Standard height for toilet roll holder?

18-22 inches.

I would install a toilet roll holder around 18" up off the floor. It should be around the height of your legs (or a bit higher) when sitting on the 'ol throne.

4 inches There is no "AVERAGE" per-se however, 30 to 36 inches high and 6 to 10 inches in front of the toilet works well. These measurements are to centerline of the roll. There are limits...

A little higher than the cat can reach.

Here is a brief copy of a blog from my site (see bio for info) which defines the heights of toilets and advantages of taller toilets Handicap toilets or ADA toilets refer to the toilets made...