Yarn usage when crocheting with yarn held double

Hi everyone, I hope someone can help me out.

I want to crochet a thick granny square jacket.

I’d need 20 skeins if I were to crochet with a single strand: 50 g, 135 m yardage, 4 mm recommended. Since I crochet very tightly, I’d use a 5 mm hook in this case.

Out of laziness—and also because I want the jacket to be thicker—I’d like to hold the yarn double.

Unfortunately, I have no idea how many skeins I’ll need then. It can’t possibly be 40, can it?

Does anyone have any advice?

P.S. By the way, I crochet without a pattern, so I don’t have to follow any pattern or gauge.  

Just to be safe, I’d assume you’ll need double the amount if you’re using the same yarn.

Or you could just get thicker yarn and check the corresponding yardage.

No, you don’t need twice as much, since the granny squares will reach their full size faster when worked with double yarn.
I think 30 skeins should be enough. 

Thank you—I just needed a rough estimate.