Adjusting for different weight yarn

I FINALLY found a pattern for a summer sweater that I like. Unfortunately, said pattern calls for worsted weight yarn. I’d like to use a finer yarn, more like a baby or sportsweight. Is there a formula I can use to figure out how much yarn to purchase of the lighter weight yarn? Do I compare the weights of yarn (how many grams per ball) or yards per ball? I’m planning on using larger needles in order to get the correct gauge.

Sometimes using the stitch gauge will give you a percentage to apply to the original yardage, but I’m not certain how close it is. I would knit thinner yarn on size 7 or 8 (or larger) but most people wouldn’t thinking it would be too loose a knit. But that’s what you want. So if you can get the same gauge and like how it looks, it should be about the same yardage.

If you don’t like the fabric, you can knit a larger size of the pattern on smaller needles if your sts/inch work out close to the sts needed for one of the other sizes. Then you’d use the yardage given for that size since you’d have the same number of sts, though you may need to do more rows.