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Another way to do this would be to make each dimension a "mouseover" field. When you hover your mouse pointer over the field the other unit(s) would display. So for instance a field that shows a dimension of 1" would display 2.54cm when hovered over. (This is similar to what happens when you hover the mouse pointer over a link; the target URL appears.)
Programming this to work universally on the REI site would take some effort I imagine but it would address the issue automatically and everywhere.