size of a knitted garment. For instance, if your size for the pattern called for 120 stitches, you would need to cast on 156 stitches to obtain the same size piece.

Thank you. You were veru helpful

Can you please link to your pattern? Are you using the same weight of yarn that it calls for? The needles listed on the label aren’t necessarily the ones you’ll need to use. Also your swatch needs to be about 6" wide so that you can measure in the middle away from the edges. It reads as though you’ve worked your swatch on 18 ats.