How many mice= a med rat?

Ok so my pet store only has xl rats but plenty of hoppers...
how many of thoes to = a med rat?

Depends Medium rats range from 90-150 grams each.

Adult mice are about 15-20 grams each
Retired Breeder Mice are 30-50 grams each.

Medium rat = 150grams or so and hopper mouse = 12grams. 12 x 13 = 156. That would mean 13 hopper mice would be equivalent to 1 medium rat.

I would say that 6-7 adult mice equals one medium rat. (However, like others, I don't weigh rodents, so I am just guessing by thinking about sizes).