Riv bike measures the bars they sell center of the curve to center of the curve right where the levers mount not at the ends. On my road bikes I will use a 42cm bar, on bikes that will see trail duty such as my cross bikes I will go up one size to 44 regardless of who makes the bar. In other words I ride Nitto bars the same width as I would any road bar.

Riv's sizing suggestion works well with the kind of bikes they sell that put you in a relatively upright position for a drop bar equipped bike, not so well for a bike that has aggressive racing geometry.