Our front railing is 3.75 inches and the planter fits perfectly. Snug and won’t tip. Our back railing is 5.5 inches wide and it fits less snuggly. I think also because there’s less planter bottom to hold onto the wider railing, I don’t think it would be as secure. They accidentally sent us the chocolate color which I think actually looks nice with our house so I did not call or return for terra cotta.