If you’re not an expert in building stairs, you may find the math a little bit challenging. The part most people find difficult is calculating the angle of the stair stringers and where they meet with the upper and lower landings . There are some excellent tools out there to make the task of planning stair stringers or calculating the rise and run of the stairs.
This calculator gives a great visual and even provides a cutting diagram for your stair stringer and it’s FREE!
Checkout 
this great tool.
