The angle you need can be computed by knowing what is the rise of the
stairs and what is the run. Most stairs have something on the order of
a 9" tread (run) and a 7 1/2" rise. Using a carpenters framing square
makes the job easy to figure out. Put a straight-edge on the markings
on the square you measured for rise and run to create a triangle. You
can then either use a protractor to measure the angle or simply compute
it mathematically using plane geometry.

Now that I have said all that, when you cut the bannisters you cannot
assume that your posts are plumb. Even if they are, I recommend you use
a bevel square and measure directly from the posts to find your cutting
angle. Clamp a level to each post and set it to true level. Then take
your measurements with the bevel and transfer this angle to your saw for
making your cuts. Make your first cut and place the rail against the
outside of the post and eyeball your angle. If it looks good you can
make the second cut with the angle you measured from the next post. I
suggest you make this cut approximately 1/2" long. That should give you
enough material to finesse a third and final cut so the rail is snug.
And you will have to do this all over again with your bottom rail. Just
remember to ensure to maintain identical drops under each juncture with
your rails and posts. BTW - don't be surprised if the posts do not have
faces that are truly parallel to each other. In fact, it is likely they
won't be. This means you have a compound angle to fit. A belt sander
makes this job easier, but it can be done by hand. Best of luck ...

Amy L. wrote:
We just purchased our home and the deck off the house was not
completed. Everything was done except the handrails. We have all the
handrails completed around the deck except for the stairs.

I was wondering if their is a standard angle for the stair railing?
Is their a formula that should be used to calculate this?

Thanks
Amy.