Solid line = floor (spellings that unambiguously belong to the group). Shaded band up to the dashed edge = ceiling (the most it could reach if every ambiguous spelling counted here).
Chelsea has been slowly declining from its former popularity. All-time it's been given to 216,121 U.S. babies, the 1,155th most common name by sound today, peaking at 20,857 in 1992, with 377 in 2025.
By sound, Chelsea also includes the spellings Chelsey, Chelsie, Chelsi, Chelsy.