If you want to easily replace a yarn in a pattern there are 4 easy steps to success.
1. Yarn weight: If a pattern is written for a DK or an aranweight, you will end up with a very different fabric is you try to knit it in 4-ply. Although you can adapt a pattern, you are best substituting DK for DK, chunky for chunky, etc.
2. Fibre content: For best results, try to use a yarn that is made up from similar fibres as the one in the pattern. Cotton yarn behaves very differently to wool so swapping one for another might give you a very different outcome than you were expecting.
3. Tension: We’re back to the tension square again. You do need to check that you can get the same tension as the pattern with your substitute yarn.
4. Yarn amounts: To work out how much of your new yarn you require, you need to know the meterage (metres per ball) for both the yarn in the pattern and your chosen yarn.
Time for a small sum: If the pattern requires six balls of yarn with 120m per 50g, you need 720m of yarn. The yarn you have chosen has 210m in a 100g ball. Three balls would give you 630m (probably not enough) and four balls 840m.