Converting gauge

Is there a universal rule for using a different gauge from the one asked for in the pattern. EG suppose the pattern gauge is 20 st, 30 rows per 4 inches. If you have a yarn which is 10 st/15 rows per 4 inches can you halve everything in the pattern?

Or, if the pattern says 15 st/20 rows and your yarn is 30 st/4- rows, can you use two strands to get the right gauge.

Or does all this depend on the type of yarn etc, so that one has to do a swatch etc in order to determine the viability of the change?

Thanks, Cathy

It’s a place to start. I don’t think I’d jump in on anything elaborate, but if I was making a scarf, for example, I’d probably dive in with that type of comparison in mind. If it doesn’t work, there’s not too much to take out.

If it was something large, a swatch would be in order.