The average stair tread size is within 8 to 10 inches long; however, there are longer stair treads for bigger staircases. 9-inch stair treads are the ones most used. To be sure you're purchasing the correct size for your staircase, follow the steps below:
Measure from the left of your step to the right. This is the max stair tread length you can purchase -- if you want full coverage, stick with this measurement.
Measure from the back of your step to the front. This is the max stair tread width you can purchase. For a narrower appearance, subtract a couple of inches from this measurement.