Floor space beats height. Rats use horizontal running space more than sheer tower height, so a wide cage with solid full-width shelves is better than a narrow tall one. A common rule of thumb is around 2 cubic feet (roughly 55-60 litres) of usable space per rat, but treat that as a floor, not a target: more is always better, and a stated "capacity" from a manufacturer is usually optimistic. Bar spacing: about 1.2 cm (half an inch) for babies and small females, up to about 2 cm for adult males. If a head fits, the rat fits. Horizontal bars are a bonus because rats climb them. Levels should be solid or fully covered. Bare wire mesh floors cause sore hocks and trapped legs; cover them with fleece, lino or wood. Keep drops between levels under about 30 cm, and break up longer falls with hammocks, shelves or a hanging bed. Older and heavier rats need shorter climbs and ramps. Look for a deep base tray so you can add digging substrate, doors wide enough to get your arm and a sick rat out easily, and powder-coated metal rather than plastic that gets chewed. Avoid glass tanks and aquariums: ventilation is poor and ammonia builds up fast, which is a genuine respiratory risk. Placement matters. Out of direct sun, away from draughts, away from kitchen fumes and aerosols, and ideally in a room where the household spends time.