King beds are too large and unwieldy to protect properly with a mattress bag. That’s why we recommend and sell King-size mattress boxes. The right protection for your bed. Get one for your mattress, and if you have just one King-size foundation/box-spring, another for that.
Some King Mattresses come with a split foundation of two box-springs of approximately Twin size. If that’s what you have for your King bed, get one King box for the mattress and two of our Twin bags for the split foundation.