Grip diameter of a riser bar?
Have a look at Sheldon Brown's handlebar reference. It lists typical clamp and outer bar diameters.

Unfortunately it does not contain oversized mountain bike handlebars, like your 31.8 mm riser bar. I suspect that the typical diameter for mountain bike bars at the handles at the grips is still 22.2 mm. You may see in the table that while the clamp diameter changed often, the outer diameter stayed constant at 22.2 mm and 23.8, respectively, for mountain bikes and road bikes.