Usually, the measure of the handrail is equal to the distance nose to nose of your stair.

 

Here a link to help you to measure your handrails

 

Stair and handrail code

https://www.thespruce.com/stair-handrail-and-guard-code-1822015

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Stairs Wikipedia stair. You could refer to the stair terminologies.

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https://en.wikipedia.org/wiki/Stairs#:~:text=The%20slope%20or%20pitch%20of,the%20nosing%20of%20the%20treads.