Gripable Handrail Height
To measure the handrail height correctly, measure
from the tip of the stair tread nosing, to the top of the
gripable handrail. This measurement must be between 34 and 38
If you're having a problem measuring this distance, lay a Straight edge on top of the stair treads, at an angle, as shown in the picture above. Then you can measure from the bottom of the Straight edge to the top of the handrail. Remember that this measurement must be plumb.
In simpler terms, use a level to make sure that your measurement is perfectly vertical (up and down), otherwise you might have a problem with the building department, if you're handrailing isn't within 34 and 38 inches.
Remember that a gripable handrail is something that you hold on to, while walking up and down the stairs and should be secured properly.