How large should a cage be for a BP?

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How large should a cage be for a ball python? Whenever I look this up they always say 20-30 gallons and how am I supposed to know how much that is?

Because they don't have legs, the dimensions of width and depth are more important than the overall volume. Essentially you want something in the range of 1sqft of floor space (wxd) for each foot of snake occupying the space, so a 6ft adult would need 6sqft of floor space.