Length to weight?

I am getting my first BP in a few weeks and it’s listed as 18-22”. I know there is no age to weight ratio, but is there a ballpark weight for that length? It’s mainly for feeder size purposes as I want to have them ordered before I get the snake.

You should first find out if your BP is being fed on mice or rats...even IF you want to change that, you don't want to do that now, you want to feed a new
snake what they're used to eating for best feeding results. Also find out if the snake is being fed (hopefully!) f/t (frozen-thawed) prey.