It’s important that your steps have the correct ratio of tread to rise going up and down safely and easily. Generally a good rule of thumb is, the smaller the rise, the greater allowable tread. You should choose what’s comfortable for you but outdoor steps should have a rise of 5 1/2 to 7 1/2 inches and a tread of 11 to 18 inches. A good standard step is typically referred to as 7 - 11, most importantly you want to have all the steps consistent.
Below is an easy formula to find the best combination depending on your total height of the steps and the material you choose.

Step tread + (2 × riser) = 25 or 26

Steps are made up of the tread, which is the part you step on, and the rise, which is the part you trip on! 
In hardscaping, these terms can be referred to as Caps and Risers.

Step tread + (2 × riser) = 25 or 26 inches
Step treadStep riser
18 in.4 in.

17 in.4.5 in.
16 in.5 in.

15 in.5.5 in.
14 in.6 in.

13 in.6.5 in.
12 in.7 in.
11 in.7 in.

It's about what works best for you, there are many combinations and endless possibilities with all the materials available. You don't want any surprises when the project is completed. 

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