Grip diameter of a riser bar? I bought a riser bar (Easton EA30) and I want to buy grips and brake levers to put on it. I know the clamp diameter is 31.8mm, but how do I figure out the bar end diameter? I don't have a caliper to measure it and it's not written anywhere (box, instructions manual, ...).

As suggested by one of the answer, the grip size is likely going to be 22.2 What makes me not sure is the fact that Easton (the same manifacturer of the bar) only sells grips of size 30 and 33 (Ref: https://www.eastoncycling.com/products/details/lock-on-grips)
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.