Pick the design and metal, enter the size it is and the size it needs to be. You get the bench answer with its reasons, the metal to add or take out, and where the jeweller would cut.
The ring at its real size. The coloured collar marks the back of the shank, where a jeweller cuts, adds or removes metal and solders it again; amber shows a piece added, a gap a piece removed.
GIA, Ring size guide: pavé and channel settings cannot be resized easily; eternity rings very difficult; twisted bands harder; small diamonds in the band make it harder and more expensive; easier to remove metal than to add; sizing beads; mechanical shanks; wide shanks fit more snugly.
GIA, Best ring material for everyday wear: gold and silver up to 2 sizes, platinum expensive to resize, titanium limited to half a size, tungsten cannot be resized. GIA 4Cs, engagement ring settings: tension settings very difficult and sometimes impossible to resize.
Stuller, Ring sizing (2007): 2.61 mm of shank per US size in bench practice; stretching (swaging) not for flat, patterned or tapered bands, previously sized bands or gallery tops, and not for more than two sizes. Stuller Bench Jeweler: cut away from the karat stamp and trademark. B. W. Simon, Fit to be sized (Bench, 2002, via Ganoksin): technique for taking down three sizes or more.
Size to circumference: US inside diameter 0.458 in + 0.032 in per size (2.55 mm of circumference per size), as JewelLoop's ring size converter. Densities: JewelLoop metal tables. The verdict is general bench guidance: a jeweller who sees the ring decides. Estimates for guidance; verify before production.