I usually just go by Big Cheese Rodents' sizes since that's who I ordered from before I started raising rats. Of course, I don't really weigh my rats (I raise my own) so I just choose rats that I feel are appropriate for each snake. I do think petshop rats tend to be on the small side, though. Whenever I've had to buy small rats I've gotten what I would call weanlings, and the mediums were more like smalls. There's no way my 600g girl could eat Big Cheese mediums but she could eat the pet shop mediums I've bought in the past.

Big Cheese sizes:
Weanling: 36-50g
Small: 50-89g
Med: 90-150g
Lg: 180-279

Hope this helps,
Shelly

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