买戒指怎么量尺寸

我想买个戒指怎样测量手指知道尺寸指内径(mm)...周长(mm)《1》15mm........47mm《2》15.5mm......48.7mm《3》16mm........50.2mm《4》16.5mm......52mm《5》17mm........53.5mm《6》17.5mm......55mm《7》18mm........56.5mm《8》18.5mm......58mm《9》19mm........59.7mm《10》19.5mm......61.2mm《11》20mm........63...