The RSPCA recommend that hamsters should have a cage with 80x50 cm of unbroken floor space. Any cages smaller than this must be marketed as extension cages only.
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Hamsters have been wrongly marketed for years as small pocket pets ideal for children. This is simply not the case. They need decent sized cages. It is cruel for hamsters to be kept in cages that do not meet the minimum requirements of floor space that a hamster needs. It causes stress and depression.