Riser CalculatorPlease click on the desired spiral stair diameter, then select the floor to floor height. 3 1/2 Foot Diameter (42″) 84″ floor to floor height 85″ floor to floor height 4 Foot Diameter (48″) 84″ floor to floor height 4 1/2 Foot Diameter (54″) 84″ floor to floor height 85″ floor to floor height 5 Foot Diameter (60″) 84″ floor to floor height 5 1/2 Foot Diameter (66″) 84″ floor to floor height 6 Foot Diameter (72″) 84″ floor to floor height