The tread angle of a spiral staircase with 19 steps and a spiral angle of 360° is 20°. We get that value by subtracting 1 from the number of steps (to get the number of treads). For that, we get 19 − 1 = 18. Then divide the spiral angle by the number of treads to find the tread angle to be: 360° / 18 = 20°. Read more