transposing yarn weights from one pattern to another

I am trying to determine how much yarn I will need for a pattern if I do not use the yarn recommended in the pattern. The pattern class for Drops Big Merino: 3 50gram balls, 175 meters, 190 yards. I would like to use Berrocco Ultra Alpaca 100 gram skein, 215 yards. Can I substitute with just the one skein of Berrocco?

The Drops yarn and any of the Ultra Alpaca I see are different gauges which will affect how much yarn you need and the size of the project. Which Ultra Alpaca are you wanting to use?

I have and want to use as a substitute for the Drops Big Merino yarn, Berocco Ultra Alpaca.The gauges are different. The Drops Merino gauge using size 8US needles is 17 stitches& 37 rows over 10x10cm. The Berocco gauge is 20 sts. 26rows over 10x10 cm. Pattern is in garter stitch. Does that help?

Well, what I meant was is the yarn worsted, DK, sport, but the gauge is most important. It’s difficult to determine how much yarn you need because the gauge is different. What are you making?

The yarn I want to use is Aran (also called Worsted). Seems like these terms are interchangeable. I am trying to make a baby’s romper for my new grand baby girl. Does that help?

The gauge difference will affect size of the garment likely making it smaller, but your own gauge will affect that as well. Personally I would get two skeins of whatever you want and if you don’t need the second one you can use it for a hat or booties. If it’s not wound into a ball some stores will also allow you to return it, but do ask first.