Fast-Stairs provide a unique, quick and easy way to build long lasting, free-standing 2½ to 13 ft high stairs. No stair stringer calculator required. No more figuring out the rise and run of steps. Simply measure your floor to floor vertical height and then choose the Fast-Stairs adjustable model that fits the desired height of your stairs. Make sure you have the necessary bottom floor horizontal run and that your top floor opening is large enough (usually 70% of the horizontal run measurement) for an 80″ head clearance.